small population paradigm (lecture 5) Flashcards
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
- 0.50.50.5*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?
mutation
migration
- 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
why?
- 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
