Risk of extinction grows

By MICHAEL SMITH-- CNEWS Science

Extinction, so we're told, is forever. That's why conservationists and biologists worry (among a whole lot of other things) about the fragmentation of habitat that our burgeoning species is causing for other species.

If you look at southern Ontario, for example, cities and farms have cut up the forest primeval into a zillion little patches - a copse here, a glade there. What happens to the species caught in these island habitats?

If they're lucky, they may be fine. But every species goes through fluctuations in number; if the numbers are small, a bad winter can wipe a species out. And the risk is accentuated when the whole species is confined in a bunch of small habitats, instead of their whole ancestral range.

In the past, a bad winter in the Ottawa area might have wiped out the local population of naked mole rats (I don't think they're found in Ontario, but I like the name. And they'll do for an example.) But that wouldn't have been a problem, because other mole rats would have survived elsewhere and eventually re-populated the area.

In a set of island habitats, there's no chance of re-populating one if the local mole rats go under - the others can't get there.

So for years there has been a debate about connecting habitats with what are called conservation corridors - an expensive undertaking, but one that might well provide a safety cushion for many of the species now living in separate and fragile habitats.

But it's not quite as simple as opening a conservation corridor, says mathematician David Earn of Hamilton's McMaster University. "The idea is sensible, but it's just an idea," he says. "You don't really know if opening a conservation corridor is going to help."

Earn, with Simon Levin of Princeton and Pejman Rohani of Cambridge, has devised a mathematical model of the interaction between members of a species with several habitats that are connected. The details are in this week's edition of the journal Science.

And the bad news is that conservation corridors could actually make things worse.

The problem is this: Ordinarily, separate groups of (say) naked mole rats tend to have different reproduction patterns - now high in this place, with lots of little mole rats, and now low in that place.

But if their habitats were connected, the different populations could start reproducing in the same pattern - all high or all low at the same time.

Then, if a bad winter or a disease hit when all the groups were at a low ebb - bang! No more mole rats.

"It's a potential danger of introducing a conservation corridor," Earn says. The model is the first time it has ever been demonstrated that the danger exists; until now, it has been just something biologists worried about in the coffee room.

But the model does more than confirm their fears, Earn says. It actually allows some insight into what would happen if two or more habitats were re-connected by conservation corridors. "Before you make a huge investment," he says, "you want to be able to predict whether it will help."

Plugging a few numbers - average reproduction rates, for instance - into Earn's model would generate those predictions.

This is not an idle bit of mathematical doodling. I did a Web search on the phrase "conservation corridor" and found literally hundreds of articles - most of them about existing or planned corridors.