Aggregative feeding behavior in sea urchins leads to destructive grazing in a Nova Scotian kelp bed

Grazing aggregations of the sea urchinStrongylocentrotus droebachiensisdrive the transition between alternative ecosystem states in Nova Scotia, from productive kelp beds to less productive barrens. This transition can be initiated by the formation of gaps within a kelp bed, containing dense aggregations of sea urchins. We examined the importance of local density of sea urchins and pre-existing gaps in a kelp canopy in mediating the formation of destructive grazing aggregations of sea urchins in a kelp bed. We transplanted 14 000 adult sea urchins from a barrens on the Atlantic coast of Nova Scotia into ~4.5 m² plots within a nearby kelp bed, at densities above and below a predicted threshold value for destructive grazing, and simulated disturbance to the kelp bed by removing the kelp canopy in half of the plots. Sea-urchin abundance and gap formation and expansion (as loss of kelp canopy cover) were monitored in and around plots weekly for 9 wk. Grazer-mediated gap formation began 3 wk after sea urchins were introduced, and increased for the remainder of the experiment. Our results indicate a direct linear relationship between sea-urchin abundance and increase in gap area within undisturbed treatments. Gaps expanded in the kelp bed at sea-urchin densities below the putative threshold for destructive grazing, indicating that the kelp bed was less resilient to grazing than predicted. Our findings provide insights into mechanisms controlling the stability of the kelp-bed ecosystem state and mediating shifts from kelp beds to barrens in Nova Scotia.