Learning Little Things about Extinction

Article excerpt

Wog Wog Mountain sits calm, dark and shrouded in cloud, overlooking a battlefield. We're in that most famous of forests in southeast New South Wales, Coolangubra, which has seen so many skirmishes between conservationists and loggers in recent years.

The forest is quiet now; much of it has been made a national park, and the protagonists have moved elsewhere. But nearby, on some odd, square patches of native bush in the midst of pine forest, another battle is going on: the struggle of populations against extinction.

The patches arc part of an extraordinary experiment created by CSIRO scientist Chris Margules. The experiment began in 1985, but the battle dates back to January 1788, when Arthur Phillip's motley collection of settlers took their first nervous steps onto the Australian continent.

Axes and saws did the damage early, but in time Australia's European pioneers used a suite of machinery in their charge across the continent. Vast tracts of bushland were cleared and carved into blocks. Embryonic modern Australia stamped its imprint on the landscape so indelibly that you might think the original had vanished completely.

But it hasn't, not quite. Fly over Australia's agricultural and suburban heartland today and you'll see that the open spaces are studded with little patches of bush. They vary in size, from a few trees huddling in the corner of a paddock to larger tracts that take half a minute or longer to fly across. It might remind you of the face of a man who has shaved in a hurry. The tufts he missed have sharp, angular boundaries and they seem vulnerable to the next pass of the blade.

When you realise that these tufts might be all we have left of the original vegetation of an entire region, they suddenly take on special significance. They might be the last strongholds for dozens, hundreds or even thousands of plant and animal species that otherwise could have disappeared when the bulk of their habitat was destroyed.

What is happening in these remnant patches of bush? What has the act of fragmentation done to them? Are species persisting, or are they gradually going extinct? What is their conservation value, and how can it be enhanced?

Questions such as these were being asked with increasing urgency in the late 1970s and 1980s. They coincided with debate about an intriguing idea laid out in The Theory of Island Biogeography, a book by two leading biologists, Robert H MacArthur and Edward O Wilson.

It was a seminal work that brought about a fundamental change in the science of ecology. The basis of the theory is this: the number of species on an offshore island is a function of the size of the island and the distance it is from the mainland. While species composition might change as species go extinct and others colonise, the total number would stay the same depending on distance and size.

Conservation biologists quickly realised that the theory could have implications for the persistence of species in habitat remnants in cleared landscapes. On that basis, bold statements were made that in a fragmented landscape, a single large reserve would be better than several small ones adding up to the same area. But some had doubts.

`One of the criticisms of this theory was that it dealt with characterless species on featureless plains,' Margules says.

Nobody knew if it would be valid in the real world with real species. There were counter-arguments that smaller reserves summing to the same area as a large reserve would sample more species because, being dispersed, they sampled more of the environmental variation in a landscape.

`At the time this was a big issue in conservation biology,' Margules says. `So I set up this field experiment to test one of the predictions of the theory, which was that groups of small reserves with more-or-less similar total area to a large reserve will retain fewer species over time. …