Grazing by sea urchins at the margins of barren patches on Mediterranean rocky reefs
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ORIGINAL PAPER
Grazing by sea urchins at the margins of barren patches on Mediterranean rocky reefs Fabio Bulleri
Received: 20 December 2012 / Accepted: 10 April 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract The role played by the urchins, Paracentrotus lividus and Arbacia lixula, in the formation and persistence of barren areas dominated by encrusting coralline macroalgae is yet to be fully elucidated. This study, carried out in the NW Mediterranean (43° 300 N, 10° 200 E) between February 2005 and April 2006, investigated how the loss or density decrease in one or both urchin species influences the recovery of erect macroalgal stands (dominated by filamentous forms) at the margins of barren areas. At a depth of 4–6 m, three barren patches were assigned to each of the following treatments: (1) control (natural densities of A. lixula and P. lividus); (2) 50 % of the natural density of A. lixula and natural density of P. lividus; (3) total removal of A. lixula and natural density of P. lividus; (4) 50 % of the natural density of P. lividus and natural density of A. lixula; (5) total removal of P. lividus and natural density of A. lixula; (6) 50 % of the natural densities of both A. lixula and P. lividus; (7) total removal of both A. lixula and P. lividus. The effects of the herbivore treatments were evaluated either in the presence or the absence of encrusting corallines. The partial or total removal of A. lixula, P. lividus or both favored the proliferation of filamentous macroalgae at the margins of barren patches. The presence of encrusting corallines reduced the development of these macroalgae. The results of this study suggest that a
Communicated by J. Grassle.
Electronic supplementary material The online version of this article (doi:10.1007/s00227-013-2244-2) contains supplementary material, which is available to authorized users. F. Bulleri (&) Dipartimento di Biologia, Universita` di Pisa, CoNISMa, Via Derna 1, 56126 Pisa, Italy e-mail: [email protected]
moderate decrease in the density of just one of the two species can decrease the ability of the herbivore assemblage to control the proliferation of filamentous macroalgae at the margins of barren patches. The extent of barren areas appears, therefore, to be regulated by the outcome of density-dependent interactions between the two species of sea urchins.
Introduction Sea urchins are among the most efficient marine herbivores and their key ecological role has been widely documented (Lawrence 1975; Hawkins and Hartnoll 1983). On temperate rocky reefs, grazing by sea urchins can remove erect algal forms or prevent their re-establishment after disturbance events, thus favoring the formation and persistence of areas dominated by encrusting corallines, generally referred to as urchin barrens (Lawrence 1975; Scheibling 1986; Andrew 1993; Benedetti-Cecchi et al. 1998; Guidetti et al. 2003). Grazing by sea urchins can be variable both in time and space (Schiel 1982; Dean et al. 1984; Bulleri et al. 1999). For instance, they can switch from a
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