Productivity and species richness in an arid ecosystem: A long-term perspective

There is little consensus on the form of the relationship between biodiversity and productivity, and most of the research examining it has been done in temperate grasslands, with arid ecosystems receiving comparatively little attention. Using 9 years of data collected using standardized sampling methods from five different community types in the Chihuahuan Desert (Jornada Basin Long-Term Ecological Research site, New Mexico, USA), we evaluate the relationship between productivity and species richness at multiple spatial scales. The relationship was consistently unimodal at the largest scale considered; however, community types differed in both the form of the relationship and the degree to which patterns fluctuated over time. Although significant linear and unimodal patterns were evident, these models fit poorly, suggesting that annual primary productivity is a weak predictor of mean species richness in arid communities. We suggest that productivity-diversity relationships are best described by a boundary that places an upper limit on species coexistence across the gradient of productivity for a site. This upper limit often appears to be maximized at intermediate levels of productivity, but it has relatively few points on the downward portion of the boundary. By focusing efforts on characterizing boundaries, approaches should be developed to determine the factors that are reducing species richness compared to what would be expected, given the level of productivity.