In this review, we discuss the ecological and evolutionary consequences of plantherbivore 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. 305

Explaining the mechanisms that produce the enormous diversity within and between tropical tree communities is a pressing challenge for plant community ecologists. Mechanistic hypotheses range from niche-based deterministic to dispersal-based stochastic models. Strong tests of these hypotheses require detailed information regarding the functional strategies of species. A few tropical studies to date have examined trait dispersion within individual forest plots using species trait means in order to ask whether coexisting species tend to be more or less functionally similar than expected given a null model. The present work takes an alternative approach by: (i) explicitly incorporating population-level trait variability; and (ii) quantifying the functional beta diversity in a series of 15 tropical forest plots arrayed along an elevational gradient. The results show a strong pattern of decay in community functional similarity with elevation. These observed patterns of functional beta diversity are shown to be highly nonrandom and support a deterministic model of tropical tree community assembly and turnover.

Insect outbreaks are expected to increase in frequency and intensity with projected changes in global climate through direct effects of climate change on insect populations and through disruption of community interactions. Although there is much concern about mean changes in global climate, the impact of climatic variability itself on species interactions has been little explored. Here, we compare caterpillar-parasitoid interactions across a broad gradient of climatic variability and find that the combined data in 15 geographically dispersed databases show a decrease in levels of parasitism as climatic variability increases. The dominant contribution to this pattern by relatively specialized parasitoid wasps suggests that climatic variability impairs the ability of parasitoids to track host populations. Given the important role of parasitoids in regulating insect herbivore populations in natural and managed systems, we predict an increase in the frequency and intensity of herbivore outbreaks through a disruption of enemy-herbivore dynamics as climates become more variable.climate change ͉ herbivore ͉ outbreak ͉ parasitoid ͉ top-down

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Summary 0[ To assess the degree of herbivore host!speci_city in the moist tropical forest on Barro Colourado Island\ Panama\ I conducted an extensive series of feeding trials on the common insect herbivores from 09 tree species[ 1[ The herbivores were o}ered leaves from both congeneric and confamilial plant species to their known host species\ as well as leaves from the most abundant tree species in the forest[ 2[ The amount of damage caused by these herbivores to young\ expanding leaves was also measured on nine of the tree species[ 3[ Of 35 herbivores species "seven Coleoptera\ one Orthoptera\ 27 Lepidoptera#\ 15) were specialized to a single plant species\ 11) were limited to feeding on a single genus and 26) were able to feed on several genera within a single family[ The remaining 04) were generalists\ able to feed from several di}erent plant families[ 4[ The causes of leaf damage varied extensively across the tree species[ On average\ specialist herbivores caused 47) of the damage to young leaves\ generalists herbivores 7) and fungal pathogens 23)[ For four of the tree species\ pathogens were the most important cause of leaf damage[ 5[ In this forest\ most chewing herbivores appear to have fairly narrow diets\ and these specialists are responsible for most of the insect herbivory[ Key!words] Barro Colorado Island\ diet specialization\ herbivores\ herbivory\ pathogens[ Journal of Animal Ecology "0887# 56\ 399Ð398