Drift-kelp suppresses sea urchin appetite for destruction

Sea urchins can cause widespread overgrazing of kelp habitat leading to an impoverished ‘urchin barren’ state. The transition to urchin barrens is significant, in part because it can be very difficult to rehabilitate back... [ view full abstract ]

Sea urchins can cause widespread overgrazing of kelp habitat leading to an impoverished ‘urchin barren’ state. The transition to urchin barrens is significant, in part because it can be very difficult to rehabilitate back to the seaweed-dominated state. Thus, it is vital to understand the mechanisms leading to overgrazing in order to prevent it. We conducted several experiments in Port Phillip Bay, Victoria, Australia to understand the circumstances that instigate urchin overgrazing. Time-lapse photography revealed that in both barrens and kelp bed habitats the barren forming urchin Heliocidaris erythrogramma moved actively towards kelp, but when drift algae was present there was less movement regardless of the habitat. A further experiment showed that, in barrens habitat, grazing rates were high on both drift and attached kelp, while in the kelp bed only drift kelp was consumed at high rates and there was virtually no grazing of 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.