Dynamics of small populations

Question 1

What are density-dependent effects?

Answer 1

Density-dependent effects are those that result in a negative relationship between population density and population growth rate, and can be modelled logistically [dN/dt = rN((K-N)/K), where N is population size, K is carrying capacity and r is the intrinsic rate of natural increase]. The processes that regulate population size include increased dispersal, increased intraspecific competition and decreased availability of limited resources, all at high population densities.

Question 2

What are density-independent effects?

Answer 2

Density-independent effects are those that affect population growth rate regardless of population density, and result in a fluctuating population as r and K vary over time. Environmental stochasticity, where environmental factors like resource supply and weather vary through time, is an example and can also include natural catastrophes or introductions of a predator, which may completely wipe out a small population.

Question 3

What are inverse density-dependent effects?

Answer 3

Also known as demographic or Allee effects, inverse density-dependent effects result in a positive relationship between population density and population growth rate, and can lead to an ‘extinction vortex’ if population falls below a threshold. At low population densities, the viability of the population rapidly reduces due to, for example, difficulty in locating a mate or reduced group defence against predators.

Question 4

How can metapopulations potentially cause difficulties for conservation attempts?

Answer 4

Especially in patchy habitats, sources (high quality patches of habitat where individuals can reliably produce offspring that can then disperse) may support populations in sinks (poorer quality patches of habitat that individuals may occupy but be unable to reliably produce offspring). As sink populations would become extinct if only they are protected, conservation efforts need to understand where is most productive and not just where the species is present / common.

Question 5

What is a population viability analysis?

Answer 5

Population viability analysis is a method of using stochastic models, starting with a simple deterministic model like N(t+1) = N(t) * λ, to predict likelihood of extinction by including the variety of factors that influence the size of a particular population, including demographic / genetic / environmental uncertainty and natural disasters. It is run multiple times to calculate a probability that the population will go extinct over a particular timeframe in a particular scenario.