Sea urchins have the capacity to destructively overgraze kelp beds and cause a wholesale shift to an alternative and stable 'urchin barren' reef state. However, destructive grazing appears labile and contingent on behavioural shift. Changes in supply of allochthonous 'drift-kelp' food are hypothesised as a trigger of change in urchin grazing behaviour, yet field tests are lacking. Here we conduct a suite of in situ behavioural surveys and manipulative experiments within kelp beds and on urchin barrens to examine foraging movements and evidence for a behavioural switch to an 'overgrazing mode' by the Australian urchin Heliocidaris erythrogramma (Echinometridae). Tracking urchins using time-lapse photography revealed urchin foraging to conform to a random walk model within kelp beds and on barrens. However, many individuals tended towards local movement within proximal crevices and movement was reduced in kelp beds compared to barrens. Directional movement of urchins toward newly available kelp was experimentally inducible, consistent with locally observed 'mobile-feeding-fronts' that develop at barrens-kelp interfaces. Habitat-specific feeding modes were also evidenced by herbivory assays which revealed urchin grazing rates to be high on both drift-kelp and standing kelp on barren grounds, while drift-kelp but not standing kelp was consumed at high rates within kelp beds. Time-lapse tracking of urchin foraging before/after addition of drift-kelp revealed a reduction in foraging across the reef surface after drift-kelp capture. Collectively, results indicate that the availability of drift-kelp is a pivotal trigger in determining urchin feeding modes which thus mediates the shift between alternative stable states for rocky reef ecosystems.
