Temperate reefs are sensitive to fluctuations in grazing intensity and many species lack adaptations to withstand high grazer densities. The presence of foliose macroalgae in the face of high grazing pressure can have important consequences for higher trophic levels, in part because macroalgae harbor prey for fish and macroinvertebrates. Here I show how the strawberry anemone, Corynactis californica, creates a refuge for benthic macroalgae and invertebrate fish prey on intensively-grazed shallow rocky reefs in the Santa Barbara Channel off Southern California (USA). On reefs heavily grazed by the purple urchin (Strongylocentrotus purpuratus) and red urchin (S. franciscanus), benthic macroalgae and invertebrate fish prey were relatively more abundant among Corynactis colonies than adjacent areas lacking the anemone. Field surveys revealed that foraging sea urchins avoid touching Corynactis polyps. In laboratory studies, the movement of urchins was impeded when urchins withdrew their tube feet upon contact with Corynactis. Results from field experiments showed that Corynactis facilitated the recruitment of macroalgae and tubicolous amphipods in “urchin barren” areas subjected to intensive grazing. In areas forested by giant kelp (Macrocystis pyrifera ), where grazing intensity from urchins was low, Corynactis suppressed algal recruitment but facilitated tubicolous amphipods.

A manipulation of fish and sea urchins suggested that grazing by urchins, as opposed to predation from fish (primarily surfperch Embiotocidae), suppressed tubicolous amphipods and this activity was hindered by the presence of Corynactis.

The net effect of Corynactis on turf algae was strongly non-linear over a gradient in density of sea urchins. At low intensities of urchin grazing, the anemone and macroalgae competed for space, with algae capable of overgrowing Corynactis. At intermediate grazing intensities, Corynactis provided a refuge for turf algae but not for juvenile kelp. Neither turf algae nor kelp benefited from the presence of Corynactis at the highest levels of grazing intensity, as sea urchins consumed nearly all macroalgae. The hump-shaped effect observed for Corynactis contrasts with the prevailing view in ecological theory that positive interactions are more common in harsh environmental conditions. The results reported here qualify this view and underscore the need to evaluate positive interactions over a range of consumer pressure.