▪ Abstract 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 ...

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Herbivory and plant defenses in tropical forests

Characteristics of higher plant terpenoids that result in mediation of numerous kinds of ecological interactions are discussed as a framework for this Symposium on Chemical Ecology of Terpenoids. However, the role of terpenoid mixtures, either constitutive or induced, their intraspecific qualitative and quantitative compositional variation, and their dosage-dependent effects are emphasized in subsequent discussions. It is suggested that little previous attention to these characteristics may have contributed to terpenoids having been misrepresented in some chemical defense theories. Selected phytocentric examples of terpenoid interactions are presented: (1) defense against generalist and specialist insect and mammalian herbivores, (2) defense against insect-vectored fungi and potentially pathogenic endophytic fungi, (3) attraction of entomophages and pollinators, (4) allelopathic effects that inhibit seed germination and soil bacteria, and (5) interaction with reactive troposphere gases. The results are integrated by discussing how these terpenoids may be contributing factors in determining some properties of terrestrial plant communities and ecosystems. A terrestrial phytocentric approach is necessitated due to the magnitude and scope of terpenoid interactions. This presentation has a more broadly based ecological perspective than the several excellent recent reviews of the ecological chemistry of terpenoids.

Higher plant terpenoids:a phytocentric overview of their ecological roles

Fundamental plant traits such as support, anchorage, and protection against environmental stress depend substantially on biomechanical design. The costs, subsequent trade-offs, and effects on plant performance of mechanical traits are not well understood, but it appears that many of these traits have evolved in response to abiotic and biotic mechanical forces and resource deficits. The relationships between environmental stresses and mechanical traits can be specific and direct, as in responses to strong winds, with structural reinforcement related to plant survival. Some traits such as leaf toughness might provide protection from multiple forms of stress. In both cases, the adaptive value of mechanical traits may vary between habitats, so is best considered in the context of the broader growth environment, not just of the proximate stress. Plants can also show considerable phenotypic plasticity in mechanical traits, allowing adjustment to changing environments across a range of spatial and temporal scales. However, it is not always clear whether a mechanical property is adaptive or a consequence of the physiology associated with stress. Mechanical traits do not only affect plant survival; evidence suggests they have downstream effects on ecosystem organization and functioning (e.g., diversity, trophic relationships, and productivity), but these remain poorly explored.

Plant biomechanics in an ecological context

Damage to leaves by herbivores can have a significant and extensive impact on growth and reproduction of plants (Marquis, 1984, 1992a, b; Marquis & Braker, 1993), which in turn can influence competitive outcomes and community composition (Janzen, 1970; Dirzo, 1984; Clark & Clark, 1985; Dirzo & Miranda, 1991). In tropical forests, approximately 11% of the annual leaf area produced is consumed by herbivores and pathogens (Coley & Aide, 1991), a resource loss equivalent to investments in reproduction (Bazzaz et al., 1987). This loss would be substantially higher except for the fact that plants allocate considerable resources to physical, chemical, and phenological defenses. Most of our understanding of the costs and benefits of defenses and of the interplay between defenses and herbivores is based on research on mature leaves. However, young expanding leaves are the most vulnerable stage during the life of a leaf. In tropical shade-tolerant species of plants, expanding leaves suffer 5–100 times the rates of damage from pathogens and herbivores as mature leaves (Coley & Aide, 1991). Seventy percent of the lifetime damage can occur during this small window of vulnerability. Many leaf developmental traits may therefore be the result of selection by herbivores and pathogens.

M.K. Ross

Papers

Leaf development in the tropical leguminous tree Copaifera in relation to microlepidopteran herbivory