The combined effects of scale and productivity on species richness

Summary 1  Although species–area curves are well documented, we know little about how they vary along productivity gradients, nor do we have a firm understanding of whether the richness–productivity relationship is independent of scale. In order to investigate these relationships, 10 species–area curves were constructed for 10 sets of nested quadrats (from 0.25 m 2 to 150 m 2 ) in herbaceous marshes where biomass ranged from 128 g m –2 to 1050 g m –2 . 2  Slopes ( z ) of the species–area curves (logS = logc + z logA) were positively correlated with production (whether measured as biomass or soil organic content). A possible cause of increased slope was larger potential species pools at high biomass. The constants (c) were negatively associated with production, which concurs with the general richness–productivity relationship. 4  Linear models were used to predict both c and z using biomass and soil organic content. Biomass explained about 38% of the variation in both z and logc. Soil organic content was a better predictor. It explained 60% of the variation in z and 41% of the variation in c. 5  The slopes ( z ) and constants (c) of the species–area curves were negatively correlated. Because c is the expected species richness at 1 m 2 and z strongly affects larger‐scale richness, the results suggest that small‐scale richness is not a good indicator of larger‐scale richness. There was no significant relationship between species richness in small‐scale (0.25 m 2 ) quadrats and large‐scale quadrats (150 m 2 ). 6  These results suggest that the conflicting data that have led to recent debate over the richness–productivity relationship may reflect both the effects of scale and the natural history peculiarities of the systems studied.