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Classics: Minimum Viable Population size

21 08 2008

‘Classics’ is a category of posts highlighting research that has made a real difference to biodiversity conservation. All posts in this category will be permanently displayed on the Classics page of ConservationBytes.com

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Shaffer, M.L. (1981). Minimum population sizes for species conservation. BioScience 31, 131–134

Small and isolated populations are particularly vulnerable to extinction through random variation in birth and death rates, variation in resource or habitat availability, predation, competitive interactions and single-event catastrophes, and inbreeding. Enter the concept of the Minimum Viable Population (MVP) size, which was originally defined as the smallest number of individuals required for an isolated population to persist (at some predefined ‘high’ probability) for some ‘long’ time into the future. In other words, the MVP size is the number of individuals in the population that is needed to withstand normal (expected) variation in all the things that affect individual persistence through time. Drop below your MVP size, and suddenly your population’s risk of extinction sky-rockets. In some ways, MVP size can be considered the threshold dividing the ‘small’ and ‘declining’ population paradigms (see Caughley 1994), so that different management strategies can be applied to populations depending on their relative distance to (population-specific) MVP size.

This wonderfully simply, yet fundamental concept of extinction dynamics provides the target for species recovery, minimum reserve size and sustainable harvest if calculated correctly. Indeed, it is a concept underlying threatened species lists worldwide, including the most well-known (IUCN Red List of Threatened Species). While there are a host of methods issues, genetic considerations and policy implementation problems, Shaffer’s original paper spawned an entire generation of research and mathematical techniques in conservation biology, and set the stage for tangible, mathematically based conservation targets.

Want more information? We have published some papers and articles on the subject that elaborate more on the methods, expected ranges, subtleties and implications of the MVP concept that you can access below.

Traill, LW, CJA Bradshaw, BW Brook. 2007. Minimum viable population size. In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment)
Traill, LW, CJA Bradshaw, BW Brook. 2007. Minimum viable population size: a meta-analysis of 30 years of published estimates. Biological Conservation 139: 159-166
Brook, BW, LW Traill, CJA Bradshaw. 2006. Minimum viable population size and global extinction risk are unrelated. Ecology Letters 9: 375-382

CJA Bradshaw

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Tags: biodiversity, conservation, science
Categories : biodiversity, conservation, conservation biology, extinction, extinction vortex, inbreeding depression, minimum viable population, mvp, population dynamics

Captive breeding for conservation

7 08 2008

My first attempt at this potentially rather controversial section of ConservationBytes.com. Inspired by my latest post (30/07/2008), I must comment on what I believe is one of the biggest wasters of finite conservation (financial) resources – captive breeding for population recovery. The first laureate of the Toothless category goes to 7 authors (Snyder et al.) who I believe deserve at least a round of beers for their bold paper published way back in 1996 in Conservation Biology – Limitations of captive breeding in endangered species recovery.

The paper describes basically that in most situations, captive breeding for population recovery is ill-conceived, badly planned, overly expensive and done without any notion of the particular species’ minimum viable population size (the population size required to provide a high probability of persistence over a long period). Examples of ridiculous cloning experiments done in the name of ‘conservation’ (one example with which I am familiar is the case of the SE Asian banteng cloning experiment – these conservation-challenged scientists actually claimed “We hope that the birth of these animals will open the way for a new strategy to help maintain valuable biodiversity and to respond to the challenge of large-scale extinctions ahead.” after spending amounts that would make Bill Gates blush). Come on! Minimum viable population sizes number in the thousands to tens of thousands (e.g., Brook et al. 2006; Traill et al. 2007), not to mention the genetic diversity necessary for persistence captive populations generally lack (see Frankham et al. 2004).

In the spirit of ecological triage, we must focus on conservation efforts that have a high probability of changing the extinction risk of species. Wasting millions of dollars to save a handful of inbred individuals (insert your favourite example here) WILL NOT, in most cases, make any difference to population viability (with only a few exceptions). Good on Snyder et al. (1996) for their analysis and conclusions, but zoos, laboratories and other captive-rearing organisations around the world continue to throw away millions using the ‘conservation’ rationale to justify their actions. Rubbish. I’m afraid there is little evidence that the Snyder et al. paper changed anything. (post original published in Toothless 31/07/2008).

CJA Bradshaw