Paper Of The Week: Habel et al. 2015. Fragmentation genetics of the grassland butterfly Polyommatus coridon: Stable genetic diversity or extinction debt?

27 May

Back when I was an undergraduate, I was never that interested in genetic analysis: it seemed that it was all too much about the molecules and not enough about the biology. Worse, as someone who was very much a field ecologist even then, it seemed to be carried out by a bunch of pipette pushers who rarely ventured beyond their list of four alternating letters that make up the genetic code of all living things, let alone outside their laboratories. In short, while I could understand its importance for things like taxonomy and cladistics, I found it hard to see how it could be useful for doing the types of research I wanted to do.

This all changed, however, with the introduction of landscape genetics. What is landscape genetics? Well, most simply put, it’s taking the results of genetic analyses and laying them over the spatio-temporal environment from which the samples were taken. By doing this, you can start looking at not just how animals move through their environments, but also how their genes move through it. Suddenly, all that talk of minimum viable populations, genetic bottlenecks and gene flow takes on a spatial perspective, and by including information about the landscapes alongside the genetic information we can start to understand the processes of extinction and speciation in a way that we never have before.

This brings me round to my choice of paper of the week for this week.  It’s Habel et al. 2015. Fragmentation genetics of the grassland butterfly Polyommatus coridon: Stable genetic diversity or extinction debt? Why have I selected this paper? It’s because of the way it combines genetic analyses and spatial information to produce a result that, in my opinion, is greater than the sum of each individual part.

The starting point for Habel et al. (2015) is the general assumption that habitat fragmentation affects the viability of populations and so has a pivotal role to play in conservation. Given our current understanding, this should be particularly true for habitat specialists that are more likely to have both a naturally patchy distribution, and to be impacted by fragmentation. All this means, given conventional wisdom, that habitat fragmentation should lead a fragmentation of genetic populations, a reduction in local genetic diversity, and a resulting increased risk of extinction.

So far, so straight-forward, but then we get to the results of this particular study: when the authors looked at a specialist grassland butterfly, they found that genetic diversity wasn’t linked with habitat size, habitat
connectivity, or census population size. Thus, they found something very unexpected, given our current understanding of things.

How do they go about trying to explain this? The authors come up with two solutions. The first is that despite the fragmentation between populations and the specialisation in terms of habitat preferences, the species is somehow managing to keep up the levels of gene flow expected of more widespread  generalist species. For a mobile species like a butterfly, this is possible, but if true, it would suggest that dispersion and gene flow are not as limited by the surrounding landscape as we might assume, even for habitat specialists.

The second possibility is perhaps more interesting. This is that there is lag between the fragmentation of a species with a relatively continuous historic distribution into small isolated populations and the loss of genetic diversity. If this is true, then it would suggest that we could easily overlook the negative effects of fragmentation on population viability because we might mistake the effects of historic gene flow for evidence of current gene flow even in the light of habitat fragmentation. Thus, we may make false assumptions about the impacts of specific human activities, such as habitat destruction, on the long term viability of individual species and populations, and that would not be good.

As you will have guessed from the start of this post, I am no geneticist, so I cannot say which of these is more likely to be correct, but simply the possibility that the second option could be true means that measuring the impacts of things like habitat fragmentation on population viability might be much more difficult to do in anything close to real time than we might otherwise assume. And without the further integration of GIS into genetic studies, we will have no way test which, if either, is correct. Thus, because of landscape genetics, GIS has the potential to be as important to genetic studies as it is to many other areas of ecology.

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Dr Colin D. MacLeod,
Founder, GIS In Ecology

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