Some of you who are familiar with my colleagues’ and my work will know that we have been investigating the minimum viable population size concept for years (see references at the end of this post). Little did I know when I started this line of scientific inquiry that it would end up creating more than a few adversaries.
It might be a philosophical perspective that people adopt when refusing to believe that there is any such thing as a ‘minimum’ number of individuals in a population required to guarantee a high (i.e., almost assured) probability of persistence. I’m not sure. For whatever reason though, there have been some fierce opponents to the concept, or any application of it.
Yet a sizeable chunk of quantitative conservation ecology develops – in various forms – population viability analyses to estimate the probability that a population (or entire species) will go extinct. When the probability is unacceptably high, then various management approaches can be employed (and modelled) to improve the population’s fate. The flip side of such an analysis is, of course, seeing at what population size the probability of extinction becomes negligible.
‘Negligible’ is a subjective term in itself, just like the word ‘very‘ can mean different things to different people. This is why we looked into standardising the criteria for ‘negligible’ for minimum viable population sizes, almost exactly what the near universally accepted IUCN Red List attempts to do with its various (categorical) extinction risk categories.
But most reasonable people are likely to agree that < 1 % chance of going extinct over many generations (40, in the case of our suggestion) is an acceptable target. I’d feel pretty safe personally if my own family’s probability of surviving was > 99 % over the next 40 generations.
Some people, however, baulk at the notion of making generalisations in ecology (funny – I was always under the impression that was exactly what we were supposed to be doing as scientists – finding how things worked in most situations, such that the mechanisms become clearer and clearer – call me a dreamer).
So when we were attacked in several high-profile journals, it came as something of a surprise. The latest lashing came in the form of a Trends in Ecology and Evolution article. We wrote a (necessarily short) response to that article, identifying its inaccuracies and contradictions, but we were unable to expand completely on the inadequacies of that article. However, I’m happy to say that now we have, and we have expanded our commentary on that paper into a broader review.
Led by prominent conservation geneticist, Professor Richard Frankham, and including my long-time partner in scientific crime, Professor Barry Brook, we’ve just published a comprehensive review of the ’50/500′ rule that’s been around since Franklin’s and Soulé’s papers in 1980.
Before I get into the details, I have to begin by saying what a truly pleasant and humbling experience it was to work with Dick. He really does … [Note: Dick objected to this sentence, so I have removed it at his request]. Not only is Dick’s knowledge impressive, he’s also one of the nicest blokes going. What an amazing combination of traits. I hope to be like him when I grow up.
Back to the detail. As mentioned, the so-called ’50/500′ rule has been around for over 30 years and persists as a general management guideline in nearly all small-population management circles. Basically the rule states that to avoid inbreeding depression (i.e., loss of ‘fitness’ due to genetic problems), an effective population size (Ne) of at least 50 individuals in a population is required. To avoid eroding evolutionary potential (the ability of a population to evolve to cope with environmental changes), an Ne of at least 500 is required.
The key here is that little qualifier effective. Ne is the number of individuals that would result in the same loss of genetic diversity, inbreeding, or genetic drift if they behaved in the manner of an idealised population. Great, you say? More like, ‘what the hell does that mean?’
Well, an ‘idealised’ population is just that – it’s not a real thing. In a perfect world, a breeding pair would be completely unrelated such that they had no chance of producing offspring with any genetic defects due to each parent donating no deleterious alleles to any particular locus. Of course, real populations rarely behave like this, so some pairs have a certain amount of ‘relatedness’. You know what happens – as the population gets smaller, the chance of breeding with a relative increases, and you get inbreeding.
It turns out that based on Dick’s own work and later papers by others, the ‘average’ ratio of the effective population to the census population (Nc, the number of individuals counted in a population – usually just the adults) size is about 0.1 to 0.2. In other words, for every 5 to 10 individuals counted in the population, on average there is only 1 ‘effective’ individual (genetically speaking).
So let’s do the maths. Ne = 50 will, on average, mean Nc = 250 to 500, and Ne = 500 means Nc = 2500 – 5000. Sound familiar? In fact, about 5000 is what our meta-analysis of demographic (i.e., census population) minimum viable population size suggested.
Yes, we’ve heard the arguments before – it’s not always an Ne:Nc between 0.1 and 0.2, and not all populations need 5000+ to ‘survive’. But that’s not what we’re saying at all – without rather difficult-to-measure estimate of the true Ne:Nc for a particular population, one should in fact default to the average situation to be safe.
But when you look at the genetic arguments alone, the 50/500 rule starts to break down. As a fundamental assumption in many of the IUCN Red List Criteria, getting the rule ‘right’ is incredibly important.
As our review points out – with extensive evidence and well-supported arguments – 50 is in fact too low to ensure no inbreeding depression for the majority of species that have been investigated. In fact, Ne ≥ 100 (i.e., Nc ≥ 500 to 1000) is closer to the real minimum. Similarly, Ne = 500 won’t necessarily ensure a population maintains its evolutionary potential in perpetuity; it too should be doubled to Ne ≥ 1000 (Nc ≥ 5000 to 10000).
This means of course that for some species, the Red List categories would have to change – specifically, those classed under Criterion C. More importantly, it means that if you’re not shooting for population sizes in the 1000s (preferably the high 1000s), then you are inadvertently (or intentionally) managing for extinction.
I can’t say I really buy the argument that we shouldn’t say these sorts of things because some species will never get to those sizes. Get used to it – extinctions are happening and we’ll have to get clever about where we best spend our conservation dollars.
I relish the ensuing commentary.
- Frankham R, CJA Bradshaw, BW Brook. 2014. Genetics in conservation management: revised recommendations for the 50/500 rules, Red List criteria and population viability analyses. Biological Conservation 170: 53-63. doi:10.1016/j.biocon.2013.12.036
- Frankham, R, BW Brook, CJA Bradshaw, LW Traill, D Spielman. 2013. 50/500 rule and minimum viable populations: response to Jamieson and Allendorf. Trends in Ecology and Evolution 28: 187-188. doi:10.1016/j.tree.2013.01.002
- Bradshaw, CJA, Clements, GR, WF Laurance, BW Brook. 2011. Better SAFE than sorry. Frontiers in Ecology and the Environment 9: 487-488. doi:10.1890/11.WB.028
- Brook, BW, CJA Bradshaw, LW Traill, R Frankham. 2011. Minimum viable population size: not magic, but necessary. Trends in Ecology and Evolution 26: 619-620. 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 9: 521-525. doi:10.1890/100177
- 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
- Field, IC, MG Meekan, RC Buckworth, CJA Bradshaw. 2009. Susceptibility of sharks, rays and chimaeras to global extinction. Advances in Marine Biology 56: 275-363. doi:10.1016/S0065-2881(09)56004-X
- 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, CJA Bradshaw, BW Brook (Authors); Mark McGinley (Topic Editor). 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)
- 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