Forming sea urchin barrens from the inside out: an alternative pattern of overgrazing

Overgrazing by sea urchins on temperate reefs can affect a phase shift from macro algal beds to 'barrens' habitat largely devoid of seaweeds. Existing models of barrens formation are derived largely from observations of strongylocentrotid urchins, which typically show a behavioural shift from cryptic feeding to exposed grazing fronts that move through and 'mow down' macroalgal beds. Foraging by the temperate diadematid urchin Centrostephanus rodgersii triggers a similar transition from intact macroalgal bed to widespread barren grounds but does not appear to involve a behavioural shift. Fine-scale foraging movements were observed using time-lapse photography across the urchin's range-extension region and described with respect to a random walk model. Foraging was highly nocturnal, with individuals homing strongly to available crevices. In situ monitoring of tagged individuals suggests strong fidelity to and thus high stability of barren patches, while similar behavioural patterns across habitat types representing a gradient of foraging intensities indicate no behavioural shift associated with overgrazing. Laboratory experiments showed that C. rodgersii lacks a directional chemosensory response to either macro algae or conspecifics. Combined evidence suggests a model of barrens formation fundamentally different to the well-established 'feeding front' model, with formation of widespread barrens by C. rodgersii occurring from the 'inside out' via growth and coalescence of small barrens patches that form within macroalgal beds as a result of additive localised grazing radiating from crevice shelters. Regulation of urchin density at the spatial scale of individual barrens patches is proposed as a viable option to manage the formation of widespread barrens habitat within the urchin's recent range-extension to eastern Tasmania.