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

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Flukes, EB and Johnson, CR and Ling, SD (2012) Forming sea urchin barrens from the inside out: an alternative pattern of overgrazing. Marine Ecology Progress Series, 464. pp. 179-194. ISSN Print 0171-8630 Online 1616-1599

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Abstract

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 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. 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.

Item Type: Article
Keywords: Phase shift · Kelp beds · Barrens · Centrostephanus rodgersii · Foraging ecology · Movement · Grazing effect · Range extension
Journal or Publication Title: Marine Ecology Progress Series
Page Range: pp. 179-194
ISSN: Print 0171-8630 Online 1616-1599
Identification Number - DOI: 10.3354/meps09881
Additional Information:

Copyright Inter-Research 2012 · www.int-res.com

Date Deposited: 19 Mar 2013 00:58
Last Modified: 18 Nov 2014 04:49
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