Distributional shifts and biodiversity loss are controlled by the combination of warming and Keystone predation

Background/Question/Methods One of the most striking ecological manifestations of anthropogenic climate change is the broad scale redistribution of species across gradients of latitude, elevation, and depth. Although range edges are determined by both abiotic conditions and interactions among species, we know remarkably little about how species interactions modify patterns of climatically-forced ecological change. Here, I use a 200 km spatial gradient in temperature and a comparison across 52 years in time to investigate the extent to which spatial and temporal patterns of intertidal invertebrate distribution and abundance depend on both temperature and predation. Present-day zonation patterns of sessile invertebrates and their main predator were documented and compared against in situ temperature records at six sites in the Salish Sea (Washington USA and British Columbia Canada). Patterns of temporal change were established by resurveying 11 sites for which historical zonation data were available – climatic conditions have warmed by approximately 1 °C in the half-century between surveys. The role of predation was determined with predator exclusions experiments.
Results/Conclusions The upper limits of benthic invertebrates shifted downwards with increasing temperature in both space and time, while lower limits – determined by predation by the sea star Pisaster ochraceus – remained spatially and temporally invariant. Owing to increased range overlap in warmer conditions, Pisaster was able to eliminate certain prey species when high temperatures prevailed. Pisaster exclusion experiments demonstrated that prey species can become established at a ‘hot’ site if predation pressure is reduced. Furthermore, because vulnerable prey species are important ecosystem engineers, local species richness more than doubled as a result of predator exclusion. The large-scale patterns in space (range limits and species richness) and long-term changes in time (41-51% reductions in vertical range and two local extinction events) identified here are thus explainable by the combined influence of thermal stress and predation pressure. By altering the importance of interspecific relationships such as competition, predation, parasitism, and facilitation, climate change may produce unexpected changes in species distributions, community structure, and diversity.