Presence of the Jonah crab Cancer borealis significantly reduces kelp consumption by the green sea urchin Strongylocentrotus droebachiensis

Abstract

Predators can initiate trophic cascades by reducing densities of their prey, and such density-mediated interactions are well-known in marine environments. Growing amounts of evi- dence suggest, however, that predator-induced modifications in prey behavior and subsequent effects on lower trophic levels, or trait-mediated interactions, may also be of fundamental importance in marine systems. In laboratory experiments we found that green sea urchins Strongylocentrotus droebachiensis, herbivores whose feeding activities can cause shifts between kelp forests and urchin barrens, significantly decreased kelp grazing rates (on average by nearly 80%) in the presence of the echinivorous Jonah crab Cancer borealis. The Jonah crab and the green urchin co-occur across a wide geographic range and our results suggest that C. borealis has the potential to initiate a trophic cascade by controlling the behavior of urchins, which could have important positive effects on their kelp food resources.