Conservation Science Blog

New research relevant to conservation in western North America

The Conservation Science Blog is intended to bring new and relevant research to the attention of conservation scientists, and facilitate discussion on how to apply this science to further conservation goals in western North America.

Is there a “rule of thumb” for minimum viable populations?

A new review by Curtis Flather and colleagues in the journal Trends in Ecology and Evolution questions the claim that there is a ‘magic number’ applicable across diverse taxa for setting a “minimum viable population” threshold below which a species is at high risk of extinction. The review has received some coverage in the media as implying that ambitious goals for recovery of endangered species are not supported by science. As an example, the media article cited “a coalition of environmental groups” who claimed that “to survive and thrive…the [wolf] population needed at least 2,000 and preferably 5,000 wolves.” But this conclusion misses the primary focus of Flather et al.’s review.

The review (available here) is a helpful antidote to previous studies that advanced claims for the existence of such a general MVP rule of thumb across all species. To be fair, it was again not the studies themselves (for example, Traill et al. (2009)), but the media coverage about them that focused on the ‘magic number’. The broader conclusion from Traill et al. (2009) and related papers was that a comprehensive consideration of threat factors such as genetic risk will often result in conclusions that populations numbering in the thousands of individuals are required for viability. In contrast, Flather et al. (2011) seem to downplay the relevance of genetic risks, stating for example that “the 50/500 values of Ne [genetic effective population size] are simply viability goals for maintaining genetically diverse populations; they provide little direct connection with extinction risk.” Although the 50/500 rule, like any rule of thumb, has limitations, our increasing understanding of conservation genetics has underlined the importance of the processes such as mutation-selection balance that the 50/500 rule attempts to address. The media coverage of Flather et al.’s review seems to conclude that if rules of thumb don’t hold, then small populations are out of danger.

Especially for species such as wolves, where we do have the data to rigorously conduct a MVP (or population viability (PVA)) analysis, there is no need to depend on ‘rules of thumb’. Along with a group of colleagues, I am currently conducting a PVA for Mexican wolves, in order to assess how extinction risks may vary with the size of the wolf metapopulation. No such analysis informed development of recovery goals for Northern Rocky Mountain wolves, and this omission has helped fuel the debate over delisting in that region.

Of course, no matter how rigorous a PVA is used, any decision about recovery goals involves ‘normative’ decisions by society as to what risk is acceptable. The authors of the Flather et al. review correctly make this point in the media coverage, stating “we don’t have a single standard that’s been set for what degree of risk we’re willing to accept for a species to go extinct,”…”I could make a calculation for a species and say nine times out of 10, it would be viable there, for 50 years. Would that be good enough, or would you want a 95 percent chance, or an 80 percent chance?”

However, this quote is troubling in that it proposes a range of risk thresholds that, if applied generally, would translate to extinction of a large number of species within 50 years. This contrasts with the threshold suggested by Mark Shaffer, who conducted the first MVP/PVA in 1981 for grizzly bears. Shaffer proposed a threshold of 99% persistence for 1000 years, orders of magnitude more conservative that that given above. Research from Australia on how the public considers these issues suggests that people poorly understand the implications of risk estimates expressed as x% over x years, as opposed to estimates of total species loss.

As described in an earlier post, Barnosky et al. (2011) recently calculated that “if all ‘threatened’ species became extinct within a century, and that rate then continued unabated, terrestrial amphibian, bird and mammal extinction would reach Big Five [geologic extinction episodes] magnitudes in 240 to 540 years (241.7 years for amphibians, 536.6 years for birds, 334.4 years for mammals).”

At the end of the day, the need for ambitious recovery goals becomes more rather than less evident as we learn more about the processes that maintain diversity and adaptive potential in the world’s species. But, at least in developed countries where the data that a PVA requires is available for many species, rules of thumb are unlikely to be good enough to set recovery goals when species recovery conflicts with economics or other social values.