Herbivory

The high rate of leaf production and leaf fall in the humid tropics means that there is a relatively large amount of energy that is potentially available to insect herbivores. Leaves are attractive to herbivores and pathogens because they are more nutritious than other parts of the plant. Immature leaves lose area to herbivores at 5-25 times the rate of fully enlarged, mature leaves (Turner 2001). The loss of material to herbivores, mostly insects, can be substantial, and can have a negative influence on tree growth and reproduction (Coley and Barone 1996). For example, Marquis (1987) found that heavy defoliation (50% loss of area or more) of the understory shrub Piper arieianum at La Selva, Costa Rica, caused significant reduction in seed output of plants for 2 years subsequent to the leaf loss. Several studies have focused on herbivory in understory palms, perhaps because they have lower stature and are easier to study. Most neotropical palm species seem to be quite resilient to leaf loss and in some cases can even increase reproductive rates following loss of leaf material (Turner 2001).

However, in many tropical forests on nutrient-poor sites, only a small proportion of the available energy stored in leaves is actually utilized by herbivores (Golley 1977). This may be due in part to defense mechanisms in tropical trees, which prevent herbivores from taking advantage of the high quantity of leaves. Many tropical plants produce secondary compounds such as phenolics that make their leaves unpalatable to most herbivores (Levin 1976). However, production of such compounds has an energetic cost (Jordan 2002), and insects may still overcome the defenses (Coley 1980). If the production of defenses uses resources that would otherwise be used to grow more plant tissue, then in certain circumstances there may be selection in favor of maximizing growth, even at the cost of being susceptible to herbivores or pathogens. However, most plants do have some mechanisms of defense against her-bivory (Turner 2001).

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