Extinction vortex
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Extinction Vortices are a class of models through which conservation biologists, geneticists and ecologists can understand the dynamics of and categorize extinctions in the context of their causes. Developed by M. E. Gilpin and M. E. Soulé in their (now) famous 1986 paper Minimal viable populations: Processes of species extinction, there are currently four classes of extinction vortices. The first two (R and D) deal with environmental factors that have an effect on the ecosystem or community level, such as disturbance, pollution, habitat loss etc. Whereas the second two (F and A) deal with genetic factors such as inbreeding and outbreeding depression, genetic drift etc.
Types of Vortex
- R Vortex: The R vortex is initiated when there is a disturbance which facilitates a lowering of population size (N) and a corresponding increase in Variability (Var(r)). This event can make populations vulnurable to additional disturbances which will lead to an additionally decreased value for N and an increased value for Var(r). A prime example of this would be the disruption of sex ratios in a population away from 50:50.
- D Vortex: The D vortex is initiated when N decreases and Var(r) increases such that the spatial distribution (D) of the population is increased and the population becomes 'patchy' or fragmented. Within these fragments local extinction rates increase which, through positive feedback, further increases D.
- F Vortex: The F vortex is initiated by a decrease in N which leads to an increase in heterozygosity and genetic drift, which results in increased degrees of inbreeding depression and an increase in population genetic load, which over time will result in extinction.
- A Vortex: The A vortex is a result of an increase in genetic drift and a corresponding decrease in genetic variance which leads to a decrease in population adaptive potential, and eventual extinction. This vortex can result from biological invasion, resulting in large scale hybridization and outbreeding depression.
Reference
Gilpin, M. E. and Soulé, M. E., (1986). "Minimum Viable Populations: Processes of Species Extinction." pp. 19-34 in M. E. Soulé, ed. Conservation Biology: The Science of Scarcity and Diversity. Sinauer, Sunderland, Mass.
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