Species abundance distributions reveal environmental heterogeneity in modified landscapes.
Abstract
Environmental heterogeneity influences biodiversity patterns such as species richness and community composition, but we do not know how it shapes community structure in terms of the number of rare and abundant species. This question is particularly relevant to modified landscapes such as agro-ecosystems, where intensive management creates highly homogeneous landscapes, which often lead to loss of rare taxa and dominance by a few aggressive species. It can be difficult to evaluate the effects of management on communities in different localities due to regional differences in species richness and composition. Because species abundance distributions (SADs) express assemblage structure in comparable units, they can be used to characterize communities irrespective of species composition, and for this reason provide a novel means of assessing the effects of management. We analysed the SADs of weed communities across a gradient of environmental heterogeneity generated by different management treatments. We tested the hypothesis that environmental heterogeneity promotes multimodality in SADs by comparing observed patterns with those predicted for simulated random samples from homogeneous communities. Observed SADs from a homogeneous environment did not deviate from predictions. By contrast, SADs from increasingly heterogeneous environments were progressively and consistently multimodal. These results demonstrate that environmental heterogeneity, resulting from variation in management treatments, leaves a signature on the SAD. Synthesis and applications. Our study shows that Species Abundance Distributions are informative indicators of environmental heterogeneity in modified landscapes. Weed communities include both rare species, which are often of conservation interest and pose little threat to crops, and abundant species, which can be problematic. Our analysis suggests that creating environmental heterogeneity by varying management treatments across the landscape can be an effective way of promoting biodiversity and decreasing the abundance of problematic species.