Interactions Between Woody Plants and Browsing Mammals Mediated by Secondary Metabolites

Mammals must overcome several challenges to exploit woody plants; these include variation among plant species, individuals, growth stages, and parts in their nutritional value and mechanical and chemical defenses (91). The latter are especially significant because woody plants produce a variety of secondary metabolites (56), many of which are chemical defenses against browsing by mammals and some of which appear to be an evolutionary response to browsing by mammals (12a). Not all secondary metabolites are equally effective as defenses against browsing, and none provides complete protection (98), because mammals have evolved anatomical, physiological, and behavioral counters to plant defenses (27, 61, 91). As a result, some woody plants are browsed more than others. The effects of secondary metabolites occur at different hierarchical scales, ranging from individual plants to ecosystems. There are recent reviews of the evolutionary and environmental controls of chemical defenses against browsing, and of the physiological and biochemical mechanisms mammals use to counter these defenses (61, 81). This review concerns three aspects of chemically mediated interactions between woody plants and browsing mammals: (i) chemical specificity and toxicity, (ii) learning as a counter to chemical defense, and (iii) the effects of chemical defense on plant communities and ecosystems.