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.