Trophic Rank and the Species-Area Relationship

The species-area relationship may be the strongest empirical generalization in community ecology. We explore the effect of trophic rank upon the "strength" of the species-area relationship, as measured by z, the slope of a log(species) vs. log(area) plot. We present a simple model for communities closed to immigration, composed of "stacked specialist" food chains (where each plant species supports a specialist herbivore, which in turn sustains a specialist carnivore, etc.), that predicts z should increase with trophic rank; the model brings out some of the spatial implications of sequential dependencies among species. We discuss empirical examples in which the z values of taxa differing in trophic rank were reported and lament the shortage of well-documented examples in the ecological literature. Several examples fit the expected pattern, but others do not. We outline several additional reasons why z values might increase with trophic rank, even for generalists. If the qualitative assumptions of the model are relaxed, the predicted effect of trophic rank on z should weaken or even be reversed. Trophic rank may not have a systematic effect on the species-area relationship if (1) there are strong top-down interactions leading to prey extinctions; (2) communities are open, with recurrent immigration, particularly at higher trophic ranks; (3) consumers are facultative generalists, able to exist on a wide range of resource species; or (4) systems are far from equilibrium. Our aim in this thought piece is to stimulate community ecologists to link theoretical and empirical studies of food web structure with analyses of spatial dynamics and landscape ecology, and to encourage empirical studies of the species-area relationship focused on comparisons across taxa varying in trophic rank.