Ch. 11 Population Dynamics Flashcards
Eutrophication
Excessive richness of nutrients in a lake or other body of water
Populations can change in size as a result of four processes:
Birth, death, immigration, and emigration
Population dynamics
The ways in which populations change in abundance over time
Populations exhibit a wide range of growth patterns, including..
- exponential growth
- logistic growth
- fluctuations
- regular cycles
Exponential growth usually happens when..
- when conditions are favorable
- when a species reaches a new geographic area
Logistic curve
- some populations reach a stable size that changes little over time
- fluctuate by a small amount around what appears to be carrying capacity
Logistic equation
K
- assumed to be constant
- the population size for which birth and death rates are equal
Logistic growth example
- when sheep were first introduced to Tasmania, the population increased rapidly
- later, population numbers fluctuated above and below a maximum sustainable population size
Birth and death rates vary over time, thus we expect..
Carrying capacity to fluctuate
Carrying capacity
An equilibrium with their environmental resources
Logistic growth
Near exponential growth when..
Resources are unlimited
Environmental resistance
Growth slows as species approaches carrying capacity of environment
___ factors reduce population growth rates.
Density-dependent
Density-dependent factor examples
- predation
- food abundance
- parasite load
- intraspecific competition
Population fluctuation
Rise and fall in population size over time
Population outbreak (bloom)
Populations may explode, causing a bloom
Population cycles
Some populations have alternating periods of high and low abundance at regular interval
Internal factors that may drive population cycles in rodents:
Hormonal or behavioral changes in response to crowding
External factors that may drive population cycles in rodents:
Weather, food supplies, or predators
Example of top-down regulation in lemmings
Used field observations and mathematical models to argue that their 4-year cycle is driven by predators
Example of bottom-up regulation in lemmings
In other studies, predator removal had no effect on population cycles
These cycles were driven by food supply
Effects of population density often have a ___ or delay.
Lag time
What may this lag be due to?
Need time to reproduce
Delayed density dependence
- delays in effect that density has on population size
Example of delayed density dependence
When a predator reproduces more slowly than its prey
__ can cause fluctuations in population size.
Time lags
Stable limit cycle
Pattern when the population fluctuates indefinitely about the carrying capacity
Many factors can drive populations to extinction:
- predictable factors
- fluctuation in population growth rate and population size
- chance events
If you fluctuate to zero ->
Extinction
Fluctuations increase risk of ___.
Extinction
When variable environmental conditions result in ___ in populations growth rate, extinction risk of population increases.
Large fluctuations
___ populations are at greatest risk.
Small
Implications for natural populations:
The larger the population size, the less likely a population will go extinct due to fluctuations
___ events can influence fluctuations in population growth rates over time.
Chance
Genetic drift
- chance events influence which alleles are passed on to next generation
- allele frequencies can change at random from one generation to next in small populations
___ can reduce genetic variation of small populations, but has little effect on large populations.
Drift
Small populations are vulnerable to the effects of genetic drift for three reasons:
- reduced genetic variability reduces ability of a population to respond to environmental change
- harmful alleles to occur at high frequencies
- high frequency of inbreeding -> further reduces genetic variation
___ tends to increase the frequency of homozygotes.
Inbreeding
Demographic stochasticity
“Chance” events related to survival and reproduction of individuals
When population size is large, there is little risk of extinction from demographic stochasticity because of laws of ___.
Probability
Allee effects
Population growth rate decreases as population density decreases
- individuals have difficulty finding mates or are less stimulated to breed at low population densities
Environmental stochasticity
Unpredictable changes in environment that can cause extinction of small populations
Environmental stochasticity example
Yellowstone grizzly bear population is at risk of extinction from random environmental variation of the number of females drops to 30-40
Demographic stochasticity vs. environmental stochasticity
Demographic stochasticity -> population-level birth and death rates are constant within a given year, but actual fates of individuals differ
Environmental stochasticity -> changes in average birth or death rates that occur from year to year because of random changes in environmental conditions
Both are more problematic for small populations
Natural catastrophes
Floods, fires, severe windstorms, or outbreaks of disease or natural enemies
For many species, areas of suitable habitat exist as a series of favorable sites that are ___ from one another.
Spatially isolated
Metapopulations
Set of spatially isolated populations linked by dispersal of individuals or gametes
Characterized by repeated extinctions and colonizations of small individual populations
Populations of some species are prone to extinction for two reasons:
- landscapes they live in are patchy
- environmental conditions often change in a rapid and unpredictable manner
Sinks
Populations that receive more immigrants than the number of emigrants they produce
For a metapopulation to persist for a long time, the extinction/colonization ratio must be..
Less than 1
Habitat fragmentation
Large tracts of habitat are often converted to spatially isolated habitat fragments by human activities
As patches get smaller and more isolated, colonization ___ and extinction rate ___.
Decreases; increases
Isolation by distance
Patches located far from occupied patches are less likely to be colonized than those near occupied patches
Patch area effect
- large patches tend to have larger population sizes
- small patches may be harder to find, and have higher extinction rates
Rescue effect
High rates of immigration that protects a population from extinction
Black Sea
Bottom-up control
- increased nutrients inputs caused eutrophication and increased phytoplankton biomass, decreases oxygen, fish die-offs, etc
- prior to nutrient enrichment, phytoplankton were limited by nutrient supply
Black Sea
Top-down control
- top predators Mnemiopsis and Beroe control abundance of populations
- overfishing was also a factor in the Black Sea
