Why some animals are shy of habitat corridors
Study shows how habitat shape is as important as size.
Corridors designed to connect animals in isolated and fragmented habitats are not necessarily the simple conservation solution they appear to be, suggests a new study.
Conservationists frequently argue that corridors need to be built to allow separated populations of endangered species to intermingle and breed, for the sake of maintaining the genetic health of the population. Wolf highways, panda passages and grizzly-bear gateways sound exciting, but in reality they may not be particularly useful to the 'flagship' animals that they are built for, argue researchers in a paper soon to appear in Conservation Biology1.
Worse, such corridors will almost certainly not be useful for a number of other less charismatic species clinging to survival in badly fragmented areas, they add.
The reason is simply that many of these animals can only survive deep within their native habitat, rather than around the edges where forest, for example, meets fields or roads. So making corridors — which by definition are dominated by edges — doesn't necessarily help.
Stuck in the middle
Robert Ewers, now at the University of Cambridge, UK, and Raphael Didham at the University of Canterbury in Christchurch, New Zealand, had noted in their earlier research on beetles in forest fragments that some species were most abundant near the edges of a forest, others at its heart.
As habitats shrink, their core areas shrivel and become divided, while edge environments proliferate. Ewers and Didham used their observations to model species' responses to this type of fragmentation, showing that it harms core-dwellers and favours edge-dwellers.
In the rainforest, for example, it is known that aggressive edge-dwelling leaf-cutter ants and grasses experience population explosions as forests get divided up and edge zones increase. This happens to the detriment of jaguars, tapirs, pumas, boars, ocelots, otters and other large animals that forage widely and prefer the core of the habitat.
"The trouble is that there's a lot more to corridor conservation than is immediately obvious," says Ewers. "You really need to know how the species that you are trying to recover is going to behave in an edge environment, and with endangered species we often don't have that information available," he adds.
Walk this way
The study is only theoretical for now, but should have significant practical applications. "There are certainly a lot of corridor projects out there that don't work," comments conservation biologist Nick Isaac at the Institute of Zoology in London. "A specific project that comes to mind was built for grizzly bears in Canada. The bears not only didn't use the corridor but ended up attacking people in the nearby area.
"What's great about this work is that it finally gives us some understanding of why corridors don't work for some species," Isaac adds.
Habitat area used to be the only important factor considered when designing conservation space. It is becoming increasingly clear that habitat shape, and how species' territories are divided up within that habitat, are equally important.
The next difficult step will be to look at current corridor projects, such as those being set up in China for the giant panda, and determine whether the animals they are being built to help are core-dwelling species that will never use them.
- Ewers R. M. & Didham R. K. Conservation Biology, (2007) (doi:10.1111/j.1523-1739.2007.00720.x).
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