Final Flashcards

Question

abiotic facors

Answer 1

interactions between organisms and their nonliving, physical environment. Include precipitation, temperature, pH, wind, and chemical nutrients.

Answer 2

a scientific discipline with ties to ecology, focuses on how humans interact with the environment.

Answer 3

a group consisting of members of the same species that live together in a prescribed area at the same time.

Answer 4

considers both the number of individuals of a particular species that are found in an area and the dynamics of the population.

Answer 5

the study of changes in populations—how and why those numbers increase or decrease over time.

Answer 6

the number of individuals of a species per unit of area or volume at a given may be determined in large part by biotic or abiotic factors in the environment that are external to the individuals in the population.

Answer 7

occurs when individuals in a population are spaced throughout an area in a manner that is unrelated to the presence of others Of the three major types of dispersion, random dispersion is least common and hardest to observe in nature, leading some ecologists to question its existence.

Answer 8

most common spacing clumped, also called aggregated distribution or patchiness, which occurs when individuals are concentrated in specific parts of the habitat. Clumped dispersion often results from the patchy distribution of resources in the environment. It also occurs among animals because of the presence of family groups and pairs, and among plants because of limited seed dispersal or asexual reproduction. Clumped dispersion may sometimes be advantageous because social animals derive many benefits from their association.

Answer 9

occurs when individuals are more evenly spaced than would be expected from a random occupation of a given habitat. Uniform dispersion also occurs when competition among individuals is severe, when plant roots or leaves that have been shed produce toxic substances that inhibit the growth of nearby plants, or when animals establish feeding or mating territories. Some populations have different spacing patterns at different ages.

Answer 10

Population size changes over time. On a global scale, this change is ultimately caused by two factors, expressed on a per capita (that is, per individual) basis: **natality,** the average per capita birth rate, and **mortality,** the average per capita death rate. In humans the birth rate is usually expressed as the number of births per 1000 people per year and the death rate as the number of deaths per 1000 people per year.

Answer 11

In equation (1), ∆N is the change in the number of individuals in the population, ∆t the change in time, N the number of individuals in the existing population, b the natality, and d the mortality. (1) ∆N/∆t =N(b-d) or can write w/out the N. The growth rate (r), or rate of change (increase or decrease) of a population on a per capita basis, is the birth rate minus the death rate: (2) r=b-d instantaneous growth rate (dN/dt). (The symbols dN and dt are the mathematical differentials of N and t, respectively; they are not products, nor should the d in dN or dt be confused with the death rate, d.) Using differential calculus, this growth rate can be expressed as follows: (3) dN/dt=rN where N is the number of individuals in the existing population, t the time, and r the per capita growth rate.

Answer 12

movement of individuals among populations, must be considered when examining changes in populations on a local scale.

Answer 13

The maximum rate at which a population of a given species could increase under ideal conditions, when resources are abundant and its population density is low, is known as its intrinsic rate of increase (r_max). A particular species’ intrinsic rate of increase is influenced by several factors. These include the age at which reproduction begins, the fraction of the lifespan (duration of the individual’s life) during which the individual is capable of reproducing, the number of reproductive periods per lifetime, and the number of offspring the individual is capable of producing during each period of reproduction. These factors determine whether a particular species has a large or small intrinsic rate of increase.

Answer 14

If we plot the population size versus time, under optimal conditions, the graph has a J shape that is characteristic of exponential population growth, which is the accelerating population growth rate that occurs when optimal conditions allow a constant per capita growth rate. When a population grows exponentially, the larger that population gets, the faster it grows.

Answer 15

represents the largest population that can be maintained for an indefinite period by a particular environment, assuming there are no changes in that environment.

Answer 16

When a population regulated by environmental limits is graphed over longer periods, the curve has a characteristic S shape. The curve shows the population’s initial exponential increase (note the curve’s J shape at the start, when environmental limits are few), followed by a leveling out as the carrying capacity of the environment is approached. The S-shaped growth curve, also called logistic population growth, can be modeled by a modified growth equation called a *logistic equation.* The logistic model of population growth was developed to explain population growth in continually breeding populations. Similar models exist for populations that have specific breeding seasons. The logistic model describes a population increasing from a small number of individuals to a larger number of individuals that are ultimately limited by the environment. The logistic equation takes into account the carrying capacity of the environment: (5) dN/dt=rN[(K-N)K]

Answer 17

Sometimes the influence of an environmental factor on the individuals in a population varies with the density or crowding of that population. If a change in population density alters how an environmental factor affects that population, then the environmental factor is said to be a density-dependent factor.

Answer 18

an interaction among two or more individuals that attempt to use the same essential resource, such as food, water, sunlight, or living space, that is in limited supply. Competition occurs both within a given population (**intraspecific competition**) and among populations of different species (**interspecific competition**).

Answer 19

Individuals of the same species compete for a resource in limited supply by interference competition or by exploitation competition. In interference competition, also called contest competition, certain dominant individuals obtain an adequate supply of the limited resource at the expense of other individuals in the population; that is, the dominant individuals actively interfere with other individuals’ access to resources. species in which interference competition operates experience a relatively small drop in population size, caused by the death of individuals that are unable to compete successfully.

Answer 20

In exploitation competition, also called scramble competition, all the individuals in a population “share” the limited resource more or less equally so that at high population densities none of them obtains an adequate amount. The populations of species in which exploitation competition operates often oscillate over time, and there is always a risk that the population size will drop to zero.

Answer 21

Any environmental factor that affects the size of a population but is not influenced by changes in population density is called a density-independent factor. Such factors are typically abiotic. Random weather events that reduce population size serve as density-independent factors.

Answer 22

Species that expend their energy in a single, immense reproductive effort

Answer 23

exhibit repeated reproductive cycles—that is, reproduction during several breeding seasons— throughout their lifetimes.

Answer 24

Ecologists try to understand the adaptive consequences of various life history traits, such as semelparity and iteroparity. Adaptations such as reproductive rate, age at maturity, and **fecundity** (potential capacity to produce offspring), all of which are a part of a species’ life history traits, influence an organism’s survival and reproduction. Although many different life histories exist, some ecologists recognize two extremes: r-selected species and K-selected species.

Answer 25

The ability of an individual to reproduce successfully, thereby making a genetic contribution to future generations of a population, is called its fitness

Answer 26

Populations described by the concept of r selection have traits that contribute to a high population growth rate. Recall that r designates the per capita growth rate. Because such organisms have a high r, biologists call them **r strategists** or **r-selected species**. Small body size, early maturity, short lifespan, large broods, and little or no parental care are typical of many r strategists, which are usually opportunists found in variable, temporary, or unpredictable environments where the probability of long-term survival is low. Some of the best examples of r strategists are insects & plants.

Answer 27

In populations described by the concept of K selection, traits maximize the chance of surviving in an environment where the number of individuals is near the carrying capacity (K) of the environment. These organisms, called **K strategists** or **K-selected** species, do not produce large numbers of offspring. They characteristically have long lifespans with slow development, late reproduction, large body size, and a low reproductive rate. K strategists tend to be found in relatively constant or stable environments, where they have a high competitive ability. Animals that are K strategists typically invest in parental care of their young.

Answer 28

A life table can be constructed to show the mortality and survival data of a population or **cohort**, a group of individuals of the same age, at different times during their lifespan.

Answer 29

Survivorship is the probability that a given individual in a population or cohort will survive to a particular age. Plotting the logarithm (base 10) of the number of surviving individuals against age, from birth to the maximum age reached by any individual, produces a **survivorship curve**. FIGURE 53-8 shows the three main survivorship curves that ecologists recognize. In Type I survivorship, the young and those at reproductive age have a high probability of surviving. The probability of survival decreases more rapidly with increasing age; mortality is concentrated later in life. In Type III survivorship, the probability of mortality is greatest early in life, and those individuals that avoid early death subsquently have a high probability of survival, that is, the probability of survival increases with increasing age. In Type II survivorship, which is intermediate between Types I and III, the probability of survival does not change with age. The probability of death is equally likely across all age groups, resulting in a linear decline in survivorship. This constancy probably results from essentially random events that cause death with little age bias. Although this relationship between age and survivorship is rare.

Answer 30

A population that is divided into several local populations among which individuals occasionally disperse (emigrate and immigrate).

Answer 31

Good habitats, called source habitats, are areas where local reproductive success is greater than local mortality.

Answer 32

Source populations generally have greater population densities than populations at less suitable sites, and surplus individuals in the source habitat disperse and find another habitat in which to settle and reproduce.

Answer 33

Individuals living in lower quality habitats may suffer death or, if they survive, poor reproductive success. Lower quality habitats, called sink habitats, are areas where local reproductive success is less than local mortality

Answer 34

Without immigration from other areas, a sink population declines until extinction occurs. If a local population becomes extinct, individuals from a source habitat may recolonize the vacant habitat at a later time. Source and sink habitats, then, are linked to one another by dispersal.

Answer 35

characteristic J curve of exponential population growth shown in Figure 53-13 reflects the decreasing amount of time it has taken to add each additional billion people to our numbers. Thomas Malthus (1766–1834), a British clergyman and economist, was one of the first to recognize that the human population cannot continue to increase indefinitely. He maintained that the inevitable consequences of population growth are famine, disease, and war. **zero population growth**, the point at which the birth rate equals the death rate (r = 0), will occur toward the end of the 21st century.

Answer 36

the science that deals with human population statistics such as size, density, and distribution, provides information on the populations of various countries. As you probably know, not all parts of the world have the same rates of population increase. Countries can be classified into two groups, highly developed and developing, based on their rates of population growth, degrees of industrialization, and relative prosperity

Answer 37

such as the United States, Canada, France, Germany, Sweden, Australia, and Japan, have low rates of population growth and are highly industrialized relative to the rest of the world. have the lowest birth rates in the world. Indeed, some highly developed countries such as Germany have birth rates just below that needed to sustain the population and are thus declining slightly in numbers. Highly developed countries also have low infant mortality rates. longer life expectancies

Answer 38

fall into two subcategories: moderately developed and less developed. Mexico, Turkey, Thailand, and most countries of South America are examples of moderately developed countries. Their birth rates and infant mortality rates are generally higher than those of highly developed countries, but the rates are declining. Moderately developed countries have a medium level of industrialization. Bangladesh, Niger, Ethiopia, Laos, and Cambodia are examples of less developed countries. These countries have the highest birth rates, the highest infant mortality rates, the lowest life expectancies.

Answer 39

the amount of time it would take for its population to double in size, assuming its current growth rate did not change.

Answer 40

the number of children a couple must produce to “replace” themselves.

Answer 41

the average number of children born to a woman during her lifetime

Answer 42

To predict the future growth of a population, it is important to know its age structure, which is the number and proportion of people at each age in a population. The number of males and number of females at each age, from birth to death, are represented in an **age structure diagram.** The overall shape of an age structure diagram indicates whether the population is increasing, stationary, or shrinking. The age structure diagram for less developed countries is shaped like a pyramid. Because the largest percentage of the population is in the prereproductive age group (that is, 0 to 14 years of age), the probability of future population growth is great.

Answer 43

A strong population growth momentum exists because when all these children mature they will become the parents of the next generation, and this group of parents will be larger than the previous group. Thus, even if the fertility rates in these countries decline to replacement level (that is, couples have smaller families than their parents did), the population will continue to grow for some time. Population growth momentum can have either a positive value (that is, the population will grow) or a negative value (that is, the population will decline). However, it is usually discussed in a positive context, to explain how the future growth of a population is affected by its present age distribution.

Answer 44

occurs when the environment is worsening from too many people, even if those people consume few resources per person. current problem in many developing nations.

Answer 45

occurs when each individual in a population consumes too large a share of resources. The effect of consumption overpopulation on the environment is the same as that of people overpopulation—pollution and degradation of the environment. Most affluent highly developed nations, including the United States, suffer from consumption overpopulation. Highly developed nations represent about 18% of the world’s population, yet they consume significantly more than half its resources. These nations also generate 75% of the world’s pollution and waste. According to the Worldwatch Institute, a private research institution in Washington, D.C., highly developed nations account for the lion’s share of total resources consumed, as follows: 86% of aluminum used, 76% of timber harvested, 68% of energy produced, 61% of meat eaten, and 42% of fresh water consumed.

Answer 46

consists of an association of populations of different species that live and interact in the same place at the same time. Communities exhibit characteristic properties that populations lack. These properties, known collectively as *community structure* and *community functioning*, include the number and types of species present, the relative abundance of each species, the interactions among different species, community resilience to disturbances, energy and nutrient flow throughout the community, and productivity.

Answer 47

A biological community and its abiotic environment together compose an ecosystem.

Answer 48

the description and analysis of patterns and processes within the community. Finding common patterns and processes in a wide variety of communities helps ecologists understand community structure and functioning.

Answer 49

Certain species interact in positive ways, in a process known as **facilitation**, which modifies and enhances the local environment for other species. For example, alpine plants in harsh mountain environments grow faster and larger and reproduce more successfully when certain other plants are growing nearby. Also, each organism plays one of three main roles in community life: producer, consumer, or decomposer. Unraveling the many positive and negative, direct and indirect interactions of organisms living together as a community is one of the goals of community ecologists.

Answer 50

Every species is thought to have its own ecological role within the structure and function of a community; we call this role its ecological niche. includes the local environment in which a species lives—its **habitat**. A niche also encompasses what a species eats, what eats it, what organisms it competes with, and how it interacts with and is influenced by the abiotic components of its environment, such as light, temperature, and moisture. The niche thus represents the totality of adaptations by a species to its environment, its use of resources, and the lifestyle to which it is suited. Although a complete description of an organism’s ecological niche involves many dimensions and is difficult to define precisely, ecologists usually confine their studies to one or a few niche variables, such as feeding behaviors or ability to tolerate temperature extremes.

Answer 51

The potential ecological niche of a species is its **fundamental niche**, but various factors, such as competition with other species, may exclude it from part of this fundamental niche.

Answer 52

The lifestyle that a species actually pursues and the resources it actually uses make up its realized niche.

Answer 53

Any environmental resource that, because it is scarce or unfavorable, tends to restrict the ecological niche of a species. Most of the limiting resources that have been studied are simple variables, such as the soil’s mineral content, temperature extremes, and precipitation amounts.

Answer 54

(a) Species of barnacles belonging to two genera, *Chthamalus* and *Balanus*, grow in the intertidal zone of a rocky shore in Scotland. (b) When *Chthamalus* individuals were experimentally removed, *Balanus* individuals did not expand into their section of the rock. (c) When *Balanus* individuals were experimentally removed, *Chthamalus* individuals spread into the empty area.

Answer 55

occurs when two or more individuals attempt to use the same essential resource, such as food, water, shelter, living space, or sunlight. Competition occurs among individuals within a population (**intraspecific competition**) or between different species (**interspecific competition**).

Answer 56

hypothesized that one species excludes another from its niche as a result of interspecific competition. Although it is possible for species to compete for some necessary resource without being total competitors, two species with abso- lutely identical ecological niches cannot coexist. Coexistence oc- curs, however, if the overlap between the two niches is reduced.

Answer 57

Reduced competition among coexisting species as a result of each species’ niche differing from the others in one or more ways. Resource partitioning may also include timing of feeding, location of feeding, nest sites, and other aspects of a species’ ecological niche. Difference in root depth is an example of resource partitioning in plants.

Answer 58

Sometimes populations of two similar species occur together in some locations and separately in others. Where their geographic distributions overlap, the two species tend to differ more in their structural, ecological, and behavioral characteristics than they do where each occurs in separate geographic areas. Such divergence in traits in two similar species living in the same geographic area is known as character displacement. There are several well-documented examples of character displacement between two closely related species. The flowers of two Solanum species in Mexico are quite similar in areas where either one or the other occurs. However, where their distributions overlap, the two species differ significantly in flower size and are pollinated by different kinds of bees. In other words, character displacement reduces interspecific competition, in this case for the same animal pollinator. bill sizes of Darwin’s finches provide another example of character displacement. On large islands in the Galápagos where the medium ground finch (Geospiza fortis) and the small ground finch (G. fuliginosa) occur together, their bill depths are distinctive. Geospiza fuliginosa has a smaller bill depth that en- ables it to crack small seeds, whereas G. fortis has a larger bill depth that enables it to crack medium-sized seeds. However, G. fortis and G. fuliginosa also live on separate islands. Where the two species live separately, bill depths are about the same intermediate size, perhaps because there is no competition from the other species.

Answer 59

the consumption of one species, the prey, by another, the predator

Answer 60

The reciprocal adaptation of two or more species that occurs as a result of their close interactions over a long period.

Answer 61

Conspicuous colors or patterns, which advertise a species’ unpalatability to potential predators, are known as aposematic coloration (pronounced “ap′-uh-suh-mat′-ik”; from the Greek apo, “away,” and semat, “a mark or sign”), or warning coloration.

Answer 62

colors or markings that help them hide from predators by blending into their physical surroundings.

Answer 63

Sometimes a defenseless species (a mimic) is protected from predation by its resemblance to a species that is dangerous in some way (a model). Such a strategy is known as Batesian mimicry. For example, a harmless scarlet king snake looks so much like a venomous coral snake that predators may avoid it. Interestingly, the range of the scarlet king snake extends far beyond that of the coral snake. In an area where coral snakes do not occur, having the coloration of the model confers no special advantage to the king snake and may be harmful, triggering natural selection. This seems to be the case: in 2008 scientists reported in the journal Nature that scarlet king snake populations located far from coral snakes have undergone natural selection and look less like their model.

Answer 64

different species (co-models), all of which are poisonous, harmful, or distasteful, resemble one another. Although their harmfulness protects them as individual species, their similarity in appearance works as an added advantage because potential predators more easily learn a single common aposematic coloration. Scientists hypothesize that viceroy and monarch butterflies are an example of Müllerian mimicry.

Answer 65

any intimate relationship or association between members of two or more species.

Answer 66

the partners of a symbiotic relationship, called symbionts, may benefit from, be unaffected by, or be harmed by the relationship.

Answer 67

a symbiotic relationship in which both partners benefit. Mutualism is either obligate (essential for the survival of both species) or facultative (either partner can live alone under certain conditions).

Answer 68

a type of symbiosis in which one species benefits and the other one is neither harmed nor helped.

Answer 69

a symbiotic relationship in which one member, the **parasite**, benefits, whereas the other, the **host**, is adversely affected. The parasite obtains nourishment from its host. A parasite rarely kills its host directly but may weaken it, rendering it more vulnerable to predators, competitors, or abiotic stressors. When a parasite causes disease and sometimes the death of a host, it is called a **pathogen**.

Answer 70

Ticks and other parasites that live outside the host’s body.

Answer 71

Parasites such as tapeworms that live within the host.

Answer 72

are crucial in determining the nature of the entire community, that is, its species composition and ecosystem functioning. usually not the most abundant species in the community. Although present in relatively small numbers, the individuals of a keystone species profoundly influence the entire community because they often affect the amount of available food, water, or some other resource. Thus, the impact of keystone species is greatly disproportionate to their abundance. If a keystone species disappears from a community, many other spe- cies in that community may become more common or rarer, or may even disappear. The term keystone species was coined by ecologist Robert T. Paine in 1969, based on his experimental studies along the Pacific coast in the state of Washington (FIG. 54-15). Paine removed a predatory sea star, Pisaster ochraceus, from a rocky intertidal community that included barnacles, mussels, limpets, and chitons, all of which the sea star preyed on.Paine noted that when a keystone predator such as the sea star is removed from a community, the species diversity of that community changes dramatically. One problem with the concept of keystone species is that it is often difficult to measure all the direct and indirect impacts of a keystone species on an ecosystem.

Answer 73

greatly affect the community because they are very common. Trees, the dominant species of forests, change the local environment. Typically, a community has one or a few dominant species, and most other species are relatively rare.

Answer 74

**Bottom-up processes** are based on **food webs**, the interconnected series of organisms through which energy flows in an ecosystem. Food webs always have plants or other producers at the first (lowest) trophic level.

Answer 75

regulate ecosystem function by trophic interactions, particularly from the highest trophic level. If top-down processes dominate an ecosystem, the effects of an increase in the population of top predators cascade down the food web through the herbivores and producers. In fact, top-down processes are also known as a *trophic cascade*.

Answer 76

a measure of both the number of species within a community (species richness) and the relative importance of each species, based on its abundance, productivity, or size (species evenness). Ecologists have developed various mathematical expressions, such as the *Shannon index,* to represent species diversity quantitatively. These diversity indices enable ecologists to compare species diversity in different communities.

Answer 77

the number of species in a community, is determined by counting the species of interest. Tropical rain forests and coral reefs are examples of communities with extremely high species richness. In contrast, geographically isolated islands and mountaintops exhibit low species richness.

Answer 78

tells us about the relative abundance of one species compared to other species.

Answer 79

Species richness is usually greater at the margins of distinct communities than in their centers. The reason is that an **ecotone**, a transitional zone where two or more communities meet, contains all or most of the ecological niches of the adjacent communities as well as some that are unique to the ecotone. This change in species composition produced at ecotones is known as the **edge effect.**

Answer 80

any event in time that disrupts community or population structure.

Answer 81

The process of community development over time, which involves species in one stage being replaced by different species, is called succession. An area is initially colonized by certain early successional species that give way over time to others, which in turn give way much later to late-successional species. Ecologists distinguish between two types of succession, primary and secondary.

Answer 82

the change in species composition over time in a habitat that was not previously inhabited by organisms. No soil exists when primary succession begins. Bare rock surfaces, such as recently formed volcanic lava and rock scraped clean by glaciers, are examples of sites where primary succession might take place.

Answer 83

the change in species composition that takes place after some disturbance removes the existing vegetation; soil is already present at these sites. Abandoned agricultural fields or open areas produced by forest fires are common examples of sites where secondary succession occurs.

Answer 84

by Joseph H. Connell. he proposed that species richness is greatest at moderate levels of disturbance One of the difficulties with the intermediate disturbance hypothesis is defining precisely what constitutes an “intermediate” level of disturbance.

Answer 85

U.S. botanist Frederick E. Clements’s view was that a community went through certain stages of development, like the embryonic stages of an organism, and eventually reached n adult state; the developmental process was succession, and the adult state was the climax community. This cooperative view of the community, called the organismic model, stresses the interaction of the members, which tend to cluster in tightly knit groups within discrete community boundaries,

Answer 86

U.S. ecologist Henry A. Gleason, held that biological interactions are less important in the production of communities than are environmental gradients (such as climate and soil) or even chance. Indeed, the concept of a community is questionable. It may be a classification category with no reality, reflecting little more than the tendency of organisms with similar environmental requirements to live in similar places. This school of thought, called the individualistic model, emphasizes species individuality, with each species having its own particular abiotic living requirements. It holds that communities are therefore not interdependent associations of organisms. Rather, each species is independently distributed across a continuum of areas that meets its own individual requirements.

Answer 87

A complex interconnection of all the food chains in an ecosystem.

Answer 88

The series of organisms through which energy flows in an ecosystem. Each organism in the series eats or decomposes the preceding organism in the chain.

Answer 89

Individual communities and their abiotic environments are **ecosystems**, which are the basic units of ecology.

Answer 90

a subfield of ecology that studies energy flow and the cycling of chemicals among the interacting biotic and abiotic parts of an ecosystem.

Answer 91

The passage of energy in a one-way direction through an ecosystem.

Answer 92

also called *autotrophs*, or simply, *producers*, form the beginning of the food chain by capturing the sun’s energy through photosynthesis. Producers, by incorporating the chemicals they manufacture into their own *biomass* (living material), become potential food resources for other organisms. Plants are the most significant producers on land, whereas algae and cyanobacteria are important producers in aquatic environments.

Answer 93

All other organisms in a community are **consumers**, also called *heterotrophs*, that extract energy from organic molecules produced by other organisms. **Herbivores** are consumers that eat plants, from which they obtain the chemical energy of the producers’ molecules and the building materials used to construct their own tissues. Herbivores are, in turn, consumed by **carnivores**, consumers that reap the energy stored in the herbivores’ molecules. Other consumers, called **omnivores**, eat a variety of organisms, both plant and animal.

Answer 94

Some consumers, called **detritus feeders,** or **detritivores**, eat **detritus**, which is dead organic matter that includes animal carcasses, leaf litter, and feces. Detritus feeders and microbial decomposers destroy dead organisms and waste products. **Decomposers**, also called **saprotrophs**, include microbial heterotrophs that supply themselves with energy by breaking down organic molecules in the remains (carcasses and body wastes) of all members of the food chain. They typically release simple inorganic molecules, such as carbon dioxide and mineral salts, which may be reused by producers. Most bacteria and fungi are important decomposers.

Answer 95

Food webs are divided into **trophic levels** (from the Greek *tropho*, which means “nourishment”). Producers (organisms that photosynthesize) occupy the first trophic level, primary consumers (herbivores) occupy the second, secondary consumers (carnivores and omnivores) the third, and so on.

Answer 96

Ecologists sometimes compare trophic levels by determining the number of organisms, the biomass, or the relative energy found at each level. This information is presented graphically as **ecological pyramids**. The base of each ecological pyramid represents the producers, the next level is the primary consumers (herbivores), the level above that is the secondary consumers (carnivores), and so on. The relative area of each bar of the pyramid is proportional to what is being demonstrated.

Answer 97

shows the number of organisms at each trophic level in a given ecosystem, with a larger area illustrating greater numbers for that section of the pyramid. In most pyramids of numbers, fewer organisms occupy each successive trophic level. Thus, in African grasslands the number of herbivores, such as zebras and wildebeests, is greater than the number of carnivores, such as lions. *Inverted pyramids of numbers*, in which higher trophic levels have more organisms than lower trophic levels, are often observed among decomposers, parasites, and herbivorous insects. One tree provides food for thousands of leaf-eating insects, for example. are of limited usefulness because they do not indicate the biomass of the organisms at each level and they do not indicate the amount of energy transferred from one level to another.

Answer 98

illustrates the total biomass at each successive trophic level.

Answer 99

a quantitative estimate of the total mass, or amount, of living material; it indicates the amount of fixed energy at a particular time. Units of measure vary: biomass may be represented as total volume, dry weight, or live weight. The 90% reduction in biomass is an approximation; actual biomass reduction from one trophic level to the next varies widely. From this brief exercise, you see that although carnivores do not eat producers, a large producer biomass is required to support carnivores in a food web.

Answer 100

indicates the energy content, often expressed as kilocalories per square meter per year, of the biomass of each trophic level. A common method ecologists use to measure energy content is to burn a sample of tissue in a calorimeter; the heat released during combustion is measured to determine the energy content of the organic material in the sample. The second law of thermodynamics explains why there are few trophic levels: energy pyramids are short because of the dramatic reduction in energy content that occurs at each successive trophic level.

Answer 101

**gross primary productivity (GPP)** of an ecosystem is the rate at which energy is captured during photosynthesis. Thus, GPP is the total amount of photosynthetic energy captured in a given period.

Answer 102

Energy that remains in plant tissues after cellular respiration has occurred is called **net primary productivity (NPP).** That is, NPP is the amount of biomass (the energy stored in plant tissues) found in excess of that broken down by a plant’s cellular respiration for normal daily activities. NPP represents the rate at which this organic matter is actually incorporated into plant tissues to produce growth.

Answer 103

he remaining energy—less than 20%—is avail- able to produce new biomass, that is, new tissues. This net energy available for biomass production by consumer organisms is called **secondary productivity**. An ecosystem’s secondary productivity is based on its primary productivity.

Answer 104

The **persistence** of synthetic pesticides and industrial chemicals is a result of their novel chemical structures. These toxins accumulate in the environment because ways to degrade them have not evolved in natural decomposers such as bacteria.

Answer 105

When an organism does not metabolize (break down) or excrete a persistent toxin, the toxin simply gets stored, usually in fatty tissues. Over time, the organism may accumulate high concentrations of the toxin. The buildup of such a toxin in an organism’s body is known as **bioaccumulation**.

Answer 106

Organisms at higher trophic levels in food webs tend to store greater concentrations of bioaccumulated toxins in their bodies than do those at lower levels. The increase in concentration as the toxin passes through successive levels of the food web is known as **biological magnification.**

Answer 107

Matter moves in numerous cycles from one part of an ecosystem to another—that is, from one organism to another (in food chains) and from living organisms to the abiotic environment and back again. We call these cycles of matter **biogeochemical cycles** because they involve biological, geologic, and chemical interactions.

Answer 108

The global movement of carbon between the abiotic environment, including the atmosphere and ocean, and organisms is known as **the carbon cycle**.

Answer 109

Coal, oil, and natural gas, called **fossil fuels** because they formed from the remains of ancient organisms, are vast deposits of carbon compounds, the end products of photosynthesis that occurred millions of years ago.

Answer 110

in which nitrogen cycles between the abiotic environment and organisms, has five steps: nitrogen fixation, nitrification, assimilation, ammonification, and denitrification. Bacteria are exclusively involved in all these steps except assimilation.

Answer 111

The first step in the nitrogen cycle, biological **nitrogen fixation** involves conversion of gaseous nitrogen (N₂) to ammonia (NH₃). This process fixes nitrogen into a form that organisms can use. Combustion, volcanic action, lightning discharges, and industrial processes also fix nitrogen as nitrate (NO₃⁻). *Nitrogen-fixing bacteria*, including cyanobacteria and certain other freeliving and symbiotic bacteria, carry on biological nitrogen fixation in soil and aquatic environments. Nitrogen-fixing bacteria employ an enzyme called **nitrogenase** to break up molecular nitrogen and combine the resulting nitrogen atoms with hydrogen.

Answer 112

Filamentous cyanobacteria have special oxygen- excluding cells called **heterocysts** that function to fix nitrogen.

Answer 113

The second step of the nitrogen cycle is **nitrification**, the conversion of ammonia (NH₃) or ammonium (NH₄⁺), formed when water reacts with ammonia, to nitrate (NO₃⁻). Soil bacteria are responsible for the two-phase process of nitrification, which furnishes these bacteria, called nitrifying bacteria, with energy.

Answer 114

In the third step, **assimilation**, roots absorb ammonia (NH₃), ammonium (NH₄⁺), or nitrate (NO₃⁻) formed by nitrogen fixation and nitrification, and incorporate the nitrogen into proteins, nucleic acids, and chlorophyll.

Answer 115

The fourth step, **ammonification**, is the conversion of organic nitrogen compounds into ammonia (NH₃) and ammonium ions (NH₄⁺). Ammonification begins when organisms produce 4 nitrogen-containing wastes such as urea in urine and uric acid in the wastes of birds. As these substances, along with the nitrogen compounds in dead organisms, decompose, nitrogen is released into the abiotic environment as ammonia (NH₃). The bacteria that perform ammonification in both the soil and aquatic environments are called *ammonifying bacteria*. Most available nitrogen in the soil derives from the recycling of organic nitrogen by ammonification.

Answer 116

The fifth step of the nitrogen cycle is **denitrification**, the reduction of nitrate (NO₃⁻) to gaseous nitrogen (N₂). Denitrifying bacteria reverse the action of nitrogen-fixing and nitrifying bacteria by returning nitrogen to the atmosphere as nitrogen gas. ## Footnote Dentrifying bacteria are anaerobic and therefore live and grow best where there is little or no free oxygen. For example, they are found deep in the soil near the water table, an environment that is nearly oxygen-free.

Answer 117

the high temperature of combustion converts some atmospheric nitrogen to **nitrogen oxides**. Automobile exhaust is one of the main sources of nitrogen oxides.

Answer 118

Nitrogen oxides are a necessary ingredient in the production of **photochemical smog**, a mixture of several air pollutants that injure plant tissues, irritate eyes, and cause respiratory problems in humans.

Answer 119

Nitrogen oxides also react with water in the atmosphere to form nitric acid (HNO₃) and nitrous acid (HNO₂). When these and other acids leave the atmosphere as **acid deposition**, they decrease the pH of surface waters (lakes and streams) and soils. Acid deposition has been linked to declining animal populations in aquatic ecosystems. On land, acid deposition alters soil chemistry: certain essential minerals, such as calcium and potassium, wash out of the soil and are therefore unavailable for plants. Nitrous oxide (N₂O), one of the nitrogen oxides, retains heat in the atmosphere (like CO₂) and so promotes global climate change.

Answer 120

Phosphorus does not exist in a gaseous state and therefore does not enter the atmosphere. In the **phosphorus cycle,** phosphorus cycles from the land to sediments in the ocean and back to the land. As water runs over rocks containing phosphorus, it gradually erodes the surface and carries off inorganic phosphate (PO₄³⁻). The erosion of phosphorus rocks releases phosphate into the soil, where it is taken up by roots in the form of inorganic phosphates. Once in cells, phosphates are incorporated into a variety of biological molecules, including nucleic acids, ATP, and the phospholipids that make up cell membranes. Animals obtain most of their required phosphorus from the food they eat, although in some places drinking water may contain a substantial amount of inorganic phosphate. Phosphate released by decomposers becomes part of the pool of inorganic phosphate in the soil that plants reuse. Thus, like carbon and nitrogen, phosphorus moves through the food web as one organism consumes another. Dissolved phosphate enters aquatic ecosystems through absorption by algae and aquatic plants, which zooplankton and larger organisms consume. In turn, a variety of fishes and mollusks eat the zooplankton. Ultimately, decomposers break down wastes and dead organisms to release inorganic phosphate into the water, making it available for use again by aquatic producers.

Answer 121

water continuously circulates from the ocean to the atmosphere to the land and back to the ocean. Water moves from the atmosphere to the land and ocean in the form of precipitation (rain, sleet, snow, or hail). Water that evaporates from the ocean surface and from soil, streams, rivers, and lakes eventually condenses and forms clouds in the atmosphere. In addition, **transpiration**, the loss of water vapor from land plants, adds a considerable amount of water vapor to the at- mosphere. Roughly 97% of the water a plant absorbs from the soil is transported to the leaves, where it is lost by transpiration. ## Footnote Water may evaporate from land and re-enter the atmosphere directly. Alternatively, it may flow in rivers and streams to coastal **estuaries**, where fresh water meets the ocean. The movement of surface water from land to ocean is called *runoff*, and the area of land drained by runoff is called a *watershed*. Water also percolates (seeps) downward in the soil to become *groundwater*, where it is trapped and held for a time. The underground caverns and porous layers of rock in which groundwater is stored are called *aquifers*. Groundwater may reside in the ground for hundreds to many thousands of years, but eventually it supplies water to the soil, streams and rivers, plants, and the ocean. The human removal of more groundwater than precipitation or melting snow recharges, called *aquifer depletion*, eliminates groundwater as a water resource. Regardless of its physical form (solid, liquid, or vapor) or location, every molecule of water eventually moves through the hydrologic cycle. As is true of the other cycles, water (in the form of glaciers, polar ice caps, and certain groundwater) can be lost from the cycle for thousands of years.

Answer 122

The sun powers the hydrologic cycle, carbon cycle, and other biogeochemical cycles and is the primary determinant of climate. Without the sun, almost all life on Earth would cease.

Answer 123

Earth’s rotation influences the direction that wind blows. Because Earth rotates from west to east, wind swerves to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This tendency of moving air to be deflected from its path by Earth’s rotation is known as **the Coriolis effect.**

Answer 124

The persistent prevailing winds blowing over the ocean produce mass movements of surface ocean water known as **ocean currents**. The prevailing winds generate circular ocean currents called *gyres*. For example, in the North Atlantic, the tropical trade winds tend to blow toward the west, whereas the westerlies in the midlatitudes blow toward the east. This helps establish a clockwise gyre in the North Atlantic. Thus, surface ocean currents and winds tend to move in the same direction, although there are many variations on this general rule. The Coriolis effect is partly responsible for the paths that surface ocean currents travel. Earth’s rotation from west to east causes surface ocean currents to swerve to the right in the Northern Hemisphere, producing a clockwise gyre of water currents. In the Southern Hemisphere, ocean currents swerve to the left, producing a counterclockwise gyre.

Answer 125

ENSO is a periodic warming of surface waters of the tropical eastern Pacific that alters both oceanic and atmospheric circulation patterns and results in unusual weather in areas far from the tropical Pacific. The warmer sea-surface temperatures and accompanying changes in ocean circulation patterns off the west coast of South America prevent colder, nutrient-laden deeper waters from **upwelling** (coming to the surface). The lack of nutrients in the water results in a severe decrease in the populations of anchovies and many other marine fishes. Other species, such as shrimp and scallops, thrive during an ENSO event.

Answer 126

the average weather conditions, plus extremes (records), that occur in a given place over a period of years. The two most important factors that determine an area’s climate are temperature (both average temperature and temperature extremes) and precipitation (both average precipitation and seasonal distribution). Other climate factors include wind, humidity, fog, cloud cover, and lightning-caused wildfires. Unlike weather, which changes rapidly, climate generally changes slowly, over hundreds or thousands of years. The wide variety of organisms on Earth evolved in part because of the many different climates, ranging from cold, snow-covered, polar climates to hot, tropical climates where it rains almost every day.

Answer 127

As the air mass moves down on the other side of the mountain, it is warmed and clouds evaporate, thereby lessening the chance of precipitation of any remaining moisture. This situation exists on the west coast of North America, where precipitation falls on the western slopes of mountains that are close to the coast. The dry lands on the sides of the mountains away from the prevailing wind (in this case, east of the mountain range) are called **rain shadows.** A rain shadow is the arid or semiarid land that occurs on the leeward side of a mountain.

Answer 128

Differences in elevation, in the steepness and direction of slopes, and therefore in exposure to sunlight and prevailing winds may produce local variations in climate known as **microclimates**, which are sometimes quite different from their overall surroundings.

Answer 129

the clearance of large expanses of forest for agriculture or other uses. When a forest is removed, the total amount of water and minerals that flow into streams increases drastically.

Answer 130

a conservation approach that emphasizes restoring and maintaining the quality of an entire ecosystem rather than the conservation of individual species.

Answer 131

a large, relatively distinct terrestrial region that has similar climate, soil, plants, and animals regardless of where it occurs. Because it covers such a large geographic area, a biome encompasses many interacting landscapes.

Answer 132

(also called arctic tundra) occurs in extreme northern latitudes wherever snow melts seasonally. long, harsh winters and extremely short summers. soils tend to be geologically young. These soils are usually nutrient poor and have little organic litter in the uppermost layer of soil. Although the soil surface melts during the summer, tundra has a layer of permanently frozen ground called **permafrost** that varies in depth and thickness. Because permafrost interferes with drainage, the thawed upper zone of soil is usually waterlogged during the summer. Limited precipitation, combined with low temperatures, flat topography (surface features), and permafrost, produces a landscape of broad, shallow lakes, sluggish streams, and boggy plains covered by lichens and small plants. Low species richness and low primary productivity regenerates quite slowly after it has been disturbed. Long-lasting injury, likely to persist for hundreds of years, has been done to large portions of the arctic tundra as a result of oil exploration and military use.

Answer 133

A similar ecosystem located in the higher elevations of mountains, above the tree line, is called **alpine tundra** to distinguish it from arctic tundra. ## Footnote An ecosystem located in the higher elevations of mountains, above the treel ine and below the snow line.

Answer 134

south of the tundra** (**or **taiga)**, which stretches across both North America and Eurasia. world’s largest biome. Winters are extremely cold and severe, although not as harsh as in the tundra. receives little precipitation and its soil is typically acidic, is low in minerals, and has a deep layer of partly decomposed conifer needles at the surface. contains numerous ponds and lakes in water-filled depressions that grinding ice sheets dug during the last Ice Age. Black and white spruces, balsam fir, eastern larch, and other conifers dominate the boreal forest, but **deciduous** trees such as aspen or birch, which shed their leaves in autumn, form striking stands. Animal life includes some larger species. However, most animal life is medium-sized to small. Most species of birds are seasonally abundant but migrate to warmer climates for winter. Insects are numerous, but there are few amphibians and reptiles except in the southern boreal forest. Most not suitable for agriculture because of its short growing season and mineral-poor soil. Harvested primarily by clear-cutting, is currently the primary source of the world’s industrial wood and wood fiber.

Answer 135

A coniferous biome characterized by cool weather, dense fog, and high precipitation, e.g., the north Pacific coast of North America. Winters are mild, and summers are cool. Relatively nutrient-poor soil, although its organic content may be high. Cool temperatures slow the activity of bacterial and fungal decomposers. Thus, needles and large fallen branches and trunks accumulate on the ground as litter that takes many years to decay and release inorganic minerals to the soil. The dominant vegetation type in the North American temperate rain forest is large evergreen trees, rich in epiphytic vegetation, which consists of smaller plants that grow nonparasitically on the trunks and branches of large trees. Squirrels, wood rats, mule deer, elk, numerous bird species (such as jays, nuthatches, and chickadees), several species of reptiles (such as painted turtles and western terrestrial garter snakes), and amphibians (such as Pacific giant salamanders and Pacific treefrogs) are common temperate rainforest animals. one of the richest wood producers in the world, supplies us with lumber and pulpwood. It is also one of the most complex ecosystems in terms of species richness.

Answer 136

Seasonality (hot summers and cold winters) is characteristic, which occurs in temperate areas where precipitation ranges from about 75 to 126 cm (30 to 50 in) annually. Typically, the soil consists of both a topsoil rich in organic material and a deep, clay-rich lower layer. Broad-leaf hardwood trees that lose their foliage annually dominate the northeastern and Mid-Atlantic United States. The trees form a dense canopy that overlies saplings and shrubs. Originally contained a variety of large mammals now regionally extinct, plus deer, bears, and many small mammals and birds. Both reptiles and amphibians abounded, together with a denser and more varied insect life than exists today. In Europe and North America, logging and land clearing for farms, tree plantations, and cities have removed much of the original temperate deciduous forest. Where it has regenerated, temperate deciduous forest is often in a semi-natural state—that is, highly modified by humans for recreation, livestock foraging, timber harvest, and other uses. Although these returning forests do not have the biological diversity of virgin stands, many forest organisms have successfully become re-established.

Answer 137

Summers are hot, winters are cold, fires help shape the landscape, and rainfall is often uncertain. Annual precipitation averages 25 to 75 cm. Grassland soil contains considerable organic material because surface parts of many grasses die off each winter and contribute to the organic content of the soil (the roots and rhizomes survive underground).

Answer 138

Some hilly temperate environments have mild winters with abundant rainfall, combined with extremely dry summers. This vegetation type is also known as maquis in the Mediterranean region, mallee scrub in Australia, matorral in Chile, and Cape scrub in Africa. Soil is thin and infertile. Frequent fires occur naturally in this environment, particularly in late summer and autumn. ## Footnote A dense growth of evergreen shrubs, often of drought-resistant pine or scrub oak trees, dominates. During the rainy winter season the landscape may be lush and green, but during the hot, dry summer the plants lie dormant. Trees and shrubs often have hard, small, leathery leaves that resist water loss. Many plants are also fire-adapted and grow best in the months following a fire. Such growth is possible because fire releases minerals that were tied up in the plants that burned. Denser, thicker vegetation tends to accumulate when periodic fires are prevented; then, when a fire does occur, it is much more severe.

Answer 139

Dry areas found in temperate (cold deserts) and subtropical or tropical regions (warm deserts). North America has four distinct deserts. The low water-vapor content of the desert atmosphere leads to daily temperature extremes of heat and cold, so a major change in temperature occurs in each 24-hour period. Deserts vary greatly depending on the amount of precipitation they receive, which is generally less than 25 cm (10 in) per year. A few deserts are so dry that virtually no plant life occurs in them. As a result of sparse vegetation, desert soil is low in organic material but often high in mineral content. Desert plants tend to have reduced leaves or no leaves, an adaptation that conserves water. Humans have altered North American deserts in several ways. Off-road vehicles damage desert vegetation, which sometimes takes years to recover. When the top layer of desert soil is disturbed, erosion occurs more readily, and less vegetation grows to support native animals. caused groundwater levels to drop. Aquifer depletion in U.S. deserts is particularly critical in southern Arizona and southwestern New Mexico.

Answer 140

a tropical grassland with widely scattered clumps of low trees. Found in areas of relatively low or seasonal rainfall with prolonged dry periods. precipitation regulates seasons. Annual precipitation is 85 to 150 cm (34 to 60 in). soil is low in essential minerals, in part because it is strongly leached. Often rich in aluminum, which resists leaching, and in places the aluminum reaches levels that are toxic to many plants. Wide expanses of grasses interrupted by occasional trees. In some places severe overgrazing by domestic animals has contributed to the conversion of marginal savanna into desert, a process known as **desertification, **the reduced grass cover caused by overgrazing allows wind and water to erode the soil; erosion removes the topsoil and decreases the soil’s ability to support crops or livestock.

Answer 141

occur in regions with a wet season and a dry season (usually two to three months each year). Annual precipitation is 150 to 200 cm (60 to 80 in). During the dry season, many tropical trees shed their leaves and remain dormant, much as temperate trees do during the winter. India, Brazil, Thailand, and Mexico are some of the countries that have tropical dry forests. Tropical dry forests intergrade with savanna on their dry edges and with tropical rain forests on their wet edges.

Answer 142

The annual precipitatio is 200 to 450 cm (80 to 180 in). Much of this precipitation, which occurs almost daily, comes from locally recycled water that enters the atmosphere by transpiration from the forest’s own trees. often located in areas with ancient, highly weathered, mineral-poor soil. Because temperatures are high and soil moisture is abundant year-round, decay organisms and detritus-feeding ants and termites decompose organic litter quite rapidly. ## Footnote Very productive despite the scarcity of minerals in the soil. Its plants, stimulated by abundant solar energy and precipitation, capture considerable energy by photosynthesis. Unrivaled in species richness. Tropical rainforest trees support extensive epiphytic commu- nities of smaller plants such as orchids and bromeliads. Although epiphytes grow in crotches of branches, on bark, or even on the leaves of their hosts, most use their host trees only for physical sup- port, not for nourishment. Because little light penetrates to the understory, many plants living there are adapted to climb already-established host trees. Not counting bacteria and other soil-dwelling organisms, about 90% of tropical rainforest organisms live in the middle and upper canopies.

Answer 143

the concentration of dissolved salts in a body of water, affects the kinds of organisms present in aquatic ecosystems, as does the amount of dissolved oxygen.

Answer 144

usually small or microscopic organisms that are relatively feeble swimmers. For the most part, they are carried about at the mercy of currents and waves. They are unable to swim far horizontally, but some species are capable of large vertical migrations and are found at different depths of water at different times of the day or at different seasons. Plankton are generally subdivided into two major categories: phytoplankton and zooplankton.

Answer 145

(photosynthetic cyanobacteria and free-floating algae) are producers that form the base of most aquatic food webs.

Answer 146

nonphotosynthetic organisms that include protozoa, tiny crustaceans, and the larval stages of many animals.

Answer 147

larger, actively swimming organisms such as fishes, turtles, and whales.

Answer 148

bottom-dwelling or- ganisms that fix themselves to one spot (sponges, oysters, and bar- nacles), burrow into the sand (many worms and echinoderms), or walk or swim about on the bottom (crayfish, aquatic insect larvae, and brittle stars).

Answer 149

The nature of a flowing-water ecosystem changes greatly from its source (where it begins) to its mouth (where it empties into another body of water). Headwater streams (small streams that are the sources of a river) are usually shallow, clear, cold, swiftly flowing, and highly oxygenated. In contrast, rivers downstream from the headwaters are wider and deeper, cloudy (that is, they contain suspended particulates), not as cold, slower flowing, and less oxygenated. Along parts of a stream or river, groundwater wells up through sediments on the bottom; this local input of water moderates the water temperature so that summer temperatures are cooler and winter temperatures are warmer than in adjacent parts of the flowing-water ecosystem. Streams and rivers depend on land for much of their energy.

Answer 150

Zonation characterizes standing-water ecosystems. A large lake has three basic zones: the littoral, limnetic, and profundal zones. Smaller lakes and ponds typically lack a profundal zone.

Answer 151

a shallow-water area along the shore of a lake or pond. It includes rooted, emergent vegetation, such as cattails and burreeds, plus several deeper-dwelling aquatic plants and algae. The most productive zone of the lake. Photosynthesis is greatest, in part because light is abundant and because the littoral zone receives nutrient inputs from surrounding land that stimulate the growth of plants and algae. In addition, ponds and lakes—like streams and rivers—depend on detritus carried from the land for much of their energy.

Answer 152

the open water beyond the littoral zone, that is, away from the shore; it extends down as far as sunlight penetrates to permit photosynthesis. The main organisms of the limnetic zone are microscopic phytoplankton and zooplankton. Larger fishes also spend some of their time in the limnetic zone, although they may visit the littoral zone to feed and reproduce. Because of the depth of this zone, less vegetation grows in the limnetic zone than in the littoral zone.

Answer 153

Beneath the limnetic zone of a large lake is the profundal zone. Because light does not penetrate effectively to this depth, plants and algae do not live in this zone. Food drifts into the profundal zone from the littoral and limnetic zones. Bacteria decompose dead plants and animals that reach the profundal zone, thus liberating minerals. These minerals are not effectively recycled because no photosynthetic organisms are present to absorb them and incorporate them into the food web. As a result, the profundal zone tends to be both mineral rich and anaerobic (oxygen deficient), with few organisms other than anaerobic bacteria occupying it.

Answer 154

the temperature changes sharply with depth, occurs because the summer sunlight penetrates and warms surface water, making it less dense. In summer, cool (and therefore more dense) water remains at the lake bottom and is separated from warm (and therefore less dense) water above by an abrupt temperature transition called the thermocline. Seasonal distribution of temperature and oxygen (more oxygen dissolves in water at cooler temperatures) affects the distribution of fish in the lake.

Answer 155

In summer, cool (and therefore more dense) water remains at the lake bottom and is separated from warm (and therefore less dense) water above by an abrupt temperature transition called the thermocline.

Answer 156

falling temperatures in autumn cause a mixing of the lake waters called the fall turnover. As surface water cools, its density increases, and it sinks and displaces the less dense, warmer, mineral-rich water beneath. Warmer water then rises to the surface where it, in turn, cools and sinks. This process of cooling and sinking continues until the lake reaches a uniform temperature throughout.

Answer 157

occurs as ice melts and surface water reaches 4°C. Surface water again sinks to the bottom, and bottom water returns to the surface. As summer arrives, thermal stratification occurs once again.

Answer 158

usually covered by shallow water for at least part of the year, have characteristic soils and water-tolerant vegetation. They include marshes, dominated by grasslike plants, and swamps, in which woody trees or shrubs dominate. Freshwater wetlands also include hardwood bottom- land forests (lowlands along streams and rivers that are periodically flooded), prairie potholes (small, shallow ponds that formed when glacial ice melted at the end of the last Ice Age), and peat moss bogs (peat-accumulating wetlands where sphagnum moss dominates).

Answer 159

One of their most important roles of wetlands is to trap and hold pollutants in the flooded soil, thereby cleansing and purifying the water. Such important environmental functions as these are known as ecosystem services.

Answer 160

a coastal body of water, partly surrounded by land, with access to the open ocean and a large supply of fresh water from rivers.

Answer 161

shallow wetlands in which salt-tolerant grasses dominate

Answer 162

the tropical equivalent of salt marshes, cover perhaps 70% of tropical and subtropical coastal mudflats where tides and waves fluctuate. Like salt marshes, mangrove forests provide valuable ecosystem services. Mangrove roots stabilize the sediments, preventing coastal erosion and providing a barrier against the ocean during storms.

Answer 163

the shoreline area between low and high tide.

Answer 164

the ocean floor. It is divided into zones based on distance from land, light availability, and depth. **abyssal zone** is that part of the benthic environment that extends from a depth of 4000 to 6000 m. ** hadal zone** is that part of the benthic environment deeper than 6000 m.

Answer 165

flowering plants that have adapted to complete submersion in ocean water. They are not true grasses. Sea grasses live in shallow water, to depths of 10 m (33 ft), where they receive enough light to photosynthesize efficiently, roots and rhizomes stabilize the sediments, reducing surface erosion, provide food and habitat for many marine organisms.

Answer 166

may reach lengths of 60 m (200 ft), are the largest brown algae. Kelps are common in cooler temperate marine waters of both the Northern and Southern Hemispheres. They are especially abundant in relatively shallow waters (depths of about 25 m, or 82 ft) along rocky coastlines. Photosynthetic and are therefore the primary food producers for the kelp forest ecosystem. Kelp forests also provide habitats for many marine animals. Kelp beds support a diversity of life that almost rivals that found in coral reefs.

Answer 167

are built from accumulated layers of calcium carbonate (CaCO₃), are found in warm (temperature usually greater than 21°C), shallow sea water. The living portions of coral reefs grow in shallow waters where light penetrates. ## Footnote Coral reefs grow slowly in warm, shallow water, as coral organisms build on the calcareous remains of countless organisms before them. The waters are often poor in nutrients. Other factors favor high productivity, however, including the presence of symbiotic zooxanthellae, warm temperatures, and plenty of sunlight. Coral reef ecosystems are the most diverse of all marine environments and contain hundreds or even thousands of species of fishes and invertebrates, Coral reefs are ecologically important because they both pro- vide habitat for a wide variety of marine organisms and protect coastlines from shoreline erosion.

Answer 168

open ocean that overlies the continental shelves, that is, the ocean floor from the shoreline to a depth of 200 m (650 ft). Organisms that live here are floaters or swimmers. The upper reaches make up the **euphotic zone**, which extends from the surface to a depth of approximately 100 m (325 ft). Nekton are thought to be mostly confined to the shallower neritic waters (less than 60 m, or 195 ft, deep) because that is where their food is. However, not much is known about the behavior and migration patterns of marine nekton.

Answer 169

that part of the open ocean that covers the deep-ocean basin, that is, the ocean floor at depths more than 200 m. It is the largest marine environment. Cold temperatures, high hydrostatic pressure, and an absence of sunlight characterize the oceanic province; these environmental conditions are uniform throughout the year. Most organisms of the oceanic province depend on **marine snow**, organic debris that drifts down into the **aphotic** (“without light”) **region** from the upper, lighted regions. Organisms of this little-known realm are filterfeeders, scavengers, or predators. Many are invertebrates, some of which attain great sizes. Many animals of the oceanic province have illuminated organs that enable them to see one another to mate or to capture food. Adapted to drifting or slow swimming, these fishes often have reduced bone and muscle mass.

Answer 170

The transition zone where two communities or biomes meet and intergrade.

Answer 171

The study of the geographic distribution of plants and animals. search for patterns in geographic distribution and try to explain how such patterns arose, including where populations originated, how they spread, and when. Recognize that geologic and climate changes such as mountain building, continental drift, and periods of extensive glaciation influence the distribution of species.

Answer 172

One of the basic tenets of biogeography is that each species originated only once. The particular place where this occurred is known as the species’ center of origin.

Answer 173

Such localized, native species are said to be **endemic**, that is, they are not found anywhere else in the world.

Answer 174

some species have a nearly worldwide distribution and occur on more than one continent or throughout much of the ocean. Such species are said to be **cosmopolitan**.

Answer 175

Alfred Wallace, who independently discovered the same theory of evolution by natural selection as Charles Darwin, divided Earth’s land areas into six major biogeographic realms: the Palearctic, Nearctic, Neotropical, Ethiopian, Oriental, and Australian. A major barrier separates each of the six biogeographic realms from the others and helps maintain each region’s biological distinctiveness. The *Neotropical* *realm* was almost completely isolated from the Nearctic realm and other landmasses for most of the past 70 million years. During this time, many marsupial species evolved. The isthmus of Panama, which formed a dry-land connection about 3 million years ago, linked North and South America and provided a route for dispersal. Only three species, the opossum, armadillo, and porcupine, are descendants of animals that survived the northward dispersal from South America, but many species, such as the tapir and llama, are descendants of animals that survived the southward dispersal from North America. Competition from these species caused many of South America’s marsupial species to go extinct. The Sahara Desert separates the *Ethiopian* *realm* from other landmasses. The Ethiopian realm contains the most varied vertebrates of all six realms. Some overlap exists between the Ethiopian realm and *Oriental* *realm* because a land bridge with a moist climate linked Africa to Asia during the Miocene and Pliocene epochs. The Oriental realm has the fewest endemic species of all the tropical realms. The *Australian* *realm* has not had a land connection with other regions for more than 85 million years. It has no native placental mammals, and marsupials and monotremes, including the duckbilled platypus and the spiny anteater, dominate it. Adaptive radiation of the marsupials during their long period of isolation led to species with ecological niches similar to those of placental mammals of other realms.

Answer 176

the ability to meet humanity’s current needs without compromising the ability of future generations to meet their needs.

Answer 177

also called biodiversity, is the variation among organisms. ccurs at all levels of ecological organization, from populations to ecosystems. takes into account genetic diversity, the genetic variety within a species, both among individuals within a given population and among geographically separate populations. (An individual species may have hundreds of genetically distinct populations.) Biological diversity also includes ecosystem diversity, the variety of ecosystems found on Earth—the forests, prairies, deserts, lakes, coastal estuaries, coral reefs, and other ecosystems.

Answer 178

the number of species of archaea, bacteria, protists, plants, fungi, and animals.

Answer 179

The smallest population that has a high chance of enduring into the future.

Answer 180

as stipulated by the U.S. Endangered Species Act, is a species in imminent danger of extinction throughout all or a significant part of its range.

Answer 181

a species likely to become endangered in the foreseeable future, throughout all or a significant part of its range.

Answer 182

the breakup of large areas of habitat into small, isolated segments (that is, islands), is a major threat to the lonterm survival of many populations and species

Answer 183

the introduction of a foreign species into an area where it is not native, often upsets the balance among the organisms living in that area and interferes with the ecosystem’s normal functioning. Unlike other forms of pollution, which may be cleaned up, biotic pollution is usually permanent.

Answer 184

If the foreign species causes economic or environmental harm, it is known as an invasive species.

Answer 185

the collection of live organisms from nature. Most commercially harvested organisms end up in zoos, aquaria, biomedical research labs, circuses, and pet stores.

Answer 186

the scientific study of how humans impact organisms and of the development of ways to protect biological diversity. Conservation biologists develop models, design experiments, and perform fieldwork to address a wide range of questions. For example, what are the processes that influence a decline in biological diversity? How do we protect and restore populations of endangered species? If we are to preserve entire ecosystems and landscapes, which ones are the most important to save? Conservation biology includes two problem-solving approaches that save organisms from extinction: in situ and ex situ conservation.

Answer 187

which includes the establishment of parks and reserves, concentrates on preserving biological diversity in nature. A high priority of in situ conservation is identifying and protecting sites that harbor a great deal of diversity. With increasing demands on land, however, in situ conservation cannot preserve all types of biological diversity. Sometimes only ex situ conservation can save a species.

Answer 188

conserves individual species in human- controlled settings. Breeding captive species in zoos and storing seeds of genetically diverse plant crops are examples of ex situ conservation.

Answer 189

The subdiscipline of ecology that studies the connections in a heterogeneous **landscape** consisting of multiple interacting ecosystems.

Answer 190

strips of habitat connecting isolated habitat patches. Habitat corridors allow wildlife to move about so they can feed, mate, and recolonize habitats after local extinctions take place. Research has also shown that habitat fragments linked by habitat corridors retain more native plant species than do isolated fragments.

Answer 191

Ch. 57 Restoration ecology in which the principles of ecology are used to return a degraded environment to one that is more functional and sustainable, is an important part of in situ conservation.

Answer 192

sperm is collected from a suitable male of a rare species and used to impregnate a female, perhaps located in another zoo in a different city or even in another country.

Answer 193

a female of a rare species is treated with fertility drugs, which cause her to produce multiple eggs. Some of these eggs are collected, fertilized with sperm, and surgically implanted into females of a related but less rare species, which later give birth to offspring of the rare species

Answer 194

*Convention on Biological Diversity*, requires that each signatory nation inventory its own biodiversity and develop a **national conservation strategy**, a detailed plan for managing and preserving the biological diversity of that specific country.

Answer 195

The most serious problem facing the world’s forests and their biological diversity is **deforestation**, which is the temporary or permanent clearance of large expanses of forest for agriculture or other uses. Deforestation increases soil erosion and thus decreases soil fertility. Soil erosion causes increased sedimentation of waterways, which harms downstream aquatic ecosystems by reducing light penetration, covering aquatic organisms, and filling in waterways. Uncontrolled soil erosion, particularly on steep deforested slopes, causes mudflows that endanger human lives and property and reduces production of hydroelectric power as silt builds up behind dams. In drier areas, deforestation can lead to the formation of deserts.

Answer 196

in which a family produces enough food to feed itself, accounts for perhaps 60% of tropical deforestation. In many developing countries where tropical rain forests are located, the majority of people do not own the land on which they live and work. Most subsistence farmers have no place to go except into the forest, which they clear to grow food. Land reform in Brazil, Madagascar, Mexico, the Philippines, Thailand, and many other countries would make the land owned by a few available to every- one, thereby easing the pressure of subsistence farmers on tropical forests. This scenario is unlikely because wealthy landowners have more economic and political clout than impoverished peasants.

Answer 197

Subsistence farmers often follow loggers’ access roads until they find a suitable spot. They first cut down the trees and allow them to dry; then they burn the area and plant crops immediately after burning. Yields from the first crop are often quite high because the nutrients that were in the trees are now available in the soil. However, soil productivity declines rapidly. In a few years the farmer must move to a new part of the forest and repeat the process. Cattle ranchers often claim the abandoned land for grazing because land that is not fertile enough to support crops can still support livestock.

Answer 198

Because carbon dioxide and other gases trap the sun’s radiation somewhat like glass does in a greenhouse, the natural trapping of heat in the atmosphere is called the **greenhouse effect**, and the gases that absorb infrared radiation are known as **greenhouse gases**. This natural heating of the atmosphere prevents Earth from becoming a frozen planet. However, the additional warming produced when increased levels of gases produced by human activities absorb additional infrared radiation, called the **enhanced greenhouse effect**, has potentially catastrophic implications.

Answer 199

some of the probable effects of climate change, including changes in sea level; changes in precipitation patterns; effects on biological diversity, including humans; and effects on agriculture. These changes will persist for centuries because many greenhouse gases remain in the atmosphere for hundreds of years. Furthermore, even after greenhouse gas concentrations have stabilized, scientists think that Earth’s mean surface temperature will continue to rise because the ocean adjusts to climate change on a delayed time scale.

Answer 200

encircles the planet some 10 to 45 km (6 to 28 miles) above the surface, contains a layer of ozone that shields the surface from much of the ultraviolet radiation from the sun. If ozone disappeared from the stratosphere, Earth would become unlivable for most forms of life. Both chlorine- and bromine-containing substances catalyze ozone destruction. The primary chemicals responsible for ozone loss in the stratosphere are a group of chlorine compounds called chlorofluorocarbons (*CFCs*), have been used as propellants in aerosol cans, coolants in air conditioners & refrigerators, foam-blowing agents for insulation & packaging, & solvents & cleaners for the electronics industry. Additional compounds that also attack ozone include halons (found in many fire extinguishers), methyl bromide (a pesticide), methyl chloroform (an industrial solvent), & carbon tetrachloride (used in many industrial processes, including the manufacture of pesticides and dyes).

Answer 201

a form of oxygen that is a human-made pollutant in the lower atmosphere but a naturally produced, essential part of the stratosphere.

Answer 202

In 1987, the **Montreal Protocol**, an agreement that originally stipulated a 50% reduction of CFC production by 1998. After scientists reported that decreases in stratospheric ozone occurred over the heavily populated midlatitudes of the Northern Hemisphere in all seasons, the Montreal Protocol was modified to include stricter measures to limit CFC production. Industrial companies that manufacture CFCs quickly developed substitutes.

Answer 203

what an animal does and how it does it, usually in response to stimuli in its environment.

Answer 204

the study of behavior in natural environments from an evolutionary perspective.

Answer 205

immediate causes such as the genetic, developmental, and physiological processes that permit the animal to carry out the particular behavior.

Answer 206

which have evolutionary explanations, ask why a particular behavior has evolved. Ultimate considerations address costs and benefits of behavior patterns. When studying ultimate causes, we may ask what the adaptive value of a particular behavior might be.

Answer 207

A behavior may help an animal obtain food or water, protect itself, reproduce, or acquire and maintain territory in which to live. The benefits typically contribute to direct fitness.

Answer 208

an individual’s reproductive success, measured by the number of viable offspring it produces. Reproduction is, of course, the key to evolutionary success.

Answer 209

(inborn behavior, popularly referred to as instinct) as genetically programmed.

Answer 210

behavior that has been modified in response to environmental experience.

Answer 211

Many behaviors that we think of as automatic depend on coordinated sequences of muscle actions.

Answer 212

A classic example of a motor program in vertebrates is egg rolling in the European graylag goose. Once activated by a simple sensory stimulus, egg-rolling behavior continues to completion regardless of sensory feedback. There is little flexibility. Behavioral ecologists call this behavior a **behavioral** **pattern**. Certain behavioral patterns can be elicited by a **sign** **stimulus**, or releaser, a simple signal that triggers a specific behavioral response. A wooden egg is a sign stimulus that elicits egg-rolling behavior in the graylag goose.

Answer 213

The conspicuous coloring of a poisonous or distasteful organism that enables potential predators to easily see and recognize it.

Answer 214

a type of learning in which an animal learns to ignore a repeated, irrelevant stimulus, that is, one that neither rewards nor punishes.

Answer 215

a type of social learning based on early experience, has been studied in some mammals as well as birds. It occurs during a **critical period,** usually within a few hours or days after birth (or hatching).

Answer 216

an association forms between some normal body function and a new stimulus.

Answer 217

the animal must do something to gain a reward (positive reinforcement) or avoid punishment.

Answer 218

the process of gaining knowledge; it includes thinking, processing information, learning, reasoning, and awareness of thoughts, perceptions, and self.

Answer 219

Some animals have a form of cognition called **insight learning**, the ability to adapt past experiences that may involve different stimuli to solve a new problem.

Answer 220

The biochemical, physiological, and behavioral responses animals make to the periodic changes in the environment. control various behaviors, including activity, sleep, feeding, drinking, reproduction, and migration.

Answer 221

The behavior of many animals, like the activities of many plants, is organized around **circadian** (meaning “approximately 1 day”) **rhythms**, which are daily, 24-hour cycles of activity. **Diurnal** animals, such as honeybees and pigeons, are most active during the day. Most bats, moths, and cats are **nocturnal** animals, most active during the hours of darkness. **Crepuscular** animals, like many mosquitoes and fiddler crabs, are busiest at dawn or dusk, or both.

Answer 222

Feeding behavior, or **foraging**, involves locating and selecting food, as well as gathering and capturing food.

Answer 223

the most efficient way for an animal to obtain food. According to *optimal foraging theory*, when animals forage, they maximize their energy intake per unit of foraging time. When animals maximize energy obtained per unit of foraging time, they may maximize their reproductive success. Many factors, such as avoiding predators while foraging, must be considered in deter- mining efficient or optimal strategies.

Answer 224

the interaction of two or more animals, usually of the same species.

Answer 225

an actively cooperating group of individuals belonging to the same species and often closely related.

Answer 226

chemical signals secreted into the environment that convey information between members of a species.

Answer 227

a ranking of social status in which each wasp has more status than the wasps that are lower in rank.

Answer 228

a geographic area they seldom leave

Answer 229

a type of natural selection for successful mating.

Answer 230

individuals of the same sex actively compete for mates.

Answer 231

females select their mates on the basis of some physical trait or some re- source offered by the winning suitors. The physical trait typically indicates genetic quality or good health.

Answer 232

males gather in a small display area called a **lek**, where they compete for females.

Answer 233

ensure that the male is indeed a male and is a member of the same species.

Answer 234

a mating system in which males fertilize the eggs of many females during a breeding season.

Answer 235

one female mates with several males.

Answer 236

The male guards his partner after copulation to ensure that she does not copulate with another male. Mate guarding behavior is likely to occur when the female is receptive and has eggs that might be fertilized by another male.

Answer 237

stable relationships that ensure cooperative behavior in mating and the rearing of the young.

Answer 238

one individual behaves in a way that seems to benefit others rather than itself with no potential payoff.

Answer 239

the sum of direct fitness (which can be measured by the number of alleles an animal perpetuates in its offspring) plus indirect fitness.

Answer 240

the probability that two individuals inherit the same uncommon allele from a recent common ancestor.

Answer 241

According to **Hamilton’s rule,** an altruistic act is adaptive if (1) its indirect fitness benefits are high for the animals that profit; (2) the recipients are close relatives of the altruist; and (3) the direct fitness cost to the altruistic animal is low.

Answer 242

**Kin selection**, also called indirect selection, is a form of natural selection that increases inclusive fitness through the breeding success of close relatives.

Answer 243

According to this hypothesis, one animal helps a nonrelative with no immediate benefit, but at some later time the animal that was helped repays the debt. In this way, the original helper experiences a net fitness benefit.

Answer 244

behavior common to a population, learned from other members of the group, and transmitted from one generation to another.

Answer 245

focuses on the evolution of social behavior through natural selection.

Answer 246

**penis** is an erectile copulatory organ that delivers sperm into the female reproductive tract. It is a long shaft that enlarges to form an expanded tip, the **glans**. Part of the loose-fitting skin of the penis folds down and covers the proximal portion of the glans, forming a cuff called the **prepuce**, or foreskin. Under the skin, the penis consists of three parallel columns of **erectile tissue**: two **cavernous** **bodies** and one **spongybody** spongy body surrounds the portion of the urethra that passes through the penis. Erectile tissue contains numerous blood vessels.

Answer 247

The process of ovum production, called **oogenesis**, begins in the ovaries. Before birth, hundreds of thousands of **oogonia** are present in the ovaries. All of a female’s oogonia form during embryonic development. No new oogonia are formed after birth. During prenatal development, the oogonia increase in size and become **primary oocytes**. By the time of birth, they are in the prophase of the first meiotic division. At this stage, they enter a resting phase that lasts throughout childhood and into adult life.

Answer 248

A primary oocyte and the granulosa cells surrounding it together make up a **follicle**

Answer 249

granulosa cells are connected by tight junctions that form a protective barrier around the oocyte.

Answer 250

Connective tissue cells surrounding the granulosa cells differentiate to form a layer of **theca cells**.

Answer 251

oogonium (diploid) --\> primary oocyte (diploid) --\> secondary oocyte + 1st polar body (both haploid) --\> (after fertilization) ovum + 2nd polar body (both haploid)

Answer 252

As an oocyte develops, it becomes separated from its sur- rounding follicle cells by a layer of glycoproteins called the **zona pellucida.**

Answer 253

During **ovulation**, the secondary oocyte ejects through the ovary wall and into the pelvic cavity. The portion of the follicle that remains in the ovary develops into the **corpus** **luteum**, a temporary endocrine gland that secretes estrogen and **progesterone**.

Answer 254

Almost immediately after ovulation, the secondary oocyte is swept into the funnel-shaped opening of the **oviduct**, also called the uterine tube or fallopian tube.

Answer 255

thickens each month in preparation for possible pregnancy.

Answer 256

a painful disorder in which fragments of the endometrium migrate to other areas, such as the oviducts or ovaries. Like pelvic inflammatory disease, endometriosis causes scarring that can lead to infertility.

Answer 257

the head of the haploid sperm then swells to form the **male pronucleus** and fuses with the **female pronucleus** to form the diploid nucleus of the zygote.

Answer 258

**human chorionic gonadotropin (hCG),** a peptide hormone that signals the mother’s corpus luteum to continue to function.

Answer 259

he mechanisms that terminate pregnancy and initiate the birth process, called **parturition**, depend on several hormones. High levels of estrogen secreted by the placenta greatly increase the number of receptors for oxytocin in the uterine wall. As a result, the uterus becomes about one hundred times more responsive to oxytocin, the hormone that stimulates uterine contractions.

Answer 260

In terrestrial vertebrates, the three germ layers also give rise to four **extraembryonic membranes**: the chorion, amnion, allantois, and yolk sac.

Answer 261

The outermost membrane, the **chorion**, encloses the entire embryo as well as the other membranes. Lying underneath the eggshell in birds and reptiles, it functions as the major organ of gas exchange.

Answer 262

the **amnion** encloses the entire embryo. The amniotic cavity, the space between the embryo and the amnion, becomes filled with amniotic fluid secreted by the membrane.

Answer 263

**allantois** is an outgrowth of the developing digestive tract. In reptiles and birds, it stores nitrogenous wastes. In humans, the allantois is small and nonfunctional, except that its blood vessels contribute to the formation of umbilical vessels joining the embryo to the placenta.

Answer 264

encloses the yolk, slowly digests it, and makes it available to the embryo. A yolk sac, connected to the embryo by a yolk stalk, develops even in mammalian embryos that have little or no yolk. Its walls serve as temporary centers for the formation of blood cells.

Answer 265

cells arrange themselves, forming a blastula, which in mammals is called a **blastocyst**.

Answer 266

outer layer of cells, the **trophoblast**, eventually forms the chorion and amnion that surround the embryo.

Answer 267

A little cluster of cells, the **inner cell mass**, projects into the cavity of the blastocyst. The inner cell mass gives rise to the embryo proper.

Answer 268

On about the seventh day of development, the embryo begins the process of **implantation**, in which it embeds in the endometrium of the uterus

Answer 269

information transfer by networks of nerve cells, called **neurons**. In most animals, neural signaling involves four processes: reception, transmission, integration, and action by effectors (muscles or glands).

Answer 270

the process of detecting a stimulus, is the job of neurons and of specialized *sensory receptors* such as those in the skin, eyes, and ears.

Answer 271

the process of sending messages along a neuron, from one neuron to another or from a neuron to a muscle or gland.

Answer 272

In vertebrates, a neural message is transmitted from a receptor to the **central nervous system (CNS)**, which consists of the brain and spinal cord. Neurons that transmit information to the CNS are called **afferent** (meaning “to carry toward”) **neurons**, or sensory neurons.

Answer 273

Afferent neurons generally transmit information to **interneurons**, or association neurons, in the CNS. Most neurons, perhaps 99%, are interneurons. Their function is to integrate input and output.

Answer 274

involves sorting and interpreting incoming sensory information and determining the appropriate response.

Answer 275

Neural messages are transmitted from the CNS by **efferent** (meaning “to carry away”) **neurons** to **effectors**—muscles and glands. Efferent neurons that signal skeletal muscle are called **motor neurons.**

Answer 276

the actual response to the stimulus. Sensory receptors and afferent and efferent neurons are part of the **peripheral nervous system (PNS)**.

Answer 277

In summary, information flows through the nervous system in the following sequence: reception by sensory receptor --\> transmission by afferent neuron --\> integration by interneurons in CNS --\> transmission by efferent neuron --\> action by effectors

Answer 278

unique to the nervous system. _Neurons_ are specialized to receive and send information. Glial cells support and protect neurons and carry out many regulatory functions.

Answer 279

Neurons produce and transmit electrical signals called **nerve impulses**, or *action potentials*. The neuron is distinguished from all other cells by its long **processes** (cytoplasmic extensions). Examine the structure of a common type of neuron, the multipolar neuron shown in FIGURE 41-2.

Answer 280

typically short, highly branched processes specialized to receive stimuli and send signals to the cell body.

Answer 281

the largest portion of the neuron, the **cell body**, contains the nucleus, the bulk of the cytoplasm, and most of the organelles. integrates incoming signals.

Answer 282

microscopic, may divide, forming branches called **axon collaterals. **conducts nerve impulses away from the cell body to another neuron or to a muscle or gland. At its end the axon divides, forming many terminal branches that end in synaptic terminals.

Answer 283

release **neurotransmitters**, chemicals that transmit signals from one neuron to another or from a neuron to an effector. The junction between a synaptic terminal and another neuron (or effector) is called a **synapse**. Typically, a small space exists between these two cells.

Answer 284

In vertebrates, the axons of many neurons outside the CNS are surrounded by a series of **Schwann cells.** The plasma membranes of these glial cells contain **myelin**, a white, fatty material. Schwann cells wrap their plasma membranes around the axon, forming an insulating covering called the **myelin sheath**. Gaps in the myelin sheath, called **nodes of Ranvie**r, occur between successive Schwann cells. At these points the axon is not insulated with myelin. Axons more than 2 μm in diameter have myelin sheaths and are described as myelinated. Those of smaller diameter are generally unmyelinated.

Answer 285

consists of hundreds or even thousands of axons wrapped together in connective tissue Within the CNS, bundles of axons are called **tracts** or **pathways** rather than nerves.

Answer 286

Outside the CNS, the cell bodies of neurons are usually grouped in masses called **ganglia** (sing., ganglion). Inside the CNS, collections of cell bodies are generally called nuclei rather than *ganglia*.

Answer 287

star-shaped glial cells that provide physical support for neurons and provide neurons with nutrients.

Answer 288

glial cells that envelop neurons in the CNS, forming insulating myelin sheaths around them.

Answer 289

ciliated glial cells that line the internal cavities of the CNS. Ependymal cells help produce and circulate the cerebrospinal fluid that bathes the brain and spinal cord of vertebrates

Answer 290

actually specialized macrophages (phagocytic cells that ingest and digest cell debris and bacteria). Microglia respond to signals from neurons and are important in mediating responses to injury or disease. These cells are found near blood vessels. When the brain is injured or infected, microglia multiply and move to the affected area. There they remove bacteria and cell debris by phagocytosis. They also release signaling molecules (also produced by macrophages and certain other cells in the immune system) that mediate inflammation.

Answer 291

he voltage measured across the plasma membrane.

Answer 292

The membrane potential in a resting (not excited) neuron or muscle cell

Answer 293

permit the passage of specific ions such as Na+, K+, Cl−, and Ca2⁺. Unlike voltage-activated and chemically activated ion channels, passive ion channels are not controlled by gates.

Answer 294

** equilibrium potential** for any particular ion is a steady state in which opposing chemical and electrical fluxes are equal and there is no net movement of the ion.

Answer 295

sodium– potassium pumps that actively transport Na⁺ out of the cell and K⁺ into the cell

Answer 296

the neural action proceeds as follows: neuron in resting state --\> stimulus causes depolarization --\> threshold reached --\> action potential transmits signal --\> repolarization annd return to resting state.

Answer 297

a nerve impulse travels rather slowly. the smooth, progressive impulse transmission just described, called **continuous** **conduction**, occurs in unmyelinated neurons

Answer 298

The action potential appears to jump from one node of Ranvier to the next . This type of impulse transmission is known as **saltatory conduction** (from the Latin word sal- tus, which means “to leap”).

Answer 299

a junction between two neurons or between a neuron and an effector, such as between a neuron and a muscle cell. neuron that terminates at a specific synapse is called a presynaptic neuron, whereas a neuron that begins at that synapse is a post-synaptic neuron.

Answer 300

the presynaptic and postsynaptic neurons occur very close together (within 2 nm of one another) and form gap junctions. The interiors of the two cells are physically connected by a protein channel. Electrical synapses let ions pass from one cell to another, permitting an impulse to be directly and rapidly transmitted from a presynaptic to a postsynaptic neuron.

Answer 301

Presynaptic and postsynaptic neurons are separated by a space, t**he synaptic cleft**, about 20 nm wide (less than one-millionth of an inch). Because depolarization is a property of the plasma membrane, when an action potential reaches the end of the axon, it cannot jump the gap. The electrical signal must be converted into a chemical one. Neurotransmitters are the chemical messengers that conduct the neural signal across the synapse and bind to chemically activated ion channels in the membrane of the postsynaptic neuron. This binding triggers specific gated ion channels to open (or close), resulting in changes in permeability of the postsynaptic membrane. When a postsynaptic neuron reaches its threshold level of depolarization, it transmits an action potential.

Answer 302

a low-molecular-weight neurotransmitter that is released from motor neurons and triggers muscle contraction. Cells that release acetylcholine are called **cholinergic neurons.**

Answer 303

affect mood, and their imbalance has been linked to several disorders, including major depression, anxiety disorders, attention deficit hyperactivity disorder (ADHD), and schizophrenia.

Answer 304

Adenylyl cyclase converts ATP to **cyclic AMP (cAMP)**, which acts as a second messenger. Cyclic AMP activates a kinase that phosphorylates a protein, which then closes K+ channels.

Answer 305

**radial nervous system** of the sea star and other echinoderms is a modified nerve net. This system shows some degree of selective organization of neurons into more than a diffuse network. It consists of a nerve ring around the mouth, from which a large radial nerve extends into each arm. Branches of these nerves, which form a network somewhat similar to the nerve net of Hydra, coordinate the animal’s movement.

Answer 306

Hydra and other cnidarians have a **nerve net** consisting of interconnected neurons with no central control organ.

Answer 307

The vertebrate nervous system has two main divisions: the **central nervous system (CNS)** and the **peripheral nervous system (PNS)**. The CNS consists of a highly developed brain that is continuous with the dorsal, tubular **spinal cord**. Serving as central control, these organs integrate incoming information and determine appropriate responses.

Answer 308

made up of the sensory receptors (for example, tactile, auditory, and visual receptors) and the nerves, which are the communication lines. PNS is subdivided into somatic and autonomic divisions. Most of the receptors and nerves concerned with changes in the external environment are somatic. Those that regulate the internal environment are autonomic. Both divisions have afferent nerves, which transmit messages from receptors to the CNS, and efferent nerves, which transmit information back from the CNS to the structures that respond. The autonomic division has two kinds of efferent pathways: sympathetic and parasympathetic nerves

Answer 309

In the early vertebrate embryo, the brain and spinal cord differentiate from a single tube of tissue, the **neural tube**. Anteriorly, the tube expands and develops into the brain. Posteriorly, the tube becomes the spinal cord. Brain and spinal cord remain continuous, and their cavities communicate.

Answer 310

the medulla, pons, and midbrain make up the **brain stem**, the elongated portion of the brain that looks like a stalk holding up the cerebrum.

Answer 311

subdivides to form the metencephalon, which gives rise to the cerebellum and **pons**, and the myelencephalon, which gives rise to the medulla.

Answer 312

most posterior part of the brain, is continuous with the spinal cord. Its cavity, the fourth ventricle, is continuous with the central canal of the spinal cord and with a channel that runs through the midbrain. walls are thick and made up largely of nerve tracts (bundles of axons) that connect the spinal cord with various parts of the brain. contains centers that regulate life-sustaining functions such as respiration, heartbeat, and blood pressure. Other reflex centers in the medulla regulate activities such as swallowing, coughing, and vomiting.

Answer 313

responsible for muscle tone, posture, and equilibrium. It also refines and coordinates muscle activity. In humans, provides input to motor areas in the cerebral cortex, and in this way helps plan and initiate voluntary activity. Certain regions of the cerebellum store implicit memories (unconscious memories for perceptual and motor skills, such as swimming or skating). The size and shape of the cerebellum vary among the vertebrate classes. Development of the cerebellum is roughly correlated with the extent and complexity of muscular activity, reflecting the principle that the relative size of a brain part correlates with its importance to the behavior of the species. Injury or removal of the cerebellum results in impaired muscle coordination.

Answer 314

In mammals, **pons** contains a thick bundle of fibers that transmits information between the two sides of the cerebellum. The pons also serves as a bridge that connects the medulla and cerebellum with other regions of the brain. The pons contains centers that help regulate respiration and nuclei that relay impulses from the cerebrum to the cerebellum.

Answer 315

In fishes and amphibians, the **midbrain **is the most prominent part of the brain and serves as the main association area. It receives incoming sensory information, integrates it, and sends decisions to appropriate motor nerves. The dorsal portion of the midbrain is differentiated to some extent. For example, the optic lobes are specialized for visual interpretations. In reptiles, birds, and mammals, many functions of the optic lobes are assumed by the cerebrum, which develops from the forebrain. In mammals, the **midbrain** consists of the **superior colliculi**, centers for visual reflexes such as pupil constriction, and the **inferior colliculi**, centers for certain auditory reflexes. The inferior colliculi are major integration centers for incoming auditory information. The mammalian midbrain also contains a center (the *red nucleus*) that helps maintain muscle tone and posture.

Answer 316

subdivides to form the *telencephalon* and *diencephalon*. The **diencephalon** gives rise to the thalamus and hypothalamus.

Answer 317

a relay center for motor and sensory messages. In mammals, all sensory messages except those from the olfactory receptors are delivered to the thalamus, where they are integrated before they are relayed to the sensory areas of the cerebrum.

Answer 318

which lies below the thalamus, forms the floor of the third ventricle. The hypothalamus is a major coordinating center for regulating autonomic and somatic responses. It integrates incoming information and provides input to centers in the medulla and spinal cord that regulate activities such as heart rate, respiration, and digestive system function. In birds and mammals, the hypothalamus controls body temperature. It also contains olfactory centers, regulates appetite and water balance, and is important in emotional and sexual responses.

Answer 319

telencephalon gives rise to the **cerebrum**. The *lateral* *ventricles* (also called the first and second ventricles) lie within the cerebrum. Each lateral ventricle connects with the third ventricle (within the diencephalon) by way of a channel. In most vertebrate groups, the telencephalon also gives rise to the **olfactory bulbs**. These structures are important in the chemical sense of smell— the dominant sense in most aquatic and terrestrial vertebrates.

Answer 320

**somatic division** of the PNS includes the receptors that react to changes in the external environment, the sensory neurons that inform the CNS of those changes, and the motor neurons that adjust the positions of the skeletal muscles that help maintain the body’s posture and balance.

Answer 321

**autonomic division** helps maintain homeostasis in the internal environment. For example, it regulates the heart rate and helps maintain a constant body temperature. The autonomic system works automatically and without voluntary input. Its effectors are smooth muscle, cardiac muscle, and glands. Like the somatic system, it is functionally organized into reflex pathways. Receptors within the viscera relay information via afferent nerves

Answer 322

The efferent portion of the autonomic division is subdivided into **sympathetic** and parasympathetic systems. Many organs are innervated by both types of nerves. In general, sympathetic and parasympathetic systems have opposite effects. For example, the heart rate is speeded up by messages from sympathetic nerve fibers and slowed by impulses from its parasympathetic nerve fibers. In many cases sympathetic nerves operate to stimulate organs and to mobilize energy, especially in response to stress.

Answer 323

**parasympathetic** nerves influence organs to conserve and restore energy, particularly during quiet, calm activities.

Answer 324

sympathetic ganglia are paired, and a chain of them, the **paravertebral sympathetic ganglion chain**, runs on each side of the spinal cord from the neck to the abdomen. Some sympathetic preganglionic neurons do not end in these ganglia but pass on to **collateral ganglia** in the abdomen, close to the aorta and its major branches. Parasympathetic preganglionic neurons synapse with postganglionic neurons in terminal ganglia near or within the walls of the organs they innervate.

Answer 325

detect information about changes in the external and internal environment. These receptors consist of specialized neuron endings or specialized cells in close contact with neurons. Sensory receptors, along with other types of cells, make up complex **sense organs,** such as eyes, ears, nose, and taste buds.

Answer 326

Sensory receptors transduce, or convert, the energy of the stimu- lus into electrical signals, the information currency of the nervous system. This process is known as **energy transduction**.

Answer 327

A change in membrane potential is a **receptor potential.** A receptor potential does not directly trigger an action potential. Like an excitatory postsynaptic potential, or EPSP, the receptor potential is a **graded response** in which the magnitude of change depends on the energy of the stimulus.

Answer 328

If the receptor is a separate cell, receptor potentials stimulate the release of a neurotransmitter, which flows across the synapse and binds to receptors on a **sensory neuron**, also known as an *afferent neuron*. When a sensory neuron becomes sufficiently depolarized to reach its threshold level, an action potential is generated. Thus, receptor potentials can generate action potentials that transmit information to the central nervous system (CNS).

Answer 329

stimulus (such as light energy) --\> sensory receptor absorbs energy from stimulus --\> transduces energy of stimulus into electrical energy --\> receptor potential --\> action potential in sensory neuron --\> signal transmitted to CNS

Answer 330

Many sensory receptors do not continue to respond at the initial rate, even if the stimulus continues at the same intensity. This change in response rate, called **sensory adaptation**, occurs for two reasons. First, during a sustained stimulus, the receptor sensitivity decreases and produces a smaller receptor potential (resulting in a lower frequency of action potentials in the sensory neurons). Second, changes take place at synapses in the neural pathway activated by the receptor.

Answer 331

receive stimuli from the outside environment, enabling an animal to know and explore the world, search for food, find and attract a mate, recognize friends, and detect enemies.

Answer 332

sensory receptors within body organs that detect changes in pH, osmotic pressure, body temperature, and the chemical composition of the blood.

Answer 333

The exchange of gases between an organism and its environment.

Answer 334

oxygen from the environment is taken up by the animal and delivered to its individual cells. At the same time, carbon dioxide generated during cellular respiration is excreted into the environment.

Answer 335

takes place in mitochondria. Oxygen is essential because it serves as the final electron acceptor in the mitochondrial electron transport chain. Carbon dioxide is a metabolic waste product of cellular respiration.

Answer 336

Sponges, hydras, flatworms, and many other small, aquatic organisms exchange gases entirely by simple **diffusion**, the passive movement of particles (atoms, ions, or molecules) from a region of higher concentration to a region of lower concentration, that is, down a concentration gradient.

Answer 337

moist, thin structures that extend from the body surface. They are supported by the buoyancy of water but tend to collapse in air.

Answer 338

In bony fishes, the fragile gills are protected by an external bony plate, the **operculum.**

Answer 339

A capillary network delivers blood to the gill filaments, facilitating diffusion of oxygen and carbon dioxide between blood and water. This system is extremely efficient be- cause blood flows in a direction opposite to the movement of the water. This arrangement, called a **countercurrent exchange system**, maximizes the difference in oxygen concentration between blood and water throughout the area where the two remain in contact.

Answer 340

respiratory structures that develop as ingrowths of the body surface or from the wall of a body cavity such as the pharynx (throat region).

Answer 341

enclosed in an inpocketing of the abdominal wall. These lungs consist of a series of thin, parallel plates of tissue (like the pages of a book) filled with hemolymph. The plates of tissue are separated by air spaces that receive oxygen from the outside environment through a spiracle

Answer 342

An opening in the floor of the throat region or **pharynx** leads into the **larynx**. Because the larynx contains the vocal cords, it is also referred to as the “voice box.” Cartilage embedded in its wall prevents the larynx from collapsing and makes it hard to the touch when felt through the neck.

Answer 343

During swallowing, a flap of tissue called the **epiglottis** automatically closes off the larynx so that food and liquid enter the esophagus rather than the lower airway.

Answer 344

**trachea**, or windpipe, which is kept from collapsing by rings of cartilage in its wall. The trachea divides into two branches, the bronchi (sing., bronchus); one bronchus connects to each lung. Both trachea and bronchi are lined by a mucous membrane containing ciliated cells.

Answer 345

air passes through the following sequence of structures after it enters the body: nostrils --\> nasal cavities --\> pharynx --\> larynx --\> trachea --\> bronchi --\> bronchioles --\> alveoli

Answer 346

During inhalation, the volume of the thoracic cavity is increased by the contraction of the **diaphragm**, the dome-shaped muscle that forms its floor.

Answer 347

a detergent-like phospholipid mixture secreted by specialized epithelial cells in the lining of the alveoli.

Answer 348

in a mixture of gases the total pressure of the mixture is the sum of the pressures of the individual gases. Each gas exerts, independently of the others, a **partial pressure**— the same pressure it would exert if it were present alone.

Answer 349

explains that the amount of oxygen or carbon dioxide that diffuses across the membrane of an alveolus depends on the differences in partial pressure on the two sides of the membrane, and on the surface area of the membrane. The gas diffuses faster if the difference in pressure or the surface area increases.

Answer 350

combine reversibly with oxygen and greatly increase the capacity of blood to transport it.

Answer 351

copper-containing proteins dispersed in the hemolymph of many species of mollusks and arthropods. Without oxygen, these pigments are colorless. When oxygen combines with the copper, hemocyanins are blue.

Answer 352

**Hemoglobin** and **myoglobin** are the most common respiratory pigments in animals. Hemoglobin is the pigment found in the blood of vertebrates. It is also present in many invertebrate species, including annelids, nematodes, mollusks, and arthropods. In some of these animals, the hemoglobin is dispersed in the plasma rather than confined to blood cells. Recall that myoglobin is a form of hemoglobin found in muscle fibers

Answer 353

In the lung (or gill), oxygen diffuses into the RBCs and combines with hemoglobin (Hb) to form **oxyhemoglobin** (HbO₂). When combined with oxygen, hemoglobin is bright red; without oxygen, it appears dark red, imparting a purplish color to venous blood.

Answer 354

illustrates the relationship between partial pressure of oxygen and percent saturation of hemoglobin. As oxygen concentration increases, there is a progressive increase in the percentage of hemoglobin that is combined with oxygen. The ability of oxygen to combine with hemoglobin and be released from oxyhemoglobin is influenced by several factors in addition to percent O2 saturation. These factors include pH, carbon dioxide concentration, and temperature.

Answer 355

Displacement of the oxygen–hemoglobin dissociation curve by a change in pH is known as the **Bohr effect**. Lactic acid released from active muscles also lowers blood pH and has a similar effect on the oxygen–hemoglobin dissociation curve—more oxygen is unloaded and available for muscle contraction.

Answer 356

Shallow breathing, which occurs in anxiety and in many respiratory diseases, causes **hypoxia**, a deficiency of oxygen.

Answer 357

Fluid between the cells, called **interstitial fluid**, or tissue fluid, bathes the cells and provides a medium for diffusion of oxygen, nutrients, and wastes.

Answer 358

The evolution of specialized **circulatory systems** allowed animals to increase in size and become many cells thick. A circulatory system reduces the diffusion distance that needed materials must travel. It transports oxygen, nutrients, hormones, and other materials to the interstitial fluid surrounding all the cells and removes metabolic wastes. In most animals, a circulatory system interacts with every organ system in the body.

Answer 359

The human circulatory system, known as the **cardiovascular system**, is the focus of extensive research because cardiovascular disease is the leading cause of death in the United States and throughout the world.

Answer 360

A circulatory system typically has the following components: (1) **blood**, a connective tissue consisting of cells and cell fragments dispersed in fluid, usually called *plasma*; (2) a pumping organ, generally a **heart**; and (3) a system of **blood vessels** or spaces through which blood circulates. Two main types of circulatory systems are open and closed systems.

Answer 361

Many invertebrates have an **open circulatory system****,**in which the heart pumps blood into vessels that have open ends. Their blood and interstitial fluid are collectively referred to as**hemolymph**. This fluid spills out of the open ends of the blood vessels, filling large spaces, called *sinuses*. The sinuses make up the**hemocoel** (blood cavity), which is not part of the coelom. The hemolymph bathes the cells of the body directly. Hemolymph re-enters the circulatory system through openings in the heart (in arthropods) or through open-ended vessels that lead to the gills (in mollusks).

Answer 362

Annelids, some mollusks (cephalopods), and echinoderms have a **closed circulatory system**. In them, blood flows through a continuous circuit of blood vessels. The walls of the smallest blood vessels, the capillaries, are thin enough to permit diffusion of gases, nutrients, and wastes between blood in the vessels and the interstitial fluid that bathes the cells.

Answer 363

the tiniest blood vessels, have very thin walls that permit exchange of materials between blood and interstitial fluid.

Answer 364

vertebrate circulatory system consists of heart, blood vessels, blood, lymph, lymph vessels, and associated organs such as the thymus, spleen, and liver. This system performs several functions: 1. Transports nutrients from the digestive system and from storage depots to each cell 2. Transports oxygen from respiratory structures (gills or lungs) to the cells 3. Transports metabolic wastes from each cell to organs that excrete them 4. Transports hormones from endocrine glands to target tissues 5. Helps maintain fluid balance 6. Helps distribute metabolic heat within the body, which helps maintain a constant body temperature in endothermic animals 7. Helps maintain appropriate pH 8. Defends the body against invading microorganisms

Answer 365

blood consists of a pale yellowish fluid called **plasma**, in which red blood cells, white blood cells, and platelets are suspended

Answer 366

Plasma contains several kinds of **plasma proteins**, each with specific properties and functions: **fibrinogen**; alpha, beta, and gamma globulins; and albumin.

Answer 367

one of the proteins involved in the clotting process. When the proteins involved in blood clotting have been removed from the plasma, the remaining liquid is called **serum**.

Answer 368

**Alpha globulins** include certain hormones and proteins that transport hormones; prothrombin, a protein involved in blood clotting; and high-density lipoproteins (HDL), which transport fats and cholesterol. **Beta** **globulins** include other lipoproteins that transport fats and cholesterol, as well as proteins that transport certain vitamins and minerals. The **gamma globulin** fraction of plasma contains many types of antibodies that provide immunity to diseases such as measles and infectious hepatitis. Purified human gamma globulin is sometimes used to treat certain diseases or to reduce the possibility of contracting a disease

Answer 369

**Erythrocytes**, informally called **red blood cells (RBCs)**, are highly specialized for transporting oxygen.

Answer 370

a deficiency in hemoglobin (often accompanied by a decrease in the number of RBCs). When hemoglobin is insuf- ficient, the amount of oxygen transported is inadequate to supply the body’s needs.

Answer 371

**leukocytes**, or **white blood cells (WBCs)**, are specialized to defend the body against harmful bacteria and other microorganisms. Leukocytes are amoeba-like cells capable of independent movement. Some types routinely slip through the walls of blood vessels and enter the tissues. Human blood contains three kinds of granular leukocytes and two types of agranular leukocytes. Both types are manufactured in the red bone marrow.

Answer 372

characterized by large, lobed nuclei and distinctive granules in their cytoplasm. The three varieties of granular leukocytes are the neutrophils, eosinophils, and basophils.

Answer 373

the principal phagocytic cells in the blood, are especially adept at seeking out and ingesting bacteria. They also phagocytose dead cells, a cleanup task that is especially demanding after injury or infection.

Answer 374

have large granules that stain bright red with eosin, an acidic dye. Eosinophils increase in number during allergic reactions and during parasitic (for example, tapeworm) infestations.

Answer 375

exhibit deep blue granules when stained with basic dyes. Like eosinophils, these cells play a role in allergic reactions. Granules in their cytoplasm contain **histamine**, a substance that dilates blood vessels and makes capillaries more permeable. Basophils release histamine in injured tissues and in allergic responses. Other basophil granules contain **heparin**, which speeds up fat removal from the blood after you eat a meal high in fat. Heparin is an anticoagulant that may help prevent blood from clotting inappropriately within the blood vessels.

Answer 376

lack large, distinctive granules, and their nuclei are rounded or kidney-shaped. Two types of agranular leukocytes are lymphocytes and monocytes. Some lymphocytes are specialized to produce antibodies, whereas others directly attack foreign invaders such as bacteria or viruses

Answer 377

largest WBCs. During infection, **monocytes** migrate from the blood into the tissues. They can phagocytose cells and remove toxic molecules. Monocytes can also differentiate into macrophages or dendritic cells. Macrophages are giant scavenger cells that voraciously engulf bacteria, dead cells, and debris. Dendritic cells are also important cells of the immune system

Answer 378

In most vertebrates other than mammals, the blood contains small, oval, nucleated cells called **thrombocytes** that function in blood clotting. Mammals have platelets, tiny spherical or disc-shaped bits of cytoplasm that lack nuclei.

Answer 379

The vertebrate circulatory system includes three main types of blood vessels: **arteries, capillaries,** and **veins**.

Answer 380

An **artery** carries blood away from a heart chamber toward other tissues. When an artery enters an organ, it divides into many smaller branches called **arterioles**. The arterioles deliver blood into the microscopic **capillaries**. After blood circulates through capillary networks within an organ or tissues, capillaries merge to form veins that channel the blood back toward the heart.

Answer 381

The vertebrate heart has one or two **atria** (sing., atrium), chambers that receive blood returning from the tissues, and one or two **ventricles** that pump blood into the arteries. Vertebrates in some groups have additional chambers.

Answer 382

pattern of blood circulation in birds and mammals can be summarized as follows: veins (conduct blood from organs) --\> right atrium --\> right ventricle --\> pulmonary arteries --\> capillaries in the lungs --\> pulmonary veins --\> left atrium --\> left ventricle --\> aorta --\> arteries (conduct blood to organs) --\> arterioles --\> capillaries --\> veins

Answer 383

a tough connective tissue sac, encloses the heart.

Answer 384

A smooth layer of endothelium covers the inner surface of the pericardium and the outer surface of the heart. Between these two surfaces is a small **pericardial cavity** filled with fluid, which reduces friction to a minimum as the heart beats.

Answer 385

The volume of blood one ventricle pumps during one beat is the **stroke volume**. Stroke volume depends mainly on **venous return**, the amount of blood the veins deliver to the heart.

Answer 386

if the veins deliver more blood to the heart, the heart pumps more blood into the arteries (within physiological limits). Increased venous return stretches the cardiac muscle fibers more and they contract with greater force, increasing stroke volume. Norepinephrine released by sympathetic nerves and epinephrine released by the adrenal glands during stress also increase the force of contraction of cardiac muscle fibers.

Answer 387

the force exerted by the blood against the inner walls of the blood vessels.

Answer 388

the resistance to blood flow caused by blood viscosity and by friction between blood and the blood vessel wall.

Answer 389

specialized receptors in the walls of certain arteries and in the heart wall, are sensitive to changes in blood pressure. When an increase in blood pressure stretches the **baroreceptors**, messages are sent to the cardiac and vasomotor centers in the medulla of the brain. The cardiac center stimulates parasympathetic nerves that slow the heart, lowering blood pressure. The vasomotor center inhibits sympathetic nerves that constrict arteri- oles; this action causes vasodilation, which also lowers blood pressure. These neural reflexes continuously work in a complementary way to maintain blood pressure within normal limits.

Answer 390

When the body becomes dehydrated, the osmotic concentration of the blood increases. In response, the posterior lobe of the pituitary gland releases **antidiuretic hormone (ADH).** ADH increases reabsorption of water in the kidneys (and only a small volume of concentrated urine is produced). Blood volume increases, raising blood pressure and restoring homeostasis.

Answer 391

When blood volume increases, the atria of the heart release a hormone called **atrial natriuretic peptide (ANP).** This hormone increases sodium excretion. As a result, a large volume of dilute urine is produced and blood pressure decreases.

Answer 392

Blood from the tissues returns to the right atrium of the heart. This oxygen-poor blood, loaded with carbon dioxide, is pumped by the right ventricle into the pulmonary circulation. As it emerges from the heart, the large pulmonary trunk branches to form the **pulmonary arteries** that deliver blood to the lungs. The pulmonary arteries are the only arteries in the body that carry oxygen-poor blood.

Answer 393

In the lungs the pulmonary arteries branch into smaller and smaller vessels. These blood vessels give rise to extensive networks of pulmonary capillaries that surround the air sacs of the lungs. As blood circulates through the pulmonary capillaries, carbon dioxide diffuses out of the blood and into the air sacs. Oxygen from the air sacs diffuses into the blood so that by the time it enters the **pulmonary veins** leading back to the left atrium of the heart, the blood is charged with oxygen. Pulmonary veins are the only veins in the body that carry blood rich in oxygen.

Answer 394

blood flows through the pulmonary circulation in the following sequence: right atrium --\> right ventricle --\> pulmonary arteries --\> pulmonar capillaries (in lungs) --\> pulmonary veins --\> left atrium

Answer 395

As an example of blood circulation through the systemic circuit, let us trace a drop of blood from the heart to the right leg and back to the heart: left atrium --\> left ventricle --\> aorta --\> right common iliac artery --\> smaller arteries in leg --\> capillaries in leg --\> small veins in leg --\> common iliac vein --\> inferior vena cava --\> right atrium

Answer 396

give rise to a network of capillaries within the heart wall.

Answer 397

join to form a large vein, the coronary **sinus**, which empties directly into the right atrium.

Answer 398

(1) collects and returns interstitial fluid to the blood, (2) launches immune responses that defend the body against disease organisms, and (3) absorbs lipids from the digestive tract.

Answer 399

(1) an extensive network of **lymphatic vessels**, or simply **lymphatics**, that conduct **lymph**, the clear, watery fluid formed from interstitial fluid and (2) **lymph tissue**, a type of connective tissue with large numbers of lymphocytes. Lymph tissue is organized into small masses of tissue called **lymph nodes** and lymph nodules. The tonsils, thymus gland, and spleen, which consist mainly of lymph tissue, are also part of the lymphatic system.

Answer 400

the tendency for plasma to leave the blood at the arterial end of a capillary and enter the interstitial fluid

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