Introduction: assembly rules, scale, purpose and application

One of the basic premises in ecology is that communities are composed of collections of species that are subsets of a larger pool of available species (e.g., Cody & Diamond, 1975; Diamond & Case, 1986), and that the composition of these subsets is governed, potentially at least, by ‘assembly rules’ (Diamond, 1985). In particular, and as originally envisaged by Diamond (1985), if community size varies with, e.g., island size, vegetation structure, or some other extrinsic factor, then such rules will also govern community amplification. Ideally, assembly rules should cover both species composition and relative abundance, the two more basic variables or descriptors of the community.

Conceptually, assembly rules are of two basic categories. One type, which will be called ‘type A’, governs community membership or species composition as community size increases or decreases. Thus, type A rules address which species are added as community size increases, e.g., with island size (MacArthur & Wilson, 1967), or with the stature or complexity of vegetation (MacArthur et al., 1966; Cody, 1973), or decreases in smaller or in more isolated habitat fragments with poor recolonization potential (Cody, 1973; Bolger et al., 1991), or decreases with time as former landbridge islands equilibrate, either real islands (Diamond & Mayr, 1976; Diamond et al., 1976) or habitat islands (Brown, 1971).