HERBIVORY AND PLANT DEFENSES IN TROPICAL FORESTS

In this review, we discuss the ecological and evolutionary consequences of plant-herbivore interactions in tropical forests. We note first that herbivory rates are higher in tropical forests than in temperate ones and that, in contrast to leaves in temperate forests, most of the damage to tropical leaves occurs when they are young and expanding. Leaves in dry tropical forests also suffer higher rates of damage than in wet forests, and damage is greater in the understory than in the canopy. Insect herbivores, which typically have a narrow host range in the tropics, cause most of the damage to leaves and have selected for a wide variety of chemical, developmental, and phenological defenses in plants. Pathogens are less studied but cause considerable damage and, along with insect herbivores, may contribute to the maintenance of tree diversity. Folivorous mammals do less damage than insects or pathogens but have evolved to cope with the high levels of plant defenses. Leaves in tropical forests are defended by having low nutritional quality, greater toughness, and a wide variety of secondary metabolites, many of which are more common in tropical than temperate forests. Tannins, toughness, and low nutritional quality lengthen insect developmental times, making them more vulnerable to predators and parasitoids. The widespread occurrence of these defenses suggests that natural enemies are key participants in plant defenses and may have influenced the evolution of these traits. To escape damage, leaves may expand rapidly, be flushed synchronously, or be produced during the dry season when herbivores are rare. One strategy virtually limited to tropical forests is for plants to flush leaves but delay “greening” them until the leaves are mature. Many of these defensive traits are correlated within species, due to physiological constraints and tradeoffs. In general, shade-tolerant species invest more in defenses than do gap-requiring ones, and species with long-lived leaves are better defended than those with short-lived leaves.