Species traits, interspecific interactions, and the effects of biodiversity on wetland ecosystem processes

Recent rates of species extinction have focused attention on the values of biodiversity, one of which is the role of biodiversity in maintaining ecosystem processes and stabilizing these processes. Theoretical and empirical studies have demonstrated that increased biodiversity can enhance ecosystem processes (e.g., primary production, nutrient cycling) and can increase resistance and resilience of ecosystems to disturbances. These effects are thought to be caused by a greater use of resources (“niche differentiation effect”), or by a greater chance of occurrence of a particularly competitive species with large effects on ecosystem processes (“sampling effect”) in species rich communities. I conducted several experiments that tested the effects of submersed aquatic macrophyte species, species composition, and species richness on wetland ecosystem processes. I demonstrate that greater numbers of submersed aquatic macrophyte species decreased total phosphorus losses from the mesocosms and increased total aboveground productivity (macrophyte and algal biomass), suggesting that wetland ecosystem functioning and services may indeed be enhanced by species richness. Contrary to expectations, species richness did not increase shoot, root, and total macrophyte biomass. Morphological traits differed among species, and these traits could explain a species' competitive ability for light and soil resources to some extent. However, species' effects on the physical and chemical environment, effects that could not be adequately explained by morphological traits, produced strong inter-specific competition and priority effects, where the best competitor was not the most productive species, nor the species depleting resources to the lowest levels. These strong competitive interactions between aquatic macrophytes can sometimes prevent niche differentiation and reverse the outcome of sampling effects. Thus, species richness may not always enhance ecosystem functioning, especially in systems where greater resource use by multiple species is prevented by species' effects on the chemical and physical environment.