Is biodiversity declining at local scales or not? A critique of recent high-profile claims of no net loss of local biodiversity
For many decades, the common dogma in ecology has been that human transformation of the planet is generating localized species extirpations that are reducing biodiversity. Recently, a sequence of papers published in high-profile journals have challenged this dogma and suggested that claims about biodiversity loss at local scales have been exaggerated. Specifically, Vellend et al. (Proceedings of the National Academy of Science, 2013) and Dornelas et al. (Science, 2014) both performed data syntheses using datasets in which researchers had repeatedly sampled the same ecosystem through time and measured changes in biodiversity. In their analyses, both authors found that some locations have experienced a loss of diversity, whereas others have experienced increases in diversity. Overall, they found that the mean effect has been no net change in biodiversity at local scales. Both sets of authors argued that their conclusions hold true across the planet as a whole, and they have begun to cite their own work in synthesis and review papers in which they suggest it is now well-established that biodiversity is not declining at local scales.
Here we re-evaluate the datasets collated by Vellend et al. and Dornelas et al., and critique both their analyses and findings. We argue that the conclusions of these authors about local biodiversity loss are fundamentally flawed for four reasons: (1) the datasets used for their syntheses represent a highly biased subset of locations on the planet that greatly under-represents the known drivers of biodiversity change at local scales, (2) the datasets analyzed were not intended for their present use, which is problematic because they do not establish pre-impact baselines that allow one to determine if biodiversity change is actually caused by a human impact, (3) the authors incorrectly mix studies focused on how biodiversity responds to human impacts with studies of how biodiversity recovers from human impact, which leads them to draw erroneous conclusions, and (4) the authors did not adequately account for the time-scales over which species go extinct, causing them to underestimate extinction rates. We end by outlining a path forward for improving estimates of local biodiversity change given the available data, and for obtaining the data needed to quantify and understand local-scale biodiversity change.