Small and declining populations

Question 1

What is the MVP?

Answer 1

Minimum Viable Population, defined as the critical threshold beneath which a population is ensured to decrease in size until extinction.

Question 2

What are deterministic changes?

Answer 2

Predictable changes to the population size.

Question 3

What are stochastic changes?

Answer 3

Inherently random changes to population growth.

More likely to cause extinction.

Question 4

What is demographic stochasticity?

Answer 4

Random variations in birth and death rates that occur in populations from year to year.

Can have a major effect on small populations, if individuals happen to have unusual values for a demographic trait.

Example - sex ratios.

Question 5

What is environmental stochasticity?

Answer 5

Any fluctuation in the physical environment and the biotic, such as predators, parasites and competitors.

Can affect the whole population and isn’t dependent on density. More likely to cause extinction as doesn’t require a small population.

Question 6

How does inbreeding depression affect small populations?

Answer 6

Increased probability of mating with a close relative. Results in a higher probability that any gene will be homozygous. This can lead to lower fitness in the case of deleterious recessive mutations.

Can be avoided in populations where the size decreases slowly as natural selection will eliminate the deleterious recessive alleles from the gene pool.

Question 7

How does genetic drift affect small populations?

Answer 7

Alleles can be lost by chance. This is a big issue in small populations as mutations are less likely to occur and so the rate of loss of variation to drift is larger.

Big threat to species survival if the environment changes as natural selection can’t act as quickly.

Question 8

What is the effective population size?

Answer 8

The number of individuals in a population who contribute offspring to the next generation.

Question 9

How can behavioural problems lead to extinction in small populations?

Answer 9

Allee effect.

In animals that hunt in packs. Widely dispersed animals might not find mates.

Question 10

Define extinction vortex?

Answer 10

Forces affecting small populations that cause them to spiral into a vortex of increasingly smaller populations.

Question 11

Example of a persisting small population?

Answer 11

Isolated Pacific Island has about 20 pairs of Socorro Island Hawk.

Likely to have been this size throughout its entire history of several thousand years.

Question 12

What is a Population Viability Analysis?

Answer 12

A species specific method of risk assessment frequently used in conservation biology.

Defined as the process that determines the probability that a population will go extinct within a given number of years.

Question 13

What circumstances can lead to small populations persisting?

Answer 13

Immigration can increase genetic diversity.

Question 14

Main drivers of extinction?

Answer 14

Habitat destruction/fragmentation 
Harvesting (over-exploitation) 
Chains of extinction 
Climate change 
Introduced species

Question 15

Biggest threat to bird species?

Answer 15

Habitat change and loss is the main threat to 75% of the 1100 threatened species.

Question 16

Potential responses to climate change?

Answer 16

Extinction 
Plasticity to adapt in situ 
Evolution to adapt in situ 
Shift their distributions 
Shift their phenology (timing of biological events).

Question 17

How can declining populations be identified?

Answer 17

Demographic surveys - is the population size declining?

Distribution surveys - is the area taken up by the population declining?

Question 18

Demographic surveys?

Answer 18

Measure the population size over time. Need to distinguish declines from normal stochastic variation.

Population cycles may be driven by predator prey interactions.

Important to study over a long time period.

Takes a lot of time and effort.

Question 19

Example of a distribution survey?

Answer 19

Every 10km square in Britain was surveyed for the presence or absence of species in the early 1970s and late 1980s.

Question 20

Issues with distribution surveys?

Answer 20

There may be a time lag before declines in size are translated to declines in distribution - happened with an arctiid tiger moth.

Question 21

How can spatial patterns in species distribution reveal biodiversity change?

Answer 21

Declining populations show sparse, fragmented distributions for their distribution size, reflecting the extinction process.

Expanding species show denser, more aggregated distributions, reflecting colonisation.

Can be used to assess relative levels of threat facing different species which lack other forms of data.

Question 22

An example of how extinction can be avoided?

Answer 22

Lord Howe Woodhen.

Population restricted to 10 pairs on two remote high mountains after formerly ranging over the entire island.

Low productivity and adult survival on the tops of mountains.

Pigs were eating eggs and chicks.

Removed pigs and introduced 50-60 pairs.

Habitat is now saturated.