Ecology Quiz 6

Question 1

Ecologists use population growth models to understand

Populations can change in size as a result of four processes:

N(t+1) =
Nt =

Answer 1

Ecologists use population growth models to understand how populations change over time, and what factors promote or limit population growth.

Populations can change in size as a result of four processes: birth, death, immigration, and emigration.

N(t+1) = Nt + B - D + I - E
Nt = N0 + B – D + I – E

N stands for number of individuals in population
Nt = number of people living in England at time t
Between time t=0 and some future t

Question 2

Population increases by a constant proportion - The number of individuals added is larger with each time period, and the population grows larger by ever increasing amounts

Geometric growth—
Exponential growth—

Results in a

Answer 2

Geometric growth—organisms reproduce in synchrony at discrete time periods.

Exponential growth— organisms reproduce continuously over time.

Results in a J-shaped set of points (geometric; blue dots) or continuous line (exponential; red).

Question 3

Geometric growth=discrete reproduction

Answer 3

Geometric growth: Nt+1 = lNt

λ = geometric growth rate or per capita finite rate of increase.

Predicts the size of the population after any number of discrete time periods.

Example discrete reproduction is cicadas, coral, salmon, some plants

Question 4

Exponential growth = continuous reproduction

Answer 4

Exponential growth is described by: dN/dt = rN

dN/dt = rate of change in population size at each instant in time

r = exponential population growth rate or per capita intrinsic rate of increase (Per capita considering population size relates to both time and overall population)

r = b – d [births - deaths]

Example: mice, cats, primates people monkeys - no specific time point

Question 5

Population growth rates

When λ(geometric) < 1 or r(exponential) < 0, the
population size will

When λ = 1 or r = 0, the population

When λ > 1 or r > 0, the population

Answer 5

the population size will decrease. - less than one less than zero

When λ = 1 or r = 0, the population stays the same size.

When λ > 1 or r > 0, the population grows geometrically or exponentially.

If resources are unlimited for r than expect a high r value, increase exponentially

Question 6

Effects of density
Two types of factors change population sizes and growth rates over time:

Answer 6

Density-independent factor

Density-dependent factor

Question 7

Density-independent factor:

Answer 7

Effects are independent of the number of individuals in the population - won’t affect population growth rate

most abiotic and biotic

Density-independent factors
Weather
Natural disasters
Pollution
Other chemical/physical conditions

Occur and influence population no matter what its density is

exmaple: Biotic factors - people hunting

Question 8

Density-dependent factor:

Answer 8

Their effects are dependent on the number of individuals in the population - affecting growth rate
always biotic

Density-dependent factors:
- Predation - pray the more of you will attract more predators
- Interspecific competition - with neighbors the more dandelions will compete with grass species in the plot of land. specific species so inter is two different species

• Intraspecific competition - more dandelions will compete with other individuals so competing with other dandelions for light N and P
• Accumulation of waste, and diseases - high density more pathogens. intra is single species - soy soy intra

Usually, the denser a population is, the greater its mortality rate.

When birth, death, or dispersal rates show strong density dependence, population growth rates may decline as densities increase.

If densities become high enough to cause λ = 1 (or r = 0), the population stops growing.

Density-dependent changes in the population growth rate can cause a population to reach a stable, maximum size.

Question 9

Logistic Growth
Example Fulmar

Answer 9

Assumption of no limit to population growth is unrealistic…Although some species can grow this way for long periods… - incorporates density dependant factors

Over many decades, the fulmar population grew exponentially as a result of carry capacity K

Fulmar population grew for over a century
r = 1.11 - above one increasing
11% growth each year – this is very high
Why? Carrying capacity increase A: altered fishing practices - Fulmars eat the offal (guts) discarded by commercial trawlers in North Sea - feed population of fulmar

Question 10

Resources are not limitless
carrying capacity:

Equation:

Answer 10

Maximum # of individuals supported in a habitat is known as carrying capacity:
Denoted by K.
Determined by limiting resources or factors.

The logistic equation incorporates carrying capacity into exponential growth

dN/dt=rN(1-N/K)
n: pop density
r: per capita growth rate
k: carrying capacity

Question 11

Logistic growth:
Example haber bosch

Answer 11

When densities are low, logistic growth is similar to exponential growth.
When N is small, (1 – N/ K) is close to 1, and the population increases at a rate close to r.
As density increases, the growth rate approaches zero as the population nears K.
Example: Pearl and Reed (1920) derived the logistic equation and used it to predict a carrying capacity for the U.S. population.
The logistic curve fit the U.S. data well up to 1950…
Population size continued to grow exponentially…
WHY? - tech advancement in the 1950s like medicine and haber bosch process (fertilizer - was limited to resources now can take nitrogen) - tech has increased the carry capacity

Question 12

Life Tables & Population Demography

Age class:
Age structure:

Answer 12

Summary of how survival and reproductive rates vary with age, size, or life stage of individuals. - age classes(zero to whatever age) then number of individuals in each class and can make calculations that can be used to predict future population trends.
Used to predict future population trends and develop strategies for managing populations.

Age class: Members of a population whose ages fall within a specified range.
Age structure: Proportion of a population in each age class
Influences whether population will increase or decrease in size:
- If many individuals of reproductive age à grow rapidly
- If many individuals older than reproductive age à population decline
- Population growing looks like a pyramid with more individuals in the reproductive age class and the opposite for declining population so greater population number in the old people

Question 13

Survivorship curves

Following three types of patterns:

Answer 13

Survivorship data can be graphed as a survivorship curve.
Survivorship curves can vary:
- Among populations of a species
- Between males and females
- Among cohorts that experience different environmental conditions

Following three types of patterns:
- Humans and most mammals have a Type I survivorship curve because death primarily occurs in the older years.
- Birds have a Type II survivorship curve, as death at any age is equally probable.
- Trees have a Type III survivorship curve because very few survive the younger years, but after a certain age, individuals are much more likely to survive. - trees

Question 14

Cohort life table

lx
Fx
Sum of lxFx for all age classes =

Answer 14

Follows the fate of a group of individuals all born at the same time (a cohort).
Mostly used for sessile organisms. - Organisms that are highly mobile or have long life spans are difficult to track.

lx = survivorship: proportion of individuals that survive from birth to age X

Fx = fecundity: average number of offspring produced per surviving adult per age class

Sum of lxFx for all age classes = net reproductive rate ( R0)
If R0 > 1.0, there is a net increase in offspring produced each generation, the population should increase exponentially.
If R0 < 1.0, the population declines, eventually to extinction.
If R0 = 1.0, births and deaths balance out and the population will not change in size

Question 15

Cohort life table
Example

Answer 15

Managers may want to alter population growth rates to reduce pest populations, or increase endangered species.

One method: identify the age- specific birth or death rates that most strongly influence population growth rate.

Example: Early efforts to protect endangered Loggerhead sea turtles focused on egg and hatching stages to increase population cycles and saw important in juvenile stage

But life table data indicated that the best way to increase growth rates was to increase survival rates of juveniles and adults.

These studies resulted in laws requiring Turtle Excluder Devices (TEDs) in shrimp nets to increase adult survival.

Question 16

Predation:

Carnivory

Herbivory

Parasitism

Answer 16

Predation: individuals of one species (predators) kill and/or consume individuals (or parts) of another species (its prey)

Carnivory—predator and prey are animals

Herbivory—predator is an animal, prey are plants or algae

Parasitism—predator (a parasite) lives symbiotically on or in the prey (its host) and consumes certain tissues; may not kill the host. Some parasites are pathogens that cause disease.

Question 17

Carnivory vs. Herbivory

Answer 17

Carnivory:
- Carnivores kill their prey
- Animal prey can usually move away or hide from predators
- Animal prey is less abundant but very nutritious

Herbivory:
- Herbivores usually don’t kill the plants they eat
- Plants are sessile
- Plant prey are often more abundant but have lower nitrogen content
- Herbivores can reduce the growth, survival, or reproduction of plants, including belowground herbivores
- A 40% reduction in growth was observed in bush lupines after 3 months of herbivory by root killing ghost moth caterpillars

Question 18

Dietary Preferences
Optimal foraging and dietary preferences depend on:

Answer 18

Encounter rate—if low, predators (carnivores) should be generalists

Handling time—if prey are easy to find but handling time is long, i.e., immobile but less nutritious plants, then predators (herbivores) should be specialists

Question 19

Generalists

Answer 19

Many predators are generalists

Limited only by what they can catch and fit in their mouths

Thus, predators are generally larger than their prey

Many generalist predators eat a wide range of prey types

Some predators concentrate foraging on whatever prey is most abundant

E.g., guppies ate disproportionate amounts of whichever prey was most abundant

Question 20

Specialist predators suffer with their prey

Answer 20

Most herbivores are specialists

Large herbivores may eat all aboveground parts, but most specialize on leaves, stems, roots, seeds, or sap

Leaves are most commonly eaten because they are often abundant and the most nutritious part, except for seeds

Question 21

Models of predation: Start with logistic growth model

Need 2 equations
1 for predators
1 for prey

Answer 21

dN/dt=rN(1-N/K)
n: pop density
r: carrying capacity
k: per capita growth rate

Need a loss term for prey taken by predators
Depends on three things
1. Predator density, P
2. Prey density, N
3. Capture efficiency, or rate of predation, a

Prey: dN/dt=rN-aNP
Predator: dN/dt=baNP-mP

Question 22

Mechanisms Important to Predation:
Finding prey:

Answer 22

Many carnivores forage by moving about in search of prey, e.g., wolves, sharks, hawks
Others remain in one place and attack prey that come within striking distance, e.g., moray eels, some snakes
Others set traps, such as spider webs or the modified leaves of a carnivorous plant

Question 23

Mechanisms Important to Predation:
Prey capture:

Answer 23

A cheetah’s body form enables bursts of speed that allow it to catch gazelles
Snakes can swallow prey that are larger than their heads because the skull bones are not rigidly attached to one another
Some predators subdue prey with poisons (e.g., spiders)

Question 24

Mechanisms Important to Predation:
Predator adaptations

Answer 24

Some predators use mimicry, blending into their environment so that prey are unaware of their presence
Some have inducible traits, such as a ciliate that adjusts its size to match the size of available prey
Some predators can detoxify or tolerate chemicals made by prey
Some garter snakes can eat toxic rough-skinned newts that make tetrodotoxin (TTX), a potent neurotoxin

Question 25

Mechanisms Important to Predation:
Prey adaptations to escape being eaten

Physical defenses:

Warning coloration (aposematic):

Mimicry:

Crypsis:

Many prey species change their behavior when predators are present:

There can be trade- offs between defenses:

Plant defenses: masting

Compensation:

Structural defenses:

Induced defenses:

Chemical defenses:

Answer 25

Physical defenses: Large size, body plan designed for rapid or agile movement; body armor

Warning coloration (aposematic): Predators learn not to eat organisms that have toxins

Mimicry: Prey resembles another organism that is toxic or very fierce

Crypsis: The prey is camouflaged, or resembles its background

Many prey species change their behavior when predators are present:
Don’t forage in open areas (snowshoe hares)
Make defensive circles (e.g., musk oxen)

There can be trade- offs between defenses:
Of four snail species eaten by shore crabs, the species with the thinnest shell takes refuge most quickly
Snails with thickest shells are the last to take refuge

Plant defenses:
Some produce huge numbers of seeds in some years and hardly any in other years (masting)
The plants hide (in time) from seed-eating herbivores, then overwhelm them by sheer numbers

Adaptive growth responses allow some plants to tolerate herbivory
Compensation: Removal of plant tissue stimulates new growth. Full compensation—no net loss of plant tissue
Removal of leaves can decrease self- shading; removal of apical buds allows lower buds to open and grow
In field gentians, herbivory early in the growing season results in compensation, but later in the season it does not
For any plant, if too much tissue is removed, or there are not enough resources for growth, compensation cannot occur.

Structural defenses: tough leaves, spines, thorns, sawlike edges, pernicious (nearly invisible) hairs that can pierce the skin

Induced defenses: produced in response to herbivore attack

Chemical defenses:
Toxic secondary compounds
Compounds that attract predators or parasitoids that will attack the herbivores
Some are produced all the time; others are induced

Question 26

Effects of predation on communities

All trophic interactions have AND example

Predators can change the outcome
If a predator or herbivore decreases
AND TWO examples

Herbivores can also alter communities:

Answer 26

All trophic interactions have the potential to reduce growth, survival, or reproduction of the organisms that are eaten
Example: A leaf-feeding beetle rapidly reduced density of Klamath weed, an invasive plant in the western U.S. that is poisonous to livestock

Predators can change the outcome of competition, affecting distribution or abundance of competitor species
If a predator or herbivore decreases performance of the top competitor, the inferior competitor may increase in abundance
Example: Paine (1974) removed starfish predators from a rocky intertidal zone, which led to local extinction of all large invertebrates except one mussel species. When the starfish predator was present, inferior competitors were able to persist.
Example: Arctic foxes were introduced to some Aleutian Islands around 1900. This reduced seabird density by nearly a hundredfold, which reduced the guano that fertilizes plants on the islands. Guano transfers nitrogen and phosphorus from the ocean to the land. With less guano, dwarf shrubs and herbaceous plants increased at the expense of grasses.

Herbivores can also alter communities:
Lesser snow geese summer in salt marshes of northern Canada where they feed on grasses and sedges The plants are fertilized by the goose feces and grow rapidly after low to intermediate levels of grazing. In 1970, lesser snow goose densities increased exponentially; probably because of increased crop production in their winter range. The geese completely removed the vegetation, drastically changing distribution and abundance of marsh plant species