Climate change is one of the greatest threats facing global biodiversity and can directly impact organisms via metabolic pathways, as well as indirectly through altered trophic interactions. In intertidal ecosystems, environmental stress and biological interactions are equally important drivers of community composition and distribution patterns, as heat and desiccation tolerance set upper biological limits while competition and predation set lower limits. Within these communities, foundation species and keystone predators drive biodiversity by increasing habitat and reducing competitive exclusion. Here, we assess the direct and indirect effects of environmental stress on keystone predators and foundation species to better understand how rising temperatures will impact intertidal communities and biodiversity. At 14 sites along a thermal gradient spanning the US West Coast, we surveyed the densities and vertical distributions of keystone predators (sea stars), intermediate predators (whelks), and foundation species (mussels and barnacles). Using general linear models, we analyzed the effects of thermal stress (daily, monthly, and yearly mean and max) and keystone predation (density and vertical extent) on foundation species’ vertical extents and overall ranges. We used generalized linear models to analyze the effects of temperature, tide height, and their interaction on spatial overlap between predators and prey.
Across the thermal gradient, temperature had a greater effect on intermediate predators than foundation species. Predators and prey were less likely to co-occur as temperature increased, particularly in the mid and high intertidal zones. This spatial mismatch was driven primarily by intermediate predators, as there was no relationship between temperature and the presence of foundation species. Similarly, temperature was not the primary driver of vertical distribution patterns. Keystone predator distribution patterns also did not affect either intermediate predators or foundation species. Recent declines in asteroid populations as a result of sea star wasting syndrome (SSWS) may have contributed to our findings that intermediate predators drove spatial overlap patterns and, thus, may play an increasingly important role in intertidal communities. Our study highlights the direct and indirect pathways through which climate change can affect intertidal communities, specifically the potential for spatial mismatch of predators and prey and the resulting alterations in biodiversity and community composition patterns.