Environment Magazine

To Corridor, Or Not to Corridor: Size is the Question

Posted on the 24 April 2012 by Bradshaw @conservbytes

To corridor, or not to corridor: size is the questionI’ve just read a really interesting post by David Pannell from the University of Western Australia discussing the benefits (or lack thereof) of wildlife ‘corridors’. I’d like to elaborate on a few key issues, and introduce the most important aspect that really hasn’t been mentioned.

Some of you might be aware that the Australian Commonwealth Government has just released its Draft National Wildlife Corridors Plan for public comment, but many of you might not really know what a ‘corridor’ constitutes.

Wildlife or biodiversity ‘corridors’ have been around for a long time, at least in terms of proposals. The idea is fairly simple to conceive, but very difficult to implement in practice.

At least for as long as I’ve been in the conservation biology biz, ‘corridors’ have been proffered as one really good way to make broad-scale landscape restoration plausible and effective for (mainly) forest-dwelling species which have copped the worst of deforestation trends around Australia and the world. The idea is that because of intense habitat fragmentation, isolated patches of primary (or at least, reasonably intact secondary) forest can be linked by planting some sort of long corridor of similar habitat between them. Then, all the little creatures can merrily make their way back and forth between the patches, thus rescuing each other from extinction via migration.

This appeals to a lot of people because it doesn’t necessarily require vast tracks of public or private land (makes the farmers happy, makes the urban sprawlers happy, keeps the greenies happy because it looks good for conservation, keeps the politicians happy because they don’t have to wade in and make unpopular decisions). Or at least, that’s the idea.

As it turns out, ‘connectivity’ per se between habitat patches is probably not quite as important as we once believed for population persistence. I’ve blogged before about several papers that have over-turned the ‘connectivity paradigm’ through experimental manipulation of microcosms (mini ecosystems), and coincidentally, found another by Mike Bull & colleagues that just came out today in Austral Ecology that roadside vegetation corridors did little to explain variation in reptile abundance.

Why might this be the case? Many, many papers (including many of my own) now identify that population size and habitat quality are far more important for population persistence than connectivity per se. While we have also identified that isolated and small populations are more temporally variable (and hence, more prone to extinction), it seems like the evidence is mounting that just ensuring some degree of connectivity doesn’t really do what many think it should do in terms of reducing extinction risk.

So, what are the alternatives? Well, I still think connectivity needs to be endorsed to reduce overall fragmentation, but it must be done smartly and guided by the evidence-based principles of extinction biology. As mentioned above, the most important driver of extinction risk is population size, and as overall habitat are increased, so too does population size. Thus, a thin thread of a corridor attaching fragments is insufficient – we should view these ‘corridors’ instead as patches themselves, with the aim to maximise their size (i.e., total area) instead of their connectivity enhancement per se. Think a big, fat chunk of restored habitat rather than a thin line of vegetation.

I’ve more or less indicated this to the Corridors Plan people in my submission, and I had the opportunity to meet with the committee a few weeks ago in Adelaide to reiterate my recommendations. Despite all the uncertainty about the true conservation effectiveness of wildlife corridors, we do know for certain that more area is better. Let’s at least follow this one principle.

CJA Bradshaw

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