Even though rain forests cover less than a tenth of Earth’s surface, they harbor nearly half the world’s 5 million or so species of plants and animals. How that diversity became concentrated in so circumscribed a habitat remains a mystery, yet biologists and conservationists alike assume the source of the rain forest’s burgeoning variety must lie, like its greatest density of species, in the forest’s deep interior. But now a team of biologists has found that diversity in some cases emerges not at the forest’s center but at its margins, where dense vegetation meets grassy savanna.
Tom Smith, an evolutionary biologist at San Francisco State University, has been studying the birds of the Cameroon rain forest for 15 years. He has become especially familiar with a robin-size green bird called the little greenbul. Smith has learned that greenbuls living in the transition zone between forest and savanna, an area ecologists call an ecotone, differ markedly from the various greenbul groups in the deep interior, sporting longer wings and narrower beaks. Smith wondered whether he might be witnessing speciation in action and hoped the greenbuls might yield some clues to the rain forest’s spectacular diversity.
To determine just how different greenbuls in the ecotone are from those of the interior, Smith enlisted the help of three geneticists, Derek Girman, a San Francisco State colleague, Bob Wayne of ucla, and Michael Bruford of the Zoological Society of London. To find out to what extent the bird populations were interbreeding, the geneticists decided to compare bits of bird DNA called microsatellites. These snippets of genetic material are particularly good tracers of who’s mating with whom because, unlike genes, they don’t code for any proteins and so have no effect on a bird’s survival. Microsatellites can thus pass from generation to generation, changing--becoming longer or shorter--when they replicate. Over time, different greenbul populations can be recognized by the length of their microsatellites.
Girman, Wayne, and Bruford found considerable genetic overlap not only among the six forest populations they studied, where they expected interbreeding to occur, but also between the birds of the forest and birds of the ecotone. Their results indicate that as many as eight greenbuls migrate between the two regions every generation--a rate close to that within central forest populations; yet birds from the two areas still looked very different. So even though the birds interbreed, the forces of natural selection acting on them are still working to shape different traits.
Smith and his colleagues don’t know whether the physical divergence between greenbuls at the forest’s margins and those in the forest will affect the birds’ ability to interbreed. That would be the definitive indicator that speciation is under way. But Smith’s measurements of body weight and wing length do show that the two groups differ even more in these respects than many birds of entirely separate species.
The researchers suspect that the more varied landscape of the forest edge may favor the survival of greenbuls that differ from those of the stable inner forest. Birds with longer wings, for example, can dart more quickly between the widely spaced islands of foliage on the fringes of the forest. These birds who enjoy life on the edge occasionally migrate back to the central forest and probably enrich the genetic reservoir of species there, says Girman.
Similar patterns, Girman says, may hold true for many other species of animals, and perhaps for plants as well. If so, evolutionary biologists and conservationists may have to rethink their strategies for preserving rain-forest biodiversity. Everyone is focusing on saving the areas with the greatest number of species per acre, says Girman. We’re saying that from a conservation perspective you also need to preserve what underlies the process of speciation--and that’s the forest margins. Without protection, he says, these areas often disappear as farms spread right to the forest edge. Otherwise what we will have are these little ‘banks’ of biodiversity where everything is stagnant, while the processes by which biodiversity is created are lost forever.