Minimum viable population
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Minimum viable population (MVP) is a lower bound on the population of a species, such that it can survive in the wild. This term is commonly used in the fields of biology, ecology, and conservation biology. MVP refers to the smallest possible size at which a biological population can exist without facing extinction from natural disasters or demographic, environmental, or genetic stochasticity.[1] The term "population" is defined as a group of interbreeding individuals in similar geographic area that undergo negligible gene flow with other groups of the species[2]. Typically, MVP is used to refer to a wild population, but can also be used for ex-situ conservation (Zoo populations).
Contents
EstimationEdit
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The definition for what constitutes a sufficient numbers for preservation varies between a minimum viable population models, as no population can be guaranteed survival in a stochastic environment, and as such there may be many calculated MVPs for a given species[3]. Some common perimeters for success include estimating the population size necessary to ensure between 99 percent probability of survival 1,000 years into the future, or 95 percent probability of survival over several centuries[4]. However, some models use generations as a unit of time rather than years to maintain consistency across taxa[5] The MVP can be estimated using computer simulations for population viability analyses (PVA). These analyses model populations using demographic and environmental information to project future population dynamics. The probability assigned to a PVA is arrived at after repeating the environmental simulation thousands of times.
ExtinctionEdit
Small populations are at a greater risk of extinction than larger populations due to small populations having less capacity to recover from stochastic events, which may be divided into four sources:[6]
Demographic stochasticityEdit
- Demographic stochasticity is often only a driving force toward extinction in populations with fewer than 50 individuals. Random events influence the fecundity and survival of individuals in a population, and in larger populations these events tend to be stabilized toward a steady growth rate. However, in small populations there is much more variance, which can in turn cause extinction[6].
Environmental stochasticityEdit
- Small, random changes in the abiotic and biotic components of the ecosystem that a population inhabits fall under environmental stochasticity. Examples of environmental stochasticity may include changes in climate over time, or arrival of another species that competes for resources. Unlike the other three sources of extinction, environmental stochasticity tends to effect populations of all sizes[6].
Natural catastrophesEdit
- An extension of environmental stochasticy, natural disasters are random, large scale events such as blizzards, droughts, storms, or fires that reduce a population directly over a short period of time. Natural catastrophes are the hardest events to predict, and MVP models often have difficulty factoring these in[6].
Genetic stochasticityEdit
- Small populations are vulnerable to genetic stochasticity, the random change in allele frequencies over time, also known as genetic drift. Genetic drift can cause alleles to disappear from a population, and this lowers genetic diversity. In small populations, low genetic diversity can increase rates of inbreeding which can result in inbreeding depression, where a population made up of genetically similar individuals loses fitness. Inbreeding in a population reduced fitness by causing deleterious recessive alleles to become more common in the population, and also by reducing adaptive potential. The so-called "50/500 rule", where a population needs 50 individuals to prevent inbreeding depression, and 500 individuals to guard against genetic drift at-large, is an oft-used benchmark for a MVP, but recent study suggests that this guideline falls short to cover a wide diversity of taxa[4][6].
ApplicationEdit
MVP does not take external intervention into account. Thus, it is useful for conservation managers and environmentalists; a population may be increased above the MVP using a captive breeding program, or by bringing other members of the species in from other reserves.
There is naturally some debate on the accuracy of PVAs, since a wide variety of assumptions generally are required for future forecasting; however, the important consideration is not absolute accuracy, but promulgation of the concept that each species indeed has an MVP, which at least can be approximated for the sake of conservation biology and Biodiversity Action Plans.[6]
There is a marked trend for insularity, surviving genetic bottlenecks and r-strategy to allow far lower MVPs than average. Conversely, taxa easily affected by inbreeding depression – having high MVPs – are often decidedly K-strategists, with low population densities while occurring over a wide range. An MVP of 500 to 1,000 has often been given as an average for terrestrial vertebrates when inbreeding or genetic variability is ignored.[7][8] When inbreeding effects are included, estimates of MVP for many species are in the thousands. Based on a meta-analysis of reported values in the literature for many species, Traill et al. reported concerning vertebrates "a cross-species frequency distribution of MVP with a median of 4169 individuals (95% CI = 3577–5129)."[9]
See alsoEdit
ReferencesEdit
- ^ Holsinger, Kent (2007-09-04). "Types of Stochastic Threats". EEB310: Conservation Biology. University of Connecticut. Archived from the original on 2008-11-20. Retrieved 2007-11-04.
- ^ "population | Definition of population in English by Oxford Dictionaries". Oxford Dictionaries | English. Retrieved 2019-06-08.
- ^ Shaffer, Mark L. (1981-02-01). "Minimum Population Sizes for Species Conservation". BioScience. 31 (2): 131–134. doi:10.2307/1308256. ISSN 0006-3568.
- ^ a b Frankham, Richard; Bradshaw, Corey J. A.; Brook, Barry W. (2014-02-01). "Genetics in conservation management: Revised recommendations for the 50/500 rules, Red List criteria and population viability analyses". Biological Conservation. 170: 56–63. doi:10.1016/j.biocon.2013.12.036. ISSN 0006-3207.
- ^ O’Grady, Julian J.; Brook, Barry W.; Reed, David H.; Ballou, Jonathan D.; Tonkyn, David W.; Frankham, Richard (2006-11-01). "Realistic levels of inbreeding depression strongly affect extinction risk in wild populations". Biological Conservation. 133 (1): 42–51. doi:10.1016/j.biocon.2006.05.016. ISSN 0006-3207.
- ^ a b c d e f Shaffer ML (1981). "Minimum population sizes for species conservation". BioScience. 31 (2): 131–134. doi:10.2307/1308256. JSTOR 1308256.
- ^ Lehmkuhl J (1984). "Determining size and dispersion of minimum viable populations for land management planning and species conservation". Environmental Management. 8 (2): 167–176. doi:10.1007/BF01866938.
- ^ Thomas CD (1990). "What do real population dynamics tell us about minimum viable population sizes?". Conservation Biology. 4 (3): 324–327. doi:10.1111/j.1523-1739.1990.tb00295.x.
- ^ Traill, Lochran W.; Bradshaw, Corey J.A.; Brook, Barry W. (2007). "Minimum viable population size: A meta-analysis of 30 years of published estimates". Biological Conservation. 139: 159–166. doi:10.1016/j.biocon.2007.06.011.