Endangered species: is living in a small population a bad thing???
- Endangered populations are small
- Small populations are at greater risk of extinction
- Berger (1990) studied 120 populations of the bighorn sheep in N. America. Small populations (with fewer than 50 individuals) were more likely to become extinct within 50 years.
- Multiple factors interact to drive species past point of recovery
Animal behaviour affects population dynamics often through Allee effects
The Allee effect (Allee 1931, 1938) for smaller populations, the reproduction and survival rates of individuals increases with population density
For populations below a CRITICAL SIZE, the death rate is higher than birth rate, so with every generation the population gets smaller and smaller
Behaviours that can affect extinction risk
1) Movement patterns: - Dispersal - Migration - Orientation - Immigration/ emigration rates
2) Habitat use: - Ecological traps - Ground nesting (susceptible to introduced predators) - Ranging behaviour - Breeding near edge habitats (susceptible to predators, road traffic, brood parasites)
3) Social behaviour and organization: -Hierarchies (e.g. elephants and wolves) -Communication -Staging during migration (e.g. shorebirds) -Anti-predator and vigilance behaviour
4) Breeding behaviour: - Mate choice - Nest site selection - Social facilitation of reproduction - Reproductive suppression - Synchronous and cooperative breeding - Lek mating systems - Inbreeding - Hybridization at low popnʼ levels
5) Foraging behaviour - Co-operative hunting (e.g. lions) - Conspecific attraction (e.g. vultures) - Ecological traps
DISPERSAL AND MIGRATION
Dispersal = one way movement away from natal site
Migration = round-trip movement to facilitate breeding/overwinter survival usually in non-random direction
- Higher risk of mortality (predation/ hunting)
- Isolation for species which have limited dispersal distances
- Sensitivity to reduced gene flow
- Sensitivity to habitat destruction
Short distance disperser limited by patch size and distance between patches, e.g. Silver spotter skipper butterfly.
Immigration & emigration higher in small patches.
RANGING BEHAVIOUR AND RESERVE SIZE IN LARGE CARNIVORES
Many large predators live in fragmented habitats - risk is when animals come in…