The influence of species diversity on ecosystem productivity in different environments: Experimental plant communities and theory

The relationship between species diversity and the functioning of ecosystems is one of the most critical but contentious issues in contemporary ecology, and plant ecologists are strongly divided over whether ecosystems of greater plant diversity exhibit higher rates of biomass production. Experimental evidence that plant species diversity has positive effects on production appears to conflict with correlations of species diversity and standing biomass in natural communities. This may be due to the confounding effects of environmental factors, such as resource availability, that exert strong control over both diversity and productivity in natural systems. Unfortunately, the extent to which environmental conditions influence the diversity-productivity relationship is unknown; it is, therefore, difficult to establish the particular ecosystems in which species loss most affects ecosystem functioning.; I performed two field experiments using microcosm plant communities to explore whether the influence of plant species diversity on ecosystem productivity varies with environment. In the first experiment I constructed communities of differing species diversity and composition along a soil fertility gradient. Although the effects of fertility on production were far greater than that of diversity, diversity effects became more pronounced as fertility increased. In a second experiment where similar communities with different levels of diversity were grown in different nutrient and light environments, the diversity-fertility interaction in the control of production was determined to be the result of light partitioning by species of different growth habits. These results suggest that, although diversity effects on production should generally be small compared to those associated with environmental factors, small-scale plant diversity may enhance productivity under certain environmental conditions.; I extended these experimental results by exploring three species interaction models within the context of the diversity-productivity relationship. As a whole, these theoretical models support observations that diversity effects increase with the resource availability, but by means that depend on whether species coexist by resource partitioning or processes resulting from non-equilibrium dynamics. Resource partitioning begets complementary resource use, where many species are needed to maximize resource harvesting. Coexistence by density-independent perturbations begets diversity effects that are related to dominance by species with particular resource-use traits.