"People have thought soil alone is enough to explain why you get two different communities of plants growing on those two soil types," says Fine. "But we showed that if it wasn't for their insect enemies, it's possible the same kind of plants would grow on both soil types. The insects cause the difference between habitats to become sharper."
Red clay tree species also are attacked by insects when they grow on clay soils, but the soil is so rich that the trees grow faster than they are consumed. And other insects and birds in the lush red clay forest help control plant-eating insects.
Coley says the study has implications for rainforest conservation: "White sand soils have unique species, sand patches are not as extensive as they used to be, and land conversion [development] therefore could have a big impact on diversity if it hit the relatively rarer sand forests."
In fact, during the study Fine and Mesones fended off a land grab by speculators, even though their study site in the Allpahuayo-Mishana Reserve supposedly was protected already. Mesones started an environmental group that helped stop the land grab.
Fine says tropical rainforests are known for biodiversity, with about 10,000 tree species in the Amazon compared with 500 to 600 in North America's temperate zone.
Yet the tropics really aren't different than other regions in terms of variations in soil, altitude and rainfall. So why do tropical rainforests have greater species diversity?
"In a world without herbivores, our study predicts there would be a much lower diversity of trees in the tropics," Fine says. "Trees would just follow the physical features of the environment whether soil,