Lobster (Homarus americanus) population declines, sea urchins, and 'barren grounds'' a space-mediated competition hypothesis

Abstract

Long-term declines in lobster landings on the Atlantic coast of Nova Scotia, Canada, have been correlated with decreased algal abundance, which is in turn associated with destructive grazingby sea urchins. Explanations for the lobster declines have focused on collapse of kelp-associated food webs and on loss of concealing kelp cover. However, such trophic- and predation-mediated linkages are not strongly supported by available evidence. This paper advances the hypothesis that competition between lobsters and sea urchins for shelter space is a major factor in these declines. Lobsters are strongly shelter-oriented, and shelter availability appears to be a locally limiting factor. Destructive grazing situations feature hgh densities of shelter-seeking urchins. Given the relative lobster and urchin densities involved, it is plausible that urchins could depress shelter availability and, consequently, lobster numbers. Although space for refuge is generally considered a critical resource in benthic marine systems, its importance is easily overlooked in the context of vagile benthic organisms. In light of the present argument, substrate-mediated competition involving such species deserves further attention.