Mauricio Lima for The New York Times

The Amazon rainforest in Brazil.

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's 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," Dr. Coley said. "It's a constant battle to stay alive, to stay in the game." Dr. Coley and Dr. 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," Dr. Coley said. "There must be something else going on."

Dr. Coley and Dr. Kursar came to endorse a different explanation. That shift occurred over the course of their three decades of research in the tropics, where they study how insects attack plants and how plants defend themselves.

A single tree may be home to hundreds of species of insects, many of which live by relentlessly devouring its seeds, stems and leaves. Many seedlings will die before reaching maturity thanks to the damage done by insects.

Plants are not helpless victims, however. They have evolved a staggering variety of defenses. Some grow cups of nectar on their leaves to attract sugar-hungry ants, which also attack insects feeding on the leaves. Some plants defend themselves by sprouting hairs. "To us they seem soft and fuzzy," Dr. Coley said, "but to a small caterpillar with a soft belly, they can be more like meat hooks."

The most impressive defenses in tropical plants are invisible, however. A plant may pack each of its leaves with hundreds of kinds of insect poisons. Those toxins can make up half the dry weight of a tropical plant leaf.

As farmers know all too well, insects can evolve resistance to pesticides. A similar evolution plays out in tropical forests, where insects can disarm many of the chemicals that plants use against them.

Of course, plants in temperate regions face attacks from insects, too. But Dr. Coley and Dr. Kursar argue that those plants are more adapted to the bigger threats they face, from the bitter cold of winter and other environmental challenges. In the tropics, plants enjoy a balmy climate year-round. While the physical environment poses less of a threat to tropical plants, it makes insects a bigger danger. They can grow faster in the warm, moist climate; without killing frosts, they can produce more generations each year.

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," Dr. Coley said.

Dr. Coley suggests that the evolution of new defenses speeds up the evolution of new plant species. When a population of plants evolves a new chemical to ward off its insect enemies, it may also change its scent. Pollinators can be exquisitely sensitive to the fragrance of plants, and so the plants may end up visited by different animals than before. That shift may isolate them from other members of their species and help them evolve into a separate species.

The chemical arms race may also explain how so many plant species can live side by side. "The way they're being different is who is feeding on them," Dr. Coley said.

The insects that have adapted to feeding on one species of plant may be unable to get around the defenses of another species growing nearby. With hundreds of different defenses in each plant, the possibilities for coexistence are practically endless.

A small but growing number of studies support this argument. Dr. Coley and Dr. Kursar, for example, have found some of this evidence while studying species of South American trees that belong to a genus called Inga. The scientists examined the chemicals of closely related Inga species living near each other. They were surprised to find that the arsenals were drastically different from species to species.

"They were like snowflakes," Dr. 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. Many species, for example, depend on bacteria living in their roots to supply them with nitrogen. Just as each tropical plant may fight certain pest species, they may need certain species of root bacteria. "There's just an unbelievable constellation of possibilities," Dr. Schemske said.

Still, Dr. Schemske and other scientists say that it is not possible yet for scientists to make broad generalizations about tropical plants and their insects. "It takes so much time just to identify all the chemicals in a single leaf," said Paul V. A. Fine of the University of California, Berkeley.

And when scientists capture insects from plants to study their counterdefenses, they face the unknown all over again. "Almost every single one of them is a new species," Dr. Fine said. The biggest obstacle to understanding the diversity of tropical rain forests, it turns out, is that very diversity.