Understanding how the structure and stability of food webs respond to environmental gradients remains a fundamental goal of ecology. In freshwater communities, body size is a strong organizing principle, which can determine both the presence and strength of predator-prey interactions. The distribution of interaction strength can greatly affect the stability of a community, with many weak links being generally more stable than the presence of a few strong links. However, few studies have examined how these dynamics respond to environmental gradients. Here we assessed food-web structure in 24 stream communities across a gradient of acid mine drainage (AMD) inputs. AMD is known to greatly affect community composition and abundance, with the removal of sensitive species, and the resulting communities being dominated by a few tolerant species. However, it is unknown how AMD affects the distribution of biomass within communities. All individuals had their length measured, and biomass estimated using length-weight regressions. Information on body size and relative abundances were used to estimate the probability of predator-prey interactions and metabolic theory was used to estimate the strength of pairwise species interactions, and the local stability of the communities was assessed.
Results/Conclusions
Community composition and biomass distributions were significantly altered in response to the AMD gradient. There was a reduction in species richness and total community abundance, as well as a complete removal of larger taxa in the most heavily impacted sites. Larger taxa generally occupy a higher trophic level and are more well-connected in the community, and their removal has important implications on food-web structure. The reduction in community diversity, coupled with the change in biomass distributions, resulted in food webs with fewer feeding links and weaker interaction strengths, across the AMD gradient. This in turn led to impacted communities being more stable than un-impacted communities. These findings have important implications in the management and restoration of communities which have reduced biodiversity and simplified biomass distributions, and may explain the observed lag in biotic recovery after the restoration of physical or chemical conditions.