Due to the huge numbers of insects and other herbivores living in a tropical rainforest, plants have developed some effective defense techniques to avoid being eaten. A major way that the plants defend themselves is through the production of secondary metabolites, also known as defense compounds. These are in some cases the same chemicals that make these plants medicinal or poisonous to humans as discussed in the section on medicinal plants.

One class of compounds, known as alkaloids, is familiar to most of us in the form of caffeine, and nicotine; other alkaloids include cocaine and morphine. Alkaloids are found in almost every part of tropical as well as temperate plants, including the leaves, flowers, stems, and roots. Alkaloids have a bitter taste and in mammals interfere with liver and cell membrane function; they may also cause cessation of lactation. They are hard to digest and damaging to the liver. It is thought that the bitter taste and difficulties with digestion are probably what deter herbivores the most.

Phenols and tannins are also found in both tropical and temperate vegetation. In field experiments plants with lower levels of tannins had twice the amount of damage from herbivory as those with higher levels. Younger leaves typically have higher amounts of tannins than older leaves; also, plants growing in the shade tend to have lower amounts of tannins suggesting that much more of the sun’s energy is necessary to drive the photosynthesis required for the production of tannins. Phenolic compounds are located in vacuoles of the leaf and are released as the leaf is broken by an herbivore. The phenolic compounds combine with proteins in the intruder and can combine with enzymes involved with digestion making digestion of the plant material difficult. Although tannins and phenols do provide some protection for the plants, they are by no means 100% effective. They seem to have no effect upon leaf cutter ants. Terpenoids, steroids and waxes, however, do seem to discourage leaf cutter ants; and tree sap may deter this insect by adhering to the insect’s mouth parts.

In response to this daily chemical ammunition, herbivores in the rainforest have over time developed their own counter-mechanisms. Some caterpillars, for example, are able to detoxify the potent cyanogenic glycosides found in passion flowers upon which they feed.

In addition to the chemical defenses we have briefly considered here, plants and trees have physical/mechanical defenses as well. For example leaf toughness, fiber content and nutritive value influence the use of leaves in a herbivore’s diet. Likewise, many plant species have developed extensive systems of spines and thorns lining the trunk or edges of a leaf.

The warree palm (Acrocomia vinifera ), an understroy species, exhibits such a defense system. Two-inch (5 centimeters) spines protrude out from the lower trunk. In addition to a potentially serious puncture wound, the spines are coated with lichens and microbes that may cause infection once the skin is penetrated. The warree palm also exhibits sharp spines on the underside of its leaf.

The Capirona (Calycophyllum spp.) tree employs one of the more unique plant adaptations to parasites. Each year, its bark sloughs off carrying with it an assorted variety of fungi, molds, insects, and other invasive plants. Similar to the development of the drip tip leaves of understory trees, the removal of bark is an effective way of diverting harmful organisms off of the tree.

Other plant adaptations, not related to defense from predators, address low light conditions on the forest floor. One of the more common strategies is for large leaf size to increase the surface area for photosynthesis. One look at an Elephant Ear plant (Xanthosoma purpuratum) and you will see what we mean!