Mechanisms of tolerance to herbivore damage: what do we know?

Identifying mechanisms of tolerance to herbivore damage will facilitate att empts to understand the role of tolerance in the evolutionary and ecologica l dynamics of plants and herbivores. Investigations of the physiological an d morphological changes that occur in plants in response to herbivore damag e have identified several potential mechanisms of tolerance. However, it is unlikely that all physiological changes that occur following damage are to lerance mechanisms. Few studies have made direct comparisons between the ex pression of tolerance and the relative expression of putative mechanisms. I briefly review empirical evidence for some of the better-studied potential mechanisms, including increased photosynthetic activity, compensatory grow th, utilization of stored reserves, and phenological delays. For each of th ese mechanisms I discuss reasons why the relationship between tolerance and these characters may be more complicated than it first appears. I conclude by discussing several empirical approaches, including herbivore manipulati ons, quantitative trait loci (QTL) analysis, and selection experiments, tha t will further our understanding of tolerance mechanisms.