The diversity of a tropical rain forest can be hard to fathom for people who have not seen one. Three acres of jungle may be home to more than 650 species of trees – more species than grow in the entire continental United States and Canada combined.

It is tempting to look at all those species living so close together as a picture of peaceful coexistence. But Phyllis D. Coley and Thomas A. Kursar, a husband-and-wife team of ecologists at the University of Utah, see them as war zones.

Hordes of insects threaten the survival of plants, which respond with chemical warfare. The result, they argue, is the remarkable biodiversity we see today.

“It’s not harmonious,” Coley said. “It’s a constant battle to stay alive, to stay in the game.”

Coley and Kursar outline their hypothesis in this week’s issue of Science.

This hypothesis is a departure from the classical explanation for tropical diversity. Traditionally, ecologists argued that all the species in a tropical forest could coexist through specialization to their physical environment. Some species might be able to live in deep shade, for example, while others could gain minerals beyond the reach of other plants.

But this explanation has fallen out of favor in recent years. “There just aren’t enough different ways to take advantage of light or nutrients or water,” Coley said. “There must be something else going on.”

Coley and Kursar came to endorse a different explanation. A single tree may be home to hundreds of species of insects, many of which live by relentlessly devouring its seeds, stems and leaves.

The tropics have thus become host to an arms race. Each species of plant is evolving defenses against its enemies, which evolve counterdefenses in turn. This arms race would explain why tropical plants have become so loaded with toxic compounds.

It might also help solve the mystery of tropical biodiversity. “We think this arms race between the herbivores and the plants might be the explanation for what maintains the diversity that we see now, and why so many plants have evolved in the first place,” Coley said.

“The idea they lay out makes a lot of sense to me,” said Douglas W. Schemske, an ecologist at Michigan State University. He suggests that along with insect pests, beneficial partners may also help drive the diversity of tropical plants.
 

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