Extinction vortices are a class of models through which

conservation biologists Conservation biology is the study of the conservation of nature and of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction and the erosion of biotic interactions. It is an in ...

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 study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...

s can understand the dynamics of and categorize

extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed 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 A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scale f ...

factors that have an effect on the

ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...

community A community is a social unit (a group of living things) with commonality such as place, norms, religion, values, customs, or identity. Communities may share a sense of place situated in a given geographical area (e.g. a country, village, ...

level, such as disturbance,

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

habitat loss Habitat destruction (also termed habitat loss and habitat reduction) is the process by which a natural habitat becomes incapable of supporting its native species. The organisms that previously inhabited the site are displaced or dead, thereby ...

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

inbreeding depression Inbreeding depression is the reduced biological fitness which has the potential to result from inbreeding (the breeding of related individuals). Biological fitness refers to an organism's ability to survive and perpetuate its genetic material. ...

and

outbreeding depression In biology, outbreeding depression happens when crosses between two genetically distant groups or populations result in a reduction of fitness. This is particularly likely if the subspecies have different habitats or if no genetic exchange has occ ...

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

etc.

Types of vortices

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

population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...

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 The sex ratio (or gender ratio) is usually defined as 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. Many species d ...

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 which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in th ...

, 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. Mo ...

, 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 Genetic load is the difference between the fitness of an average genotype in a population and the fitness of some reference genotype, which may be either the best present in a population, or may be the theoretically optimal genotype. The average i ...

, 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 English biologist and statistician Ronald Fisher in his fundamental theorem of natural selection. In his 1930 book ''The Genetical Theory of Natural Selection'', Fisher postulates that the rate of ch ...

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 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, 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

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