Extinction Vortex
Bradshaw
(2008) defined extinction Vortex, as the term used to describe the process that
declining population undergo when mutual reinforcement occurs among biotic and
abiotic process that drives population size downward to extinction. In the same
manner, Gilpin and Soule’ (1986), saw Extinction Vortex as 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. This is true as it is noticed that, Extinction vortex, is the kind of
extinction initiated by other causes, but random environmental fluctuations
take over to cause the final extinction regardless of what causes the
population to decline in the first place.
Example,
an extinction vortex was observed in jungle-fowl. The extinction vortex began
simply enough with an unusually long dry season. The drought takes its toll on
the jungle-fowl population, especially the young. By the time monsoon rains
began, the population stands at 45: 20 females, 25 males. At this time, peasants
encouraged by a government settlement policy cleared a patch of degraded forest
land for subsistence agriculture. With them, the peasants bring livestock
including chickens, closely related to jungle-fowl, and dogs. The chickens
harbor an avian flu, to which they are immuned, which quickly spreads among the
jungle fowl population of the nearby forest fragment. The flu decimates the
jungle-fowl; at year end 21 remain: 9 females, 12 males. The presence of dogs
increased predation of the jungle-fowl to 66% of the adult population every
year so after another year the population stands at just 16: 6 females, 10
males. The population dips to 3, and the single remaining adult female is
crushed by a tree limb that falls during a thunderstorm. Neither of her three
chicks survive without her protection. The two male birds wander the forest
looking for the mate they will never find. After the final male jungle-fowl is
eaten by a python, the species is extinct from the forest patch. Hence, the
extinction of the jungle-fowl was enhanced and completed by the environmental
conditions, and that is how extinction vortex occurs.
It is believed that, the present
ongoing 6th mass extinction of species of organisms, is a clear extinction vortex
in display. Anthropogenic factors usually has greater effect on species. This can trigger in other genetic problems that can further affect the population of the species. Gilpin and Soulé (1986), also identified four
types of Extinction Vortex
R Vortex: which is initiated by a
disturbance that facilitates a lowering of population size (N) and a
corresponding increase in variability (Var(r)). A prime example of this would
be the disruption of sex ratios in a population away from
the species optimum
D Vortex: The D vortex is initiated
when population size (N) decreases and variability (Var(r)) increases such that
the spatial distribution (D) of the population is increased.
F Vortex: This is initiated by a
decrease in population size (N) which leads to a decrease in individual heterozygosity
and increases the rate of genetic drift, resulting in increased degrees of
inbreeding depression and an increase in population genetic load, which over time will
result in extinction.
A Vortex:
this is a result of an increase in genetic drift and corresponding decrease in
genetic variance which leads to a decrease in ‘population adaptation” and
eventual extinction. Moreso, Frankel and Michael (1981) reiterated that,
extinction is the failure of a population to maintain itself through
reproduction.
REFERENCES
Gilpin ME, Soule ME
(1986). Minimum Viable Population:
Process of species extinction. In M.E Soule. Conservation biology: The
science of scarcity and diversity. Sinauer, Sunderland, mass pp. 19-34.
Wikipedia (2016). Extinction vortex. Retrieved from http://en.m.wikipedia.org/wiki/extinction_vortex
Bradshaw C.J.A.
(2008). The extinction vortex.
Retrieved from https://conservationbytes.com/2008/08/25/the-extinction-vortex/
Rhett B. (2012). The extinction vortex. Retrieved from http://rainforests.mongabay.com/09vortex.htm
Frankel O.H &
Michael E. S (1981). Conservation and
evolution: Cambridge university press, Cambridge.
