Predators Can Reverse the Effects of Warming on a Marine Ecosystem Engineer

Although there is abundant evidence of climate change impacts on distributions, population growth, and the phenology of individual species, our understanding of how climate change alters species interactions is more limited. Interactions between ectothermic predators and prey are likely to be especially sensitive to temperature due to these organisms' temperature-dependent metabolism, performance, and behavior. We used three decades of data from kelp forests in California to examine the effect of temperature on interactions between the predatory sunflower star, Pycnopodia helianthoides, and one of its prey species, the purple sea urchin, Strongylocentrotus purpuratus, an ecosystem engineer. In the absence of sunflower stars, higher temperatures increased sea urchin population growth rates. However, the negative effects of sunflower stars on sea urchin population growth rates strengthened with higher temperatures. When sunflower stars were above a threshold density, their negative effect on sea urchins at warmer temperatures outweighed the direct positive effect of temperature, leading to decreasing urchin population growth rates with increasing temperature. These results suggest that recovery of sunflower stars, which were extirpated across much of the west coast of North America between 2013 and 2014, could reduce urchin populations and contribute to the resilience of kelp forests to increasing temperatures. More broadly, these results highlight how species interactions can mediate the direct effects of warming on populations.