Population dynamics Flashcards
An ecological fact of life
Nt+1 = Nt + B - D + I – E
- N = number of individuals of a species
- T = time
- B = births
- D = deaths
- I = Immigrations
- E = emigrations
Increase population size
Decrease population size
Populations are open and dynamic entities
Individuals can move from one population to another
Population size can change from one time period to the next
Populations exhibit a wide range of growth patterns, including exponential growth, logistic growth, fluctuations and regular cycles
These four patterns of population growth are not mutually exclusive and a single population can experience each of them at different points in time
Exponential growth
A population increases by a constant proportion at each point in time
When conditions are favourable, a population can increase exponentially for a limited time
Can also occur when a species reaches a new geographic area
If conditions are favourable in the new area, the population may grow exponentially until density-dependent factors regulate its numbers
Logistic growth
Some populations reach a stable size that changes little over time
Such populations first increase in size, then fluctuate by a small amount around what appears to be the carrying capacity
Patterns of population growth
Plots of real populations rarely match the logistic curve exactly
Logistic growth is used broadly to indicate any population that increases initially, then levels off at the carrying capacity
In the logistic equation, k is assumed to be constant – k = population size for which birth and death rates are equal (carrying capacity)
For k to be constant, birth and death rates must also be constant over time at any given density, which rarely happens in nature
Birth and death rates vary over time, so carrying capacity (k) is expected to fluctuate
K = density where the birth rate intersects the death rate
Population fluctuation
A rise and fall in population size over time
Population cycles
Some populations have alternating periods of high and low abundance at regular intervals
Populations of small rodents typically reach a peak every 3-5 years
Predation can drive population cycles, as can other factors
Delayed density dependence
Can cause populations to fluctuate in size
e.g when a predator reproduces more slowly than its prey
The effects of population density often have a lag time of delay
The number of individuals born in a given time period is influenced by population densities that were present several time periods ago
If predator population is small initially, the prey population may increase, and as a result the predator population increases but with a time lag
Large numbers of predators may decrease the prey population, then the predator population decreases again
= population cycles
Population extinction
Many factors can drive populations to extinction
Predictable (deterministic) factors, as well as fluctuations in population growth rate, population size, and chance events
The risk of extinction increases greatly in small populations
Chance genetic, demographic and environmental events can play a role in making small populations vulnerable to extinction
Genetic drift – chance events influence which alleles are passed on to the next generation, can cause allele frequencies to change at random from one generation to the next in small populations, drift reduced the genetic variation of small populations, has little effect on large populations
Small populations are vulnerable to the effects of genetic drift because:
1. Loss of genetic variability reduces the ability of a population to respond to future environmental change
2. Genetic drift can cause harmful alleles to occur at high frequencies
3. Small populations show a high frequency of inbreeding (mating between related individuals)
Inbreeding increases frequency of homozygotes, including those that have two copies of a harmful allele, which can lead to reduced reproductive success
Demographic stochasticity – chance events related to the survival and reproduction of individuals, population-level birth and death rates are constant within a given year, but the actual fates of individuals differ
When population size is large, there is little risk of extinctions from demographic stochasticity because of the laws of probability
Allele effects – population growth rate decreases as population density decreases; individuals have difficulty finding mates at low population densities
In small populations, allele effects can cause the population growth rate to drop, which causes the population size to decrease further
Environmental stochasticity – unpredictable changes in the environment, changes in the average birth or death rates that occur from year to year because of random changes in environmental conditions
More likely to cause extinction when population size is small
Natural catastrophes such as floods, fires, severe windstorms, or outbreaks of disease or natural enemies can eliminate or greatly reduce populations
A species can be vulnerable to extinction when all are members of one population
Metapopulations
Many species have a metapopulation structure in which sets of spatially isolated populations are linked by dispersal
For many species, areas of suitable habitat exist as a series of favourable sites that are spatially isolated from one another
Metapopulations are characterised by repeated extinctions and colonisations
Populations of some species are prone to extinction because:
1. The landscapes they live in are patchy (making dispersal between populations difficult)
2. Environmental conditions often change in a rapid and unpredictable manner
But the species persists because the metapopulation includes populations that are going extinct and new populations established by colonisation
Extinction and colonisation of habitat patches can be described by the equation
P = proportions of habitat patches that are occupied at time t
C = patch colonisation rate
E = patch extinction rate
1. There are an infinite number of identical habitat patches
2. All patches have an equal chance of receiving colonists
3. All patches have an equal chance of extinction
4. Once a patch is colonised, its population increases to its carrying capacity more rapidly than the rates of colonisation and extinction (allows population dynamics within patches to be ignored)
This leads to fundamental insight – for a metapopulation to persist for a long time, the ratio e/c must be less than 1
Some patches will be occupied as long as the colonisation rate is greater than the extinction rate
Otherwise the metapopulation will collapse and all populations in it will become extinct
Led to research on key issues – how to estimate factors that influence patch colonisation and extinction, importance of the spatial arrangement of suitable patches, extent to which the landscape between habitat patches affects dispersal, how to determine whether empty patches are suitable habitat or not
Real metapopulations often violate the assumptions of the Levins model
Patches may vary in population size and ease of colonisation; extinction and colonisation rates can vary greatly among patches
These rates can also be influenced by non-random environmental factors – conditions and resources
Isolation by distance – patches that are located far from occupied patches are less likely to be colonised than near patches
Patch area – small patches may be harder to find, and also have higher extinction rates
A patch that is near an occupied patch may receive immigrants repeatedly, making extinction less likely
High rates of immigration to protect a population from extinction = the rescue effect
