The present is the key to the past: linking regime shifts in kelp beds to the distribution of deep-living sea urchins.

Understanding processes that drive sudden shifts in ecosystem structure and function has become an important research focus for coastal management. In kelp bed ecosystems, regime shifts occur when high densities of sea urchins destructively graze kelp and create coralline algal barrens. While the importance of predation and disease in mediating shifts between kelp beds and barrens on shallow rocky reefs has been well documented, little is known about the role of deep-living urchins in these alternative stable-state dynamics. In this study, we test the hypothesis that deep-living urchins along the central Atlantic coast of Nova Scotia move onshore and trigger shifts from kelp beds to barrens on shallow rocky reefs. We documented urchin distribution and abundance using tow-camera surveys down to 140 m depth and spanning 140 km of coast and created a predictive species-distribution model using these observations and spatial data on environmental factors that likely delineate suitable habitat for urchins. We used a random forest model to generate our predictions, which correctly classified 91% of observations into a positive or negative occurrence of urchins. Sea urchins predominantly occurred within 1.5 km of shore, in depressions and flat habitats between 40 and 85 m depth. We found that shallow regions where destructive grazing fronts have been documented over the past four decades were closer to deep-living sea urchin habitats compared to regions that remained in a kelp bed state during the same period. This supports our prediction that deep-living urchins play an important role in driving shallow regime shift dynamics, and indicates that their distribution can help identify areas of coast that are most vulnerable to a collapse to barrens.