What explains the differences between communities?

Rohde (1980a) suggested that animal communities can be arranged in a continuum from random and unstructured to highly structured, depending on ecological characteristics of species in the communities. Animals with little vagility and/or small population or individual size live in largely empty niche space. They are less subject to structuring mechanisms, in particular competition, than are large animals or animals that live in large populations with much vagility (although they may be nonrandom to a degree because of nonrandom colonization events). The latter have filled extant niche space to a greater degree, i.e., they are closer to saturation, and include the predominantly large mammals and birds, and free-living vagile insects occurring in large populations. (Saturation, however, does not exclude the possibility of further increases in diversity by subdivision of niches.) Gotelli and Rohde (2002) tested this hypothesis using null-model analysis to check for nonrandomness in the structure of metazoan ectoparasites of 45 species of marine fish, and compared the results with those for herps, birds, and mammals. In parasites, co-occurrence patterns could not be distinguished from those that might arise by random colonization and extinction. Presence–absence matrices for small-bodied taxa (parasites, herps) with low vagility and/or small population size were mostly random, whereas presence–absence matrices for large-bodied taxa with high vagility and/or large population size (birds, mammals) were highly structured, supporting Rohde's hypothesis. For Figure 11.1, some data from Gotelli and McCabe (2002) were also used, which support the hypothesis even more strongly.