Ecology Quiz 6 Flashcards
(26 cards)
Ecologists use population growth models to understand
Populations can change in size as a result of four processes:
N(t+1) =
Nt =
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
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
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).
Geometric growth=discrete reproduction
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
Exponential growth = continuous reproduction
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
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
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
Effects of density
Two types of factors change population sizes and growth rates over time:
Density-independent factor
Density-dependent factor
Density-independent factor:
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
Density-dependent factor:
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.
Logistic Growth
Example Fulmar
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
Resources are not limitless
carrying capacity:
Equation:
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
Logistic growth:
Example haber bosch
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
Life Tables & Population Demography
Age class:
Age structure:
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
Survivorship curves
Following three types of patterns:
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
Cohort life table
lx
Fx
Sum of lxFx for all age classes =
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
Cohort life table
Example
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.
Predation:
Carnivory
Herbivory
Parasitism
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.
Carnivory vs. Herbivory
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
Dietary Preferences
Optimal foraging and dietary preferences depend on:
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
Generalists
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
Specialist predators suffer with their prey
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
Models of predation: Start with logistic growth model
Need 2 equations
1 for predators
1 for prey
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
Mechanisms Important to Predation:
Finding prey:
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
Mechanisms Important to Predation:
Prey capture:
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)
Mechanisms Important to Predation:
Predator adaptations
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
