Reducing per capita food supply alters urchin condition and habitat

When food supply declines or population density increases, the per capita food availability is reduced causing a decline in condition of consumers. Many consumers alter their feeding behaviour and ultimately the surrounding community (e.g. overgrazing and formation of urchin barrens). This study tested the hypothesis that sea urchin populations are of greater density and poorer condition in barrens (little food) than forest habitat (lots of food). We then tested the hypothesis that a decrease in per capita food supply to urchins has a negative effect not only on their condition but also on their surrounding habitat. We experimentally assessed the effect of limited food supply and increased density of a subtidal Australian sea urchin (Heliocidaris erythrogramma) on their condition (i.e. gonad index) and surrounding benthic habitat (i.e. turf-forming algae). Our results show that a reduction in food supply led to poorer consumer condition and greater herbivory on surrounding local habitat. We provide evidence that per capita food reduction is one of the necessary conditions for the over-consumption by urchins (i.e. urchin barrens), a proposed but previously untested mechanism.