Abundance, diversity and body size: patterns from a grassland arthropod community

1. The empirical relationships among body size, species richness and number of individuals may give insight into the factors controlling species diver sity and the relative abundances of species. To determine these relationshi ps, we sampled the arthropods of grasslands and savannahs at Cedar Creek, M N using sweep nets (90 525 individuals of 1225 species) and pitfall traps ( 12 721 individuals of 92 species). Specimens were identified, enumerated an d measured to determine body size. 2. Both overall and within abundant taxonomic orders, species richness and numbers of individuals peaked at body sizes intermediate for each group. Ev olution could create unimodal diversity patterns by random diversification around an ancestral body size or from size-dependent fitness differences. L ocal processes such as competition or predation could also create unimodal diversity distributions. 3. The average body size of a species depended significantly on its taxonom ic order, but on contemporary trophic role only within the context of taxon omic order. 4. Species richness (S-i) within size classes was related to the number of individuals (I-i) as S-i = I-i(0.5). This relationship held across a 100 00 0-fold range of body sizes. Within size classes, abundance distributions of size classes were all similar power functions. A general rule of resource division, together with similar minimum population sizes, is sufficient to generate the relationship between species richness and number of individual s. 5. Smaller bodied species had slightly shallower abundance distributions an d may, in general, persist at lower densities than larger species. 6. Our results suggest there may be fewer undescribed small arthropod speci es than previously thought and that most undescribed species will be smalle r than arthropods.