Extinction Vortex
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.
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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.
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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.
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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.
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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.
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.
