Species richness facilitates ecosystem resilience in aquatic food webs

P>1. Many studies indicate that biodiversity in ecosystems affects stability, either by promoting temporal stability of ecosystem attributes or by enhancing ecosystem resistance and resilience to perturbation. The effects on temporal stability are reasonably well understood and documented but effects on resistance and resilience are not. 2. Here, we report results from an aquatic mesocosm experiment in which we manipulated the species richness and composition of aquatic food webs (macrophytes, macro-herbivores and invertebrate predators), imposed a pulse disturbance (acidification), and monitored the resistance (initial response) and resilience (recovery) of ecosystem productivity and respiration. 3. We found that species-rich macroinvertebrate communities had higher resilience of whole-ecosystem respiration, but were not more resistant to perturbations. We also found that resilience and resistance were unaffected by species composition, despite the strong role composition is known to play in determining mean levels of function in these communities. 4. Biodiversity's effects on resilience were probably mediated through complex pathways affecting phytoplankton and microbial communities (e.g. via changes in nutrient regeneration, grazing or compositional changes) rather than through simpler effects (e.g. insurance effects, enhanced facilitation) although these simpler mechanisms probably played minor roles in enhancing respiration resilience. 5. Current mechanisms for understanding biodiversity's effects on ecosystem stability have been developed primarily in the context of single-trophic level communities. These mechanisms may be overly simplistic for understanding the consequences of species richness on ecosystem stability in complex, multi-trophic food webs where additional factors such as indirect effects and highly variable life-history traits of species may also be important.