There are many mechanisms whereby animal activity can directly or indirectly influence the species diversity of plant communities. Most obviously, herbivory can influence the species composition directly through plant mortality or indirectly by changing the outcome of interspecific competition (Hulme 1996). Animals may also affect plant species composition by modifying the physical structure of the environment such that the flow of resources required for plant growth is altered. Such physical effects mediated by the physical structure of the environment have been termed ‘ecosystem engineering’ (defined in table 11.1; Jones et al. 1994, 1997, Lawton 1994, Lawton and Jones 1995). Animals may have other functions that influence the persistence of populations (e.g., pollination) or the colonization of new sites (seed dispersal). In this chapter we aim to provide an overview of how these diverse effects of animals influence plant species diversity, and to this end, we propose community assembly theory as a conceptual framework. Community assembly theory provides us with a schematic representation of the vital steps involved in the determination of species presence or absence at a particular site. By asking how might animal activity influence each of the steps of assembly, we ensure a comprehensive outlook on how animals affect plant species diversity. That said, we restrict ourselves in this chapter to ecological mechanisms and we do not consider evolutionary effects that are of undoubted importance at higher levels of spatial and temporal scales. Following our discussion of animal effects on community assembly, we highlight two case studies of herbivores arising from recent research in Israel. We use insights provided by these examples to suggest mechanisms that are likely to be of particular importance in arid ecosystems. For example, we suggest that since the flow of water, the primary limiting factor in these systems, is so easily influenced by structural changes in the environment, physical ecosystem engineering may be a relatively important, though previously neglected, interaction type in arid ecosystems. The processes governing which species occur in a particular plant community can be summarized very simply.
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