Sea Urchin (Strongylocentrotus droebachiensis) Aggregating Behavior Investigated by a Subtidal Multifactorial Experiment

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Abstract
We performed a multilevel factorial field experiment to identify the effects of five factors on sea urchins' (Strongylocentrotus droebachiensis) aggregating behavior. The factors were (1) source of urchins (kelpbed or barrens), (2) density of urchins (high or low), (3) location of treatment (kelpbed or barrens), (4) the presence and type of invertebrate predators (crabs or lobsters), and (5) season (summer or winter). These manipulative experiments were performed in flexible, resilient cages designed to withstand the severe wave surge in Nova Scotia's shallow subtidal environment. Interactions identified by ANOVA among the various factors showed that urchins aggregate more at high (20∙m−2) than at low (4∙m−2) density. The presence of lobsters in the kelpbed, and of crabs in the barrens, triggered the formation of even larger aggregations. These aggregations remain in the open even in the presence of predators. We argue that this behavioral mechanism (a defensive aggregation response to lobsters in the kelpbed) is the trigger that precipitates widespread destructive urchin grazing and the transformation of kelpbeds to barrens. Lobsters thus play two opposite roles. At low urchin density lobsters keep urchins in hiding and thereby contribute to kelpbed persistence. At higher urchin density, lobsters trigger the formation of large, exposed urchin aggregations that graze destructively on kelp. Urchin responses to predators are probably mediated by a combination of conditioning and sensitivity to biochemical cues. The large-scale change in community structure, from kelpbed to barrens, can thus be understood in terms of the adaptive behavioral responses of individual organisms. We review the roles of other predators in this system and show that they have varying effects on the intensity of urchin grazing, depending on urchin density, season, and habitat type.