A latent threat to biodiversity: consequences of small-scale heterogeneity loss

The threat of homogenisation to biodiversity is generally considered to occur at broad scales or in response to high-intensity impacts. Therefore, most biodiversity studies estimate local average or total species richness rather than local heterogeneity. Here we consider the potential for relative shifts between these different aspects of biodiversity at small spatial scales to be an early warning signal for biodiversity loss. In response to chronic, very low-level pollution, we observed a disjunctive response with gamma diversity (total species richness) and beta diversity (heterogeneity) decreasing while alpha diversity (average species richness) was still increasing. Homogenisation may, therefore, affect biodiversity through thresholds that alter the relationship between the average species richness and its heterogeneity, leading to the potential for regime shifts. Our stressor also had a strong negative effect on rare species, meaning that the purported importance of rare species as "insurance" in the face of environmental change may be overstated.