The Geometry of Niches

Like others in this volume, this chapter has a lot to do with scale. But it is organizational scale rather than spatial or temporal scale that I wish to consider. Much of what is commonly called community ecology deals with a very small number of species — often two. Yet communities are large sets of species — all the species in an area or at least those in a particular taxonomic or trophic group. Clearly, we need to ask how species interactions are built into large sets. Such is the aim of the various studies of food webs: food webs are the diagrams depicting which species in a community interact trophically. Among the achievements of recent food web studies is a long catalogue of food web patterns reviewed recently by Lawton (1989). These patterns can be grouped into six broad categories: the general patterns of connectance and trophic grouping across trophic levels (e.g. Briand and Cohen, 1984, Cohen, this volume, Cohen and Briand, 1984, Yodzis, 1981), the number of trophic levels (e.g. Cohen, this volume, Pimm and Lawton, 1977, Pimm and Kitching, 1987), the patterns of species which feed across trophic levels (e.g. Pimm and Lawton, 1978), the degree to which interactions are grouped into compartments (e.g. Pimm and Lawton, 1980, Yodzis, 1982), and such ratios as that between the number of species at one trophic level and the number of species at the trophic level on which these predators feed (e.g. Cohen, 1977).