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what is the small population paradigm?

- small populations: few individuals in a small area

- small populations have increased probability of extinction


why are small populations vulnerable?

- environmental stochasticity: chance events that disrupt populations

- demographic stochasticity: chance events that disrupt population growth

- behavioural effects i.e. allee effects

- genetic factors


How does environmental stochasticity affect small populations?

- environmental stochasticity, especially climatic events, is often synchronised over large areas
- affects even species with fairly big ranges

- plants are often more resistant than vertebrates due to dormant, stress tolerant life forms


How does environmental stochasticity affect small populations? Examples:

- montserrat oriole

Montserrat Oriole:

- 1995 volcanic eruption (also 2001, 2003, 2006)

- density independent
direct impacts: volcanic dust, 75% habitat loss

- density dependent indirect impacts: lower food abundance due to acid rain caused by volcano

- populations declined dramatically


How does environmental stochasticity affect small populations? Examples:

- coachella valley fringe-toed lizard

- drought induces populations in fringe-toed lizards in coachella valley


How does demographic stochasticity affect small populations?

- breeding success

- say there's an 80% chance a breeding pair's offspring will die before breeding age
- all individuals die after a year
- extinction occurs if no young live to maturity

- if 30 pairs, extinction probability is 0.8^30 = 0.001
- if 10, 0.8^10 = 0.11
- if 5, 0.8^5= 0.33

- smaller population, higher chance of extinction due to breeding failure


How does demographic stochasticity affect small populations?

- sex ratio problems

- if a population has 2 breeding pairs and each female produces 2 remaining offspring

- 0.5*0.5*0.5*0.5 = 0.0625

- 6.25% chance that offspring will all be one sex

- population will go extinct if that happens

e.g. last 6 dusky seaside sparrows were male


How do behavioural factors - allee effects affect small populations?

- allee effect = correlation between population size or density and the mean individual fitness of a population or species

- usually negative density dependence due to intraspecific competition, rarely positive


How do behavioural factors - allee effects affect small populations?

- pollination

- pollination allee effects often found in flowers pollinated by animals

- plants get further apart as population size decreases

- reduces probability of pollen transfer e.g. in rainforest trees

- smaller clumps less attractive to pollinators


How do behavioural factors - allee effects affect small populations?

- social animals

- predation rates e.g. in colonial nesting birds

- foraging effectiveness e.g. pack hunters

- willingness to mate e.g. flamingos


How do behavioural factors - allee effects affect small populations?

- lesser kestrels

- higher reproductive success and adult survival in larger colonies


How do behavioural factors - allee effects affect small populations?

- finding mates

- finding mates at low population densities can be hard

- especially if large home ranges e.g. polar bears

- competition for mates from other species


How do genetic factors affect small populations?

- smaller populations = less genetic diversity

- homozygosity (within an individual)
- allelic richness - number of alleles per locus (within a population)


What is effective population size?

- number of breeding individuals

- often much less than census population


When is effective population size reduced?

not all individuals mate/some contribute more offspring than other:
-non-monogamous species
- e.g. lek forming species or species with extra pair paternity: superb fairy wren extra pair paternity up to 75%
- uneven sex ratios

population sizes fluctuate:
- low population years have disproportionate influence

generations overlap
- e.g. small mamals


What is genetic drift?

- stochastic events that determine which alleles are passed on to the next generation

- in small populations genetic drift is more likely to lead to loss of genetic diversity


How can small populations be rescued from genetic drift?


- even if immigrations are low
- especially effective in small populations


Why is loss of genetic diversity bad?

- deleterious recessive alleles more likely to occur in combination leading to inbreeding depression

- natural selection acts on genetic diversity, reduced diversity can lead to reduced evolutionary potential

- inbreeding leads to reduction in reproductive success (e.g. atlantic salmon, native new zealand birds)

- 82% animal studies in a meta-analysis had positive correlation between fitness and genetic diversity (Reed and Frankham, 2003)


What are potential benefits of loss of genetic diversity?

- alleles with high fitness tend to be dominant so heritability can remain high in small populations

- 50% genetic variation can consist of deleterious alleles, reduced diversity may purge these

- in theory small populations with limited genetic diversity can have high fitness

- need empirical evidence for adverse impacts of low genetic diversity


Does genetic variation really matter?

- IUCN red list species way less genetically diverse than least-concern related species

- can determine ecological processes and thus services
- e.g. nutrient release and decomposition in aspen forests
- e.g. primary productivity and energy flux


How did heath hens go extinct?

1870 - 300 individals
1900 - 70 individuals (hunting)

1910 - 2000 individuals (reserve and hunting ban)

1932 - extinct

- environmental stochasticity: fire & cold winters
- demographic stochasticity: sex ration bias, severe population fluctuations
- genetic bottleneck effects: inbreeding
- disease outbreak: worsened by inbreeding? less resilience?


What is an example of a species with a naturally small population?

Socorro island hawk:
- 15-20 pairs for several thousand years