The effects of predator size and population on the urchin, Strongylocentrotus spp. in the Channel Islands
Mentor:Bob Warner, Research Professor of Evolutionary Ecology, University of California Santa Barbara
It has been found that the increased fishing pressure on two major predators, the sheephead, Semicossyphus pulcer, and the lobster, Panulirus interruptus, has lead to an increase in the abundance of a major Channel Island herbivore, the Strongylocentrotus spp. The release of top-down control has lead to a trophic cascade on their own prey, Macrocystis pyrifera, leading to a localized phenomenon known as urchin barrens. Urchin barrens are areas with a high density of exposed sea urchins and low kelp cover. Since reserve establishment in 2003 there has been an increase in predator size and abundance. This study examines whether increased size and number of predators cause the urchins to shift from exposed to cryptic habitat preference and the role of habitat complexity in this shift. We quantified the abundance of cryptic versus exposed urchins in sites with different predator population structure and habitat complexity within the Channel Islands. We found that a higher fraction of larger urchins sought refuge in areas with larger predators. This is important because urchins that seek spatial refuge do not graze on kelp at such a high frequency. This change in urchin behavior caused by the increased size of predators allows kelp densities to increase in reserve areas.