What is the observed relationship between species richness and productivity?

Understanding the relationship between species richness and productivity is fundamental to the management and preservation of biodiversity. Yet despit e years of study and intense theoretical interest, this relationship remain s controversial. Here, we, present the results of a literature survey in wh ich we examined the relationship between species richness and productivity in 171 published studies. We extracted the raw data from published tables a nd graphs and subjected these data to a standardized analysis, using ordina ry least-squares (OLS) regression and generalized linear-model (GLIM) regre ssion to test for significant positive, negative, or curvilinear relationsh ips between productivity and species diversity. If the relationship was cur vilinear, we tested whether the maximum (or minimum) of the curve occurred within the range of productivity values observed (i.e., was there evidence of a hump?). A meta-analysis conducted on the distribution of standardized quadratic reg ression coefficients showed that the average quadratic coefficient was nega tive (i.e., the average species richness-productivity relationship was curv ilinear and decelerating), and that the distribution of standardized quadra tic regression coefficients was significantly heterogeneous.(i.e., the stud ies did not sample the same underlying species richness-productivity relati onship). Looking more closely at the patterns of productivity-diversity relationship s, we found that, for vascular plants at geographical scales smaller than c ontinents, hump-shaped relationships occurred most frequently (41-45% of al l studies). A positive relationship between productivity and species richne ss was the next most common pattern, and positive and hump-shaped relations hips co-dominated at the continental scale. For animals, positive, negative , and hump-shaped patterns were common at most geographical scales, and no one pattern predominated. For both plants and animals, hump-shaped curves w ere relatively more common in studies that crossed community boundaries com pared to studies conducted within a community type, and plant studies that crossed community types tended to span a greater range of productivity comp ared to studies Within community types. Sample size and plot size did not a ffect the probability of finding a particular productivity-diversity relati onship (e.g., positive, hump-shaped, etc.). However, hump-shaped curves wer e especially common (65%) in studies of plant diversity that used plant bio mass as a measure of productivity, and in studies conducted in aquatic syst ems.