Trophy hunting; the demographic and phenotypic effects

kudu.africanskyhunting
Courtesy: Africanskyhunting

I’ve had an interest in trophy hunting since taking a gap year [after school] to work at the Sijarira Forest Area on the shores of Lake Kariba in Zimbabwe. It was a reserve set aside for trophy hunting – exclusively. What struck me about hunting at even that young age was that it was fairly low impact, viz. one camp, a small network of roads, one client at any one time, small labour force. The hunting itself was not low impact of course; animals got shot! And that is the thing – people pay money to kill animals. No surprise then that is deeply polarising. There is a lot of vitriol out there from the animal rights groups, and the pro-hunting lobby can be pretty unsavoury – but what does science tell us?

As a biologist, I think that nature is ‘red in tooth and claw’, and we’re lucky to have made it this far as hominids. That said and as a conservationist scientist, I think that hunting is useful principally because it directs money toward the conservation effort [see in particular Lindsey et al, 2007. Biological Conservation 134(4):455-469].

But – trophy hunting can be useful as long as it does not precipitate:

1] the localised decline and extirpation of a hunted species, 2] loss of genetic variance or inbreeding, 3] a shift in the targeted trait through evolutionary change, and, or 4] serious disruption to age-sex structure and animal behaviour.

Naturally, all of these negative impacts are inter-linked, but these are serious challenges to a conservation scientist. How, for example, do we determine and tease-apart demographic, plastic and evolutionary change in hunted populations? How do we know that these changes are driven by mortality through hunting, or rather through density-dependent and independent effects? Is hunting to blame for extirpation, or are other factors at play? Tough questions!

So far, science has informed us that unregulated trophy hunting can lead to population decline and even collapse, and loss of genetic variance – especially where the hunted populations are small and within closed areas [and pretty much all Protected Areas in Africa are closed]. Science has informed us that trophy hunting can have serious implications for age-sex structure in populations, particularly in big cats where infanticide will result from the displacement of a dominant male.

Science though has been a bit more vague on the demographic, plastic and evolutionary responses of targeted populations to trophy hunting. This is largely through a lack of data – which is likely why one of the very few published studies of evidence of directional selection, or rapid evolution through trophy hunting was that on Canadian bighorn sheep by Coltman et al [2003. Nature 426:655–658]. There is a lot of evidence of hunted and fished populations becoming smaller in size, or smaller horned in ungulates – but people always assume inheritance of the targeted trait.

While I was at Imperial College London, I became interested in using a recently developed two-sex integral projection model developed by the mathematical biologist, Susanne Schindler [ http://sschindlerblog.wordpress.com/2013/06/20/size-sex-and-squirrels/ ] to address the problem of demographic and evolutionary effects of hunting. We used the same long-term data on bighorn sheep that was published in the Nature paper mentioned above. I analysed the data, Susanne adapted her model, and Tim Coulson worked hard to make sure I didn’t write a load-of-rubbish. We could not use horn data, because lambs aren’t born with horns – but based our model on body mass, a trait strongly correlated with horn length. So once the model was parameterised, we could explore what the demographic and evolutionary effects of hunting were on this population

What we ended up funding has important implications: body mass is only weakly inherited from large males. So that means that no matter how many males are shot – any response to trophy hunting will be demographic – not evolutionary.

So now what? In many ways – nothing changes. Unregulated hunting will lead to smaller males in a population because there are fewer of them. So hunting still needs to be regulated. Here in Africa, or in Canada – it doesn’t matter. Shoot too many males and they will become smaller.

In the absence of data, we still think that hunting should attempt to emulate the natural mortality of senescent males [or at least be marginally additive]. Any other hunting strategy – for ungulates at least – will likely have a deleterious impact and a subsequent shift in targeted trait distribution.

See our paper here: http://www.pnas.org/content/early/2014/08/06/1407508111

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