Drift-kelp suppresses sea urchin appetite for destruction

Sea urchins can cause widespread overgrazing of kelp beds leading to an impoverished ‘urchin barren’ state, which can be very difficult to recover. It is therefore vital to understand the mechanisms leading to overgrazing in order to prevent it in the first instance. Here we show results form a suite of critical experiments in Port Phillip Bay, Victoria, Australia designed to understand triggers of overgrazing by the sea urchin Heliocidaris erythrogramma. We used time-lapse cameras to survey urchin movement in both barren and kelp habitats to reveal active movement of urchins towards kelp in both habitats. Furthermore, in the presence of drift-kelp, we observed less movement independent of habitat type. To further understand the role of drift-kelp in determining destructive grazing, grazing assays were performed, and showed that on grazing rates were high for both drift and attached kelp on barrens, whereas in kelp habitat only drift kelp was consumed and almost no attached algae. Finally, time-lapse monitoring before and after the experimental addition of drift-kelp, clearly demonstrated suppression of urchin foraging in the presence of drift-kelp. Our collective results provide critical experimental support demonstrating that destructive overgrazing of standing kelp beds is triggered when drift-kelp becomes in short supply.