In keeping with the theme of extinctions from my last post, I want to highlight a paper we’ve recently had published online early in Ecology entitled Limited evidence for the demographic Allee effect from numerous species across taxa by Stephen Gregory and colleagues. This one is all about Allee effects – well, it’s all about how difficult it is to find them!

Now, the evidence for component Allee effects abounds, but finding real instances of reduced population growth rate at low population sizes is difficult. And this is really what we should be focussing on in conservation biology – a lower-than-expected growth rate at low population sizes means that recovery efforts for rare and endangered species must be stepped up considerably because their rebound potential is lower than it should be.

We therefore queried over 1000 time series of abundance from many different species and lo and behold, the evidence for that little dip in population growth rate at low densities was indeed rare – about 1 % of all time series examined!

I suppose this isn’t that surprising, but what was interesting was that this didn’t depend on sample size (time series where Allee models had highest support were in fact shorter) or variability (they were also less variable). All this seems a little counter-intuitive, but it gels with what’s been assumed or hypothesised before. Measurement error, climate variability and the sheer paucity of low-abundance time series makes their detection difficult. Nonetheless, for those series showing demographic Allee effects, their relative model support was around 12%, suggesting that such density feedback might influence the population growth rate of just over 1 in 10 natural populations. In fact, the many problems with density feedback detections in time series that load toward negative feedback (sometimes spuriously) suggest that even our small sample of Allee time series are probably vastly underestimated. We have pretty firm evidence that inbreeding is prevalent in threatened species, and demographic Allee effects are the mechanism by which such depression can lead a population down the extinction vortex.

CJA Bradshaw

Gregory, S., Bradshaw, C.J.A., Brook, B.W., & Courchamp, F. (2009). Limited evidence for the demographic Allee effect from numerous species across taxa Ecology DOI: 10.1890/09-1128