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In April this year, some American colleagues of ours wrote a rather detailed, 10-page article in Trends in Ecology and Evolution that attacked our concept of generalizing minimum viable population (MVP) size estimates among species. Steve Beissinger of the University of California at Berkeley, one of the paper’s co-authors, has been a particularly vocal adversary of some of the applications of population viability analysis and its child, MVP size, for many years. While there was some interesting points raised in their review, their arguments largely lacked any real punch, and they essentially ended up agreeing with us.

Let me explain. Today, our response to that critique was published online in the same journal: Minimum viable population size: not magic, but necessary. I want to take some time here to summarise the main points of contention and our rebuttal.

But first, let’s recap what we have been arguing all along in several papers over the last few years (i.e., Brook et al. 2006; Traill et al. 2007, 2010; Clements et al. 2011) – a minimum viable population size is the point at which a declining population becomes a small population (sensu Caughley 1994). In other words, it’s the point at which a population becomes susceptible to random (stochastic) events that wouldn’t otherwise matter for a small population.

Consider the great auk (Pinguinus impennis), a formerly widespread and abundant North Atlantic species that was reduced by intensive hunting throughout its range. How did it eventually go extinct? The last remaining population blew up in a volcanic explosion off the coast of Iceland (Halliday 1978). Had the population been large, the small dent in the population due to the loss of those individuals would have been irrelevant.

But what is ‘large’? The empirical evidence, as we’ve pointed out time and time again, is that large = thousands, not hundreds, of individuals.

So this is why we advocate that conservation targets should aim to keep at or recover to the thousands mark. Less than that, and you’re playing Russian roulette with a species’ existence.

Back to the critique. While the title of several high-profile media outlets used the term ‘magic number‘, we never used the term or supported the idea of a ‘one-number-fits-all’ approach. MVP does, of course, vary among species, and very possibly even more among major taxonomic groups (but the data are wanting). The problem is that there are no really good ways to predict MVP from first principles (e.g., life history traits); rather, one must rely on comprehensive population viability analyses based on detailed demographic and other data that, unfortunately, simply aren’t available for most species on the planet.

Yes, we did find some remarkable consistency among species for the magnitude of MVP size, and some taxonomic differences (remember, we controlled for mass differences by standardising MVP sizes to 40 generations), but the fact remains there is always unexplained variation. However, throwing the MVP baby out with the conservation bathwater is just plain silly. You will NEVER get adequate data to develop PVAs for all species of conservation of concern, which means generalisations, or rules of thumb, are a conservation necessity.

And this is the slightly amusing part – the Flather critique essentially agrees with us; they state:

“We also suspect. . .that multiple populations totalling thousands (not hundreds) of individuals will be needed to ensure long-term persistence”

This is pretty much what we’ve been emphasising all along. You need thousands (and ~ 5000 [note the ~] is around the median mark), not hundreds, to reduce the probability of stochastic extinctions to acceptably low values. But where Flather and colleagues fall down is that they provide no viable alternative to our suggested approach.

The critique also completely glosses over the effects of genetic erosion – when few individuals are left in a population, inbreeding suppresses vital rates and reduces the population’s potential for recovery following disturbance. And yes, there’s plenty of evidence for this. It’s also interesting that genetic data suggest too that thousands, not hundreds, should be the target range to avoid inbreeding depression – an entirely independent line of evidence that supports the demographic analyses.

So I don’t really get what their critique is all about, apart from some elaborate and long-winded way of saying ‘things vary’. Yes, yes they do. But that shouldn’t stop us from developing generalisations. Now, just in case you’ve heard through the grapevine that our follow-up piece on the Species Ability to Forestall Extinction (SAFE) index has triggered the hounds to bay for blood, stay tuned shortly for our response to a series of related critiques in Frontiers in Ecology and the Environment.

CJA Bradshaw

References

• BROOK, BW, LW TRAILL, CJA BRADSHAW. 2006. Minimum viable population size and global extinction risk are unrelated. Ecology Letters 9: 375-382. doi:10.1111/j.1461-0248.2006.00883.x
• BROOK, BW, CJA BRADSHAW, LW TRAILL, R FRANKHAM. 2011. Minimum viable population size: not magic, but necessary. Trends in Ecology and Evolution doi:10.1016/j.tree.2011.09.006
• CLEMENTS, GR, CJA BRADSHAW, BW BROOK, WF LAURANCE. 2011. The SAFE index: using a threshold population target to measure relative species threat. Frontiers in Ecology and the Environment doi:10.1890/100177
• FLATHER CH, HAYWARD GD, BEISSINGER SR, STEPHENS PA. 2011. Minimum viable populations: is there a “magic number” for conservation practitioners?6. Trends in Ecology and Evolution 26: 307-316
• HALLIDAY, T. 1978. Vanishing Birds: Their Natural History and Conservation. 1st edn. Holt, Rinehart and Winston, New York
• 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. doi:10.1016/j.biocon.2007.06.011
• TRAILL, LW, BW BROOK, R FRANKHAM, CJA BRADSHAW. 2010. Pragmatic population viability targets in a rapidly changing world. Biological Conservation 143: 28-34. doi:10.1016/j.biocon.2009.09.001