Forecasting range shifts using abundance distributions along environmental gradients

Globally, many species’ distributions are shifting in response to contemporary climate change. However, the direction and rate of shifts remain difficult to predict, impeding managers' abilities to allocate resources most effectively. Here, we explore a new approach for forecasting species' range-limit shifts that requires only abundance data along environmental (eg elevational) gradients. We hypothesized that species’ abundance distributions could provide information on the likelihood of future range-limit shifts. We tested this prediction using data from several transect studies that compared historical and contemporary distributions. Consistent with our prediction, we found that strong asymmetry in abundance distributions (ie “leaning” distributions) indeed preceded species’ lower-limit range shifts (Fisher's exact test P < 0.001, R2 = 0.28). Accordingly, surveying abundances along environmental gradients may be one promising, cost-effective method for forecasting local shifts. Ideally, practitioners will be able to incorporate this approach into species-specific management planning and to inform on-the-ground conservation efforts.

Methods
Data were extracted from eight cited peer-reviewed studies in total. These studies cover a range of species, biomes, and regions across the globe. The accompanying data here have been centralized and processed to include the key metrics we discuss in our manuscript such as the historical and current range limits, species' midpoints, optimum elevations, leans in meters, leans as percentages, and lower-range limits shift rates per year and decade.