Linking species diversity to the functioning of ecosystems: on the importance of environmental context

There is currently much interest in understanding how loss of biodiversity might altar ecological processes vital to the functioning of ecosystems. Un fortunately, ecologists have reached little consensus regarding the importa nce of species diversity to ecosystem functioning because empirical studies have not demonstrated any consistent relationship between the number of sp ecies in a system and the rates of ecological processes. We present the res ults of a simple model that suggests there may be no single, generalizable relationship between species diversity and the productivity of an ecosystem because the relative contributions of species to productivity change with environmental context. The model determined productivity for landscapes var ying in species diversity (the number of species in the colonist pool), spa tial heterogeneity (the number of habitat types composing the landscape). a nd disturbance regimes (+/- a non-selective mortality). Linear regressions were used to relate species diversity and productivity for each of the envi ronmental contexts. Disturbance changed the form of the diversity/productiv ity relationship by reducing the slope (i.e. the change in productivity per species added to the colonist pool), but spatial heterogeneity increased o r decreased this slope depending on the particular habitat types composing the landscape. The cause of the diversity/productivity relationship also ch anged with environmental contest. The amount of variation in productivity e xplained bq species diversity always increased with spatial heterogeneity; rr while th; amount of variation explained by species composition (i.e. the particular species composing the colonist pool) tended to increase with di sturbance. These results lead us to conclude that the form and cause of the relationship between species diversity;ind productivity may be highly dyna mic-changing over both time and space. Because the trends resulted from wel l-known mechanisms by which environmental variation alters the absolute and relative abundances of taxa, we suspect this conclusion may be applicable to many different systems.