Global climate change drives shifts in the distribution and abundance of species. These range shifts result from direct impacts of changing environmental conditions on species' demography, but also indirect impacts mediated by changing interactions with other species. However, forecasts of the impacts of climate-driven range shifts on biological diversity largely ignore or make oversimplifying assumptions about species' interactions, despite the fact that they ultimately determine species ranges and control which species persist. Here, we use spatial coexistence theory to recast the range dynamics of competitive communities as a range-scale coexistence problem and forecast the persistence of competitors during climate change. We adapt the low-density (invasion) growth rate from theory as a range-level metric of persistence. Using phenomenological models of population growth for multiple migrating competitors, we show how mechanisms of coexistence quantify the impacts of different competitive processes on persistence. Finally, we forecast persistence at the range scale for three competing alpine plant species under future climate conditions, using field measurements across an elevational gradient made across three years.
Results/Conclusions
Spatial coexistence theory can be used to quantify the effect of different competitive processes on persistence in a way that allows numerous different range shift scenarios to be grouped into one of only four general categories. These categories are based on changing benefits of competitor aggregation, benefits of competitor overlap, and whether dispersal rates allow species to track their optimal environmental conditions through space. Although the benefits of aggregation and overlap can change independently, most climate change scenarios are likely to impact persistence through both processes. In plant communities in particular, we find that dispersal has a consistently large impact on species persistence, although its influence is somewhat idiosyncratic; limited dispersal can lead to increased benefits of competitor aggregation, but can also substantially reduce fitness when it prevents species from tracking optimal conditions.