Destructive grazing by sea urchins Strongylocentrotus spp. in a central California kelp forest: potential roles of recruitment, depth, and predation

Abstract

In January 1986, a sea-urchm mediated deforestation began in C a m e l Bay, California. USA, near the center of the California sea otter's range. The deforestation occurred on a n offshore rocky reef where depths range from l ? to > 30 m. Both purple (Strongylocentrotus purpuratus) and red (S. franciscanus) sea urchins were abundant and probably belonged to a cohort that settled in 1984. By October 1986, nearly all non-crustose algae had been removed, as were most sessile invertebrates (excluding the cnidarians Balanophyllia elegans and Corynactis californica). Concurrent samples taken from 2 nearby forested sites at comparable depths showed no evidence of overgrazing by urchins. Populations of red sea urchins at the deforested site remained fairly stable, but purple sea urchins declined to zero by July 1989. Algal spores were available during the deforestation, but recruitment of seaweeds d d not occur until after sea urchln abundance had decllned Reasons for the decline of the purple sea urchin population are not clear, although predation by sea otters 1s unlikely. Circumstances surrounding these events indicate kelp forest deforestations may b e controlled not only by mortality of adult sea urchins (from predation or disease) and fluctuation of their food supply, but also by factors affecting recruitment such a s larval abundance and early post-settlement sunrival.

Organic enrichment of submarine‐canyon and continental‐shelf benthic communities bymacroalgal drift imported from nearshore kelp forests

“…Temporal variation in M. pyrifera density and biomass were never significant, despite the conspicuous wintertime thinning or removal of the nearshore kelp canopy (Watanabe and Harrold 1991) and clearing of both attached (e.g. Seymour et al 1989) and drift (e.g.…”

Intra- and interspecific differences in palatability of Arctic macroalgae from Kongsfjorden (Spitsbergen) for two benthic sympatric invertebrates

“…Here, except for few small patches of Alaria esculenta interspersed with two Desmarestia species, most of the macroalgal canopy (mainly kelps) has disappeared due to high consumption rates. These areas have been previously described as bbarren groundsQ (Lawrence, 1975;Arnold, 1976;Chapman, 1981;Mann, 1982;Hagen, 1983;Schiel, 1982;Dean et al, 1984;Harrold and Pearse, 1987;Watanabe and Harrold, 1991).…”

Effect of temperature on settlement and postsettlement survival in a barrens-forming sea urchin

“…In subtidal ecosystems, destructive grazing due to sea urchin population outbreaks frequently causes an ecological disequilibrium, a situation that has been observed globally (Lawrence 1975, Watanabe & Harrold 1991, Hagen 1995, Guidetti & Sala 2007, Ling 2008, Hernández et al 2008a. Urchin hyperabundance results in unproductive barrens that are highly persistent (Knowlton 2004) and can damage the local economy (Hughes et al 2005).…”