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Heliconius sara butterfly in Peru. (Photo by R. Butler)
DIVERSITIES OF IMAGE
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.
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.
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.
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.
Review questions:
Why are some rainforest animals (especially insects and frogs) brightly colored?
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