Drift-kelp suppresses sea urchin appetite for destruction

Sea urchins can cause widespread overgrazing of kelp habitat leading to an impoverished ‘urchin barren’ state, which can be very difficult to recover back to kelp habitat. It is vital to understand the mechanisms leading to overgrazing in order to prevent it. Here we conducted several experiments in Port Phillip Bay, Victoria, Australia to understand what instigates urchin overgrazing. We used multiple time-lapse camera systems to examine urchin movement in both barren and kelp habitats. In both habitats the barren forming urchin Heliocidaris erythrogramma, showed active movement towards kelp, furthermore, when drift algae was present we observed less movement regardless of the habitat. To further understand the role drift kelp plays an experiment examining grazing was undertaken. In barrens habitat grazing rates were high for both drift and attached kelp, whereas in kelp habitat only drift kelp was consumed and almost no attached algae. Our results provide strong support for the notion that destructive overgrazing of standing kelp beds only occurs once the subsidy of drift kelp becomes in short supply.