Rainforest Diversity


July 31, 2012

Because plants grow year round in the tropical rainforest, they must continuously defend themselves against an array of predators. Over the course of millions of years of evolution, plants have developed a variety of mechanical and biochemical defenses. Mechanical defenses like thorns, spines, and stinging hairs appear to be secondary protection to chemical compounds produced by plants, like alkaloids, tannins, and toxic amino acids.

In response, like biochemical warfare, herbivorous insects have adapted to these compounds and insects that eat these plants are able to detoxify the chemicals. The result is that any given insect species has adapted to feed on only a limited number of plants species, while leaving these individual plant species toxic to most other insects.

Medicinal plants

Through the rigorous process of natural selection, plant species have been perfecting various chemical defenses to ensure survival over millions of years of evolution, and are proving to be an increasingly valuable reservoir of compounds and extracts of substantial medicinal merit. These plants have synthesized compounds to protect against parasites, infections and herbivores, creating acutely powerful chemical templates with which pharmacologists can create new drugs.

Interesting associations have developed between plants and insects like that of the Heliconid butterflies and passion flower vines of the genus Passiflora. Passion flower vines contain cyanide-based compounds for protection against predators. However, Heliconid caterpillars have adapted to these compounds and are able to eat the vine's leaves. Therefore, Heliconid butterflies lay their eggs directly on the passion flower vine, so the larvae will have easy access to their food source. Passion flowers have counter-adapted to the behavior by developing mechanisms to discourage Heliconid butterflies from laying eggs on their leaves. Some Passiflora have evolved structures (actually nectaries) that create housing and produce excess nectar for ants. In return, the ants attack anything, including butterfly eggs, that intrudes on their host. One- upping their predators, some Passiflora have structures that mimic the eggs of Heliconid butterflies. Since a Heliconid butterfly will not lay its eggs on leaves that already have (or appear to have) these eggs, she will move on to another plant. In this manner, Passiflora deter Heliconid butterflies without devoting any resources to the production of nectar for a guard of ants, a technique of protection adopted by many other plants as their primary means of defense. Heliconid caterpillars that develop into butterflies retain the cyanide they consumed as larvae, making the adult butterflies highly unpalatable to predators. The distinct pattern and color of Heliconid butterflies acts as a sort of warning for predators of its toxic composition. When a predator eats one of these butterflies and experiences a foul taste and other ill effects, it learns to associate the colors and patterns of the prey with the bad experience. The next time the predator recognizes the color pattern, it is likely to avoid that potential prey.

This use of warning coloration to advertise bad taste or toxic composition is employed frequently in the rainforest by a variety of animals. The toxic chemicals almost never kill the predator, but cause some irritation to violent sickness. There would be no use if the poison killed the predator, since the next predator that came along would make the same mistake and eat the prey. By making the predator ill, the toxin causes the predator to recognize and avoid the unpalatable prey and similar-looking species known as mimics.

Ranitomeya imitator poison dart frog from Peru. Photo by Rhett A. Butler

Review questions:

  • Why are some rainforest animals (especially insects and frogs) brightly colored?
  • How do plants protect themselves from predators?

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Continued / Next: MIMICRY AND CAMOUFLAGE in the Rainforest