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
