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 <i>Centrostephanus rodgersii</i> 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 timelapse 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. <i>In situ</i> 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 <i>C. rodgersii</i> 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 <i>C. rodgersii</i> 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.