Behaviour of sea urchin Strongylocentrotus droebachiensis grazing fronts: Food-mediated aggregation and density-dependent facilitation

The occurrence of destructive grazing fronts is a common phenomenon in sea urchins, but mechanisms governing front formation and dynamics remain poorly understood. We experimentally examined the effect of kelp biomass on the aggregative behavior and movement of a front of green sea urchins Strongylocentrotus droebachiensis at a wave-exposed site on the Atlantic coast of Nova Scotia. We varied kelp (Laminaria digitata and L. longicruris) abundance in 2 x 2 m plots adjacent to the front in 3 treatments: 50% plant removal, and 100% frond removal and unmanipulated control. In each treatment, we monitored the position of the front and urchin density at the leading edge over 24 d. The mean advance of the front in 24 d (2.27 m) did not differ between treatments, but urchin density was greatest in the control (74.9 urchins 0.25 m ^-2) and lower in whole plant (54.3) and frond (39.4) removal treatments. When urchin density was used as a covariate, front advance was inversely related to kelp biomass and greater in frond and plant removal treatments than in the control. Together, urchin density and kelp biomass explained 75% of variation in front advance. These findings provide the first direct evidence that urchins redistribute themselves along a front to concentrate in patches of greatest food availability. Temporal variation in urchin density at the front was inversely correlated with wave height, and individual grazing rates increased with urchin density, which may explain seasonal variation in front dynamics observed in previous studies.