Speciation by a wandering river.

Spectation by a wandering river

Textbook explanations of the fantastic elaboration of species in the Amazon and other tropical areas have relied on stability as a key factor. Seasonless

years and an invariant environment the theory goes, have provided the opportunity for great numbers of species to evolve and coexist.

But recent studies have shown that there are seasonal variations in temperature in the tropics, as well as variations between years. And according to Phyllis Coley of the University of Utah in Salt Lake City and her colleagues at the University of Turku in Finland, in the Peruvian Amazon the physical environment itself is subject to great and recurring changes. Meandering rivers cause large-scale disturbances in the Peruvian forest, the researchers say; it may be these changes, rather than stability, that are responsible for the diversity of tropical species.

The researchers, who analyzed Landsat images, report in the July 17 NATURE that more than 26 percent of the lowland forest shows signs of recent erosion and new soil deposition; 12 percent of the lowland forest is in successional stages along rivers. "People had never really thought in terms of the habitat being destroyed on such a large scale," Coley says. "But most of the Amazon is fairly low, flat country.... This is actually destroying and creating lots of new habitat."

That means the forest, far from being uniform, is a mosaic of patches of different ages, which may help maintain a diversity of species adapted to different successional stages. Soil laid down during successive floods can vary among patches as well, increasing opportunities for differing species. And the dynamic nature of the environment may have played a role in the origin of tropical diversity, the researchers say: The flipping coils of the river may at times isolate populations and allow speciation.

Conservation efforts should take into account the emerging notions about the dynamic nature of the tropics, Coley says. If parks are set aside, "You would like to include large enough areas so that when the river continues to move you're not wiping out the only mature forest left."