Grazing by kelp crabs amplifies impacts of climate-induced bryozoan outbreaks on kelp

Our ability to predict impacts of climate change on ecosystems is limited by complex species interactions. This is particularly the case among the world's temperate oceans where kelp forest ecosystems have experienced dramatic perturbations due to interacting direct and indirect effects of climate change, including climate-induced outbreaks of Membranipora spp., an epiphytic bryozoan. To test the interacting effects of ocean temperature, herbivory, and bryozoan cover on rates of kelp loss, we performed grazing experiments in the field where we manipulated bryozoan cover on kelp and the number and size of two dominant canopy grazers, kelp crabs (Pugettia producta) and turban snails (Tegula pulligo), across a naturally occurring temperature gradient. We found that the rate of kelp loss due to grazing by kelp crabs accelerated with increasing bryozoan cover, and that bryozoan percent (%) cover decreased significantly in presence of kelp crabs. Moreover, we detected a statistically significant, but weak negative effect of temperature on rates of kelp loss in the presence of crabs, functioning in the opposite direction than expected. Comparatively, the effects of turban snails on kelp loss were minimal regardless of bryozoan cover or temperature, nor was there any detectable effect of snails on bryozoan surface area. These findings reveal that grazers can exacerbate the impacts of climate-induced bryozoan cover on kelp loss at the scale of a kelp blade, but that their effects are species-specific. Our study emphasizes how trophic interactions can amplify the magnitude of climate change impacts in some cases and are thus key considerations for marine ecosystem-based management in the face of ongoing warming of temperate oceans.