Senior Researcher University of California Davis, United States
Abstract: Under climate change, we expect gradual changes in environmental conditions to gradually change species interactions and abundances. In systems where ecological feedbacks create tipping points, however, changes in species interactions can shift tipping points such that even small perturbations trigger shifts among alternative stable states. Therefore, in some systems gradual changes in environment could lead to sudden and persistent ecosystem collapse. To investigate whether this could occur on subtidal reefs dominated either by kelp or by urchin herbivores, we combine 37 years of kelp canopy satellite data across the US west coast, diver surveys, and oceanographic data.
We find that warmer ocean climate phases trigger urchin overgrazing and kelp forest loss throughout southern California. These shifts accompany large increases in the numbers of detected urchins, confirming recent work that behavioral changes in herbivore grazing activity underpin kelp forest collapse and recovery. Fitting process-based dynamical models of kelp forest collapse, we find that warmer temperatures increase herbivore grazing activity, thereby shifting tipping points. Because localized disturbances govern stochastic transitions among kelp and urchin barrens locally, the effect of climate is apparent only at >50km scales. Across the region, our best-fitting models explain 31% of variation in historic kelp forest presence and suggest that projected ocean warming in southern California could drive widespread loss of giant kelp forests. This impact could be offset by reducing urchin densities through local fisheries management.