The relationship between productivity and species richness

Recent overviews have suggested that the relationship between species richn ess and productivity (rate of conversion of resources to biomass per unit a rea per unit time) is unimodal thump-shaped). Most agree that productivity affects species richness at large scales, but unanimity is less regarding u nderlying mechanisms. Recent studies have examined the possibility that var iation in species richness within communities may influence productivity, l eading to an exploration of the relative effect of alterations in species n umber per se as contrasted to the addition of productive species. Reviews o f the literature concerning deserts, boreal forests, tropical forests, lake s, and wetlands lead to the conclusion that extant data are insufficient to conclusively resolve the relationship between diversity and productivity, or that patterns are variable with mechanisms equally varied and complex. A more comprehensive survey of the ecological literature uncovered approxima tely 200 relationships, of which 30% were unimodal, 26% were positive linea r, 12% were negative linear, and 32% were not significant. Categorization o f studies with respect to geographic extent, ecological extent, taxonomic h ierarchy, or energetic basis of productivity similarly yielded a heterogene ous distribution of relationships. Theoretical and empirical approaches inc reasingly suggest scale-dependence in the relationship between species rich ness and productivity; consequently, synthetic understanding may be conting ent on explicit considerations of scale in analytical studies of productivi ty and diversity.