Herbivory, Plant Regeneration, and Species Coexistence

Abstract

The mechanisms by which plants species coexist in botanically diverse communities has been the focus of intense theoretical and empirical interest (Harper 1977; Grubb et al. 1982; Tilman & Pacala 1993). However, although considerable empirical data exist on the structure of plant communities, there is still no consensus regarding the mechanisms underlying species diversity (Crawley 1986; Tilman & Pacala 1993). Since the number of plant species found to coexist within most habitats is often greater than the number of limiting resources, studies have focused on mechanisms limiting or preventing competitive exclusion. Although none of the principal models developed to explain species coexistence at local scales are mutually exclusive, they fall into two broad groups (Crawley 1986; Zobel 1992; Tilman & Pacala 1993): those where the principal mechanism mediates exploitative competition between established plants for resources (light, nutrients, etc.), e.g. resource partitioning, spatial and temporal heterogeneity in resource supply and neighbourhood effects; and those that focus on limiting pre-emptive competition for microsites, e.g. regeneration niches, phenological heterogeneity, patch dynamics and lottery models (Table 1). Herbivory has often been treated in these models of plant community structure as a type of disturbance (Grime 1979; Tilman 1982). However, the impact of selective herbivores on resource partitioning (by grazing plant tissues) or regeneration (by reducing numbers of propagules via preor postdispersal predation) may differ considerably from most forms of disturbance. The extensive literature describing the impact herbivores may have on the demography of the plant species they consume (Crawley 1988; Huntly 1991) has led to the suggestion that herbivory may be more important than competition in structuring terrestrial plant communities (Sih et al. 1985). Nevertheless, comparatively few studies provide tractable hypotheses regarding the role of herbivores in vegetational diversity (Huntly 1991). This paper addresses the role