Extinction vortices are a class of models through which conservation biologists,

geneticists A geneticist is a biologist or physician who studies genetics, the science of genes, heredity, and variation of organisms. A geneticist can be employed as a scientist or a lecturer. Geneticists may perform general research on genetic processes ...

and

ecologist Ecology () is the natural science of the relationships among living organisms and their environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere levels. Ecology overlaps with the closely re ...

s can understand the dynamics of and categorize

extinction Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

s in the context of their causes. This model shows the events that ultimately lead small populations to become increasingly vulnerable as they spiral toward extinction. Developed by M. E. Gilpin and M. E. Soulé in 1986, 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 An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...

community A community is a social unit (a group of people) with a shared socially-significant characteristic, such as place, set of norms, culture, religion, values, customs, or identity. Communities may share a sense of place situated in a given g ...

level, such as disturbance,

pollution Pollution is the introduction of contaminants into the natural environment that cause harm. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the component ...

habitat loss Habitat destruction (also termed habitat loss or habitat reduction) occurs when a natural habitat is no longer able to support its native species. The organisms once living there have either moved elsewhere, or are dead, leading to a decrease ...

etc. Whereas the second two (F and A) deal with genetic factors such as

inbreeding depression Inbreeding depression is the reduced biological fitness caused by loss of genetic diversity as a consequence of inbreeding, the breeding of individuals closely related genetically. This loss of genetic diversity results from small population siz ...

and

outbreeding depression In biology, outbreeding depression happens when crosses between two genetically distant groups or populations result in a reduction of fitness. The concept is in contrast to inbreeding depression, although the two effects can occur simultaneously o ...

genetic drift Genetic drift, also known as random genetic drift, allelic drift or the Wright effect, is the change in the Allele frequency, frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene va ...

etc.

Types of vortices

*R Vortex: The R vortex is initiated when there is a disturbance which facilitates a lowering of

population Population is a set of humans or other organisms in a given region or area. Governments conduct a census to quantify the resident population size within a given jurisdiction. The term is also applied to non-human animals, microorganisms, and pl ...

size (N) and a corresponding increase in variability (Var(r)). This event can make populations vulnerable to additional disturbances which will lead to further decreases in population size (N) and further increases in variability (Var(r)). A prime example of this would be the disruption of

sex ratio A sex ratio is the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. However, many species deviate from an even sex ratio, ei ...

s 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 and the population becomes "patchy" or fragmented. Within these fragments, local extinction rates increase which, through

positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop where the outcome of a process reinforces the inciting process to build momentum. As such, these forces can exacerbate the effects ...

, further increases D. *F Vortex: The F vortex is initiated by a decrease in population size (N) which leads to a decrease in

heterozygosity Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mos ...

, and therefore a decrease in genetic diversity. Decreased population size makes the effects of genetic drift more prominent, resulting in increased risk 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 the impact of genetic drift on the population, due to the population's decreased size. This corresponds with a decrease in

genetic variance Genetic variance is a concept outlined by the England, English biologist and statistics, statistician Ronald Fisher in his Fisher's fundamental theorem of natural selection, fundamental theorem of natural selection. In his 1930 book ''The Genetica ...

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.

Extinction vortex factors

Environmental factors

Many of the environmental events that contribute to an extinction vortex do so through reduction in population size. These events can include rapid loss of population size due to disease, natural disasters, and climate change. Habitat loss and/or habitat degradation can also kick start an extinction vortex. Other factors include events that occur more gradually, such over-harvesting (hunting, fishing, etc.), or excessive predation.

Genetic factors

Populations that succumb to an extinction vortex experience strong genetic factors that cause already small populations to decrease in size over time. All populations experience genetic drift, a random process that causes changes in the population genetic structure over time. Small populations are particularly vulnerable to rapid changes in population genetic structure due to the random nature of gamete sampling. When a population is small, any change in alleles can disproportionately impact the population. Thus, genetic drift leads small populations to lose genetic diversity. Additionally, when populations become small, inbreeding increases because individuals are more likely to mate with others with a genome that contains many of the same alleles. Inbreeding can lead to inbreeding depression within the population, and this can cause fewer offspring, more birth defects, more individuals prone to disease, decreased survival and reproduction (fitness), and decreased genetic diversity within the population. With a decrease in genetic diversity comes even greater likelihood of inbreeding and inbreeding depression. Another genetic factor that can lead small populations toward the spiral of extinction is limited gene flow. For example, if a population becomes isolated due to

habitat fragmentation Habitat fragmentation describes the emergence of discontinuities (fragmentation) in an organism's preferred environment (habitat), causing population fragmentation and ecosystem decay. Causes of habitat fragmentation include geological proces ...

, migration rates decrease or become non-existent, causing the population to lose genetic diversity over time and increasing inbreeding. Migration is important because new individuals from outside of the population will almost certainly add new genetic variation, which can increase overall fitness within the population. One example of the role of genetics in extinction occurs in the case of fragmented metapopulations of southern dunlins (Calidris alpine schinzii) in SW Sweden. These endangered shorebirds experienced inbreeding and loss of genetic diversity at two molecular markers examined, and this limited survival and reproduction throughout the population by increasing inbreeding. When parent dunlins with more similar genetics mated, their offspring had lower likelihood of hatching, and if they did manage to hatch, they were more likely to die soon after hatching.Blomqvist, D., Pauliny, A., Larsson, M., Flodin, L. "Trapped in the extinction vortex? Strong genetic effects in a declining vertebrate population". (2009) BMC Evolutionary Biology 10:33.

Demographic factors

Demographic factors that are involved in extinction vortices include reduced fecundity, changes in dispersal patterns, and decreased population density.

References

{{Extinction

Vortex In fluid dynamics, a vortex (: vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...