The Diversity Pyramid

Within many clades, the species richness of various trophic levels parallels productivity at those levels (figure 7.1). As noted long ago by Elton (1927), the ecological efficiency of a community dictates that only a fraction of the productivity at one trophic level will be transferred to the next highest level, producing a pyramid; herbivore production may, for example, be only 20% of plant production and first-level carnivore production may be only 15% of herbivore production, and therefore only amounts to 3% of plant production. Similarly, within at least some clades of terrestrial vertebrates (reptiles, birds, dinosaurs), species diversity varies inversely with trophic level (Allmon 1992a), implying some positive correlation between trophic resources available and resulting species richness.

Similar patterns are indicated by higher rates of origination among some taxa with particular trophic modes. Roy, McMenamin, and Alderman (1990), for example, found that suspension-feeding benthos (crinoids, bivalves, bryo-zoans) showed higher familial origination rates than nonsuspension feeders

Energy flux

FIGURE 7.1. An "ecological pyramid" representing the net productivity of each trophic level in an ecosystem. This particular pyramid represents transfers of 20, 15, and 10%, respectively, among trophic levels. From Ricklefs (1990). By analogy, a "diversity pyramid" may also exist, with greater taxonomic diversity among lower trophic levels.

Energy flux

FIGURE 7.1. An "ecological pyramid" representing the net productivity of each trophic level in an ecosystem. This particular pyramid represents transfers of 20, 15, and 10%, respectively, among trophic levels. From Ricklefs (1990). By analogy, a "diversity pyramid" may also exist, with greater taxonomic diversity among lower trophic levels.

(cephalopods and arthropods) in the late Cretaceous. Allmon et al. (1992) found the reverse for Paleozoic gastropods; suspension-feeding genera show lower origination rates than nonsuspension feeders.

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