Like neutral ecology, this model seems to be throwing biology away, by showing that the patterns in nature can arise without any of the things that scientists traditionally invoke as their causes. And like neutral ecology, Colwell’s theory made many people—including Colwell himself—uncomfortable, even hostile. “At first, I only believed it on alternate Tuesdays,” he says. But some spectacular matches between the model’s predictions and the natural world changed his mind.
The first match was in Madagascar. Going by latitude, biodiversity in Madagascar should peak at the island’s north, as this is the warmest, wettest region closest to the equator. In fact, in 1999, David Lees and his colleagues, after surveying 637 species of rain forest mammals, butterflies, birds, amphibians, and reptiles, revealed that more species are found in the middle of the island than anywhere else. Colwell was so impressed he wanted to call his theory the Perinet effect, in honor of the Madagascan reserve containing the peak of diversity, but that is a colonial name. Wanting to be politically correct and thinking the Madagascan name, Analamazoatra, too much of a mouthful, Colwell settled on the less romantic “mid-domain effect.”
The mid-domain effect can also explain how diversity varies with altitude. Ever since Humboldt, most biologists had thought that the altitudinal gradient in diversity matched the latitudinal one, with a steady decline from low to high. But as the studies accumulated, they were forced to reassess this notion. About half of the counts showed, in fact, that more species are found at intermediate altitudes, neither up nor down. Climate alone cannot explain this pattern, but the mid-domain effect can because this is where there should be most overlap between the ranges of lowland and mountain species. Something similar might happen in the sea. It was long thought that diversity should decline with depth, with most species in shallow waters and fewest in the abyss. But there is some evidence that middling depths, of about 2,000 meters, are most diverse, although we know so little about marine diversity that it is impossible to draw firm conclusions.
Sometimes biology does not comply with the mid-domain effect’s predictions. Mid-domain models predict that endemic species with small ranges should be spread around at random, just as the short