Hereu B et al. (2012), Multiple processes regulate long-term populatio...

ECB-ART-42438

PLoS One 2012 Jan 01;75:e36901. doi: 10.1371/journal.pone.0036901.

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Multiple processes regulate long-term population dynamics of sea urchins on Mediterranean rocky reefs.

Hereu B , Linares C , Sala E , Garrabou J , Garcia-Rubies A , Diaz D , Zabala M .

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We annually monitored the abundance and size structure of herbivorous sea urchin populations (Paracentrotus lividus and Arbacia lixula) inside and outside a marine reserve in the Northwestern Mediterranean on two distinct habitats (boulders and vertical walls) over a period of 20 years, with the aim of analyzing changes at different temporal scales in relation to biotic and abiotic drivers. P. lividus exhibited significant variability in density over time on boulder bottoms but not on vertical walls, and temporal trends were not significantly different between the protection levels. Differences in densities were caused primarily by variance in recruitment, which was less pronounced inside the MPA and was correlated with adult density, indicating density-dependent recruitment under high predation pressure, as well as some positive feedback mechanisms that may facilitate higher urchin abundances despite higher predator abundance. Populations within the reserve were less variable in abundance and did not exhibit the hyper-abundances observed outside the reserve, suggesting that predation effects maybe more subtle than simply lowering the numbers of urchins in reserves. A. lixula densities were an order of magnitude lower than P. lividus densities and varied within sites and over time on boulder bottoms but did not differ between protection levels. In December 2008, an exceptionally violent storm reduced sea urchin densities drastically (by 50% to 80%) on boulder substrates, resulting in the lowest values observed over the entire study period, which remained at that level for at least two years (up to the present). Our results also showed great variability in the biological and physical processes acting at different temporal scales. This study highlights the need for appropriate temporal scales for studies to fully understand ecosystem functioning, the concepts of which are fundamental to successful conservation and management.

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Genes referenced: LOC100887844

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	Figure 1. Study site.Medes Islands Marine Reserve. Locations of study sites inside (Tascons, Freueto, Vaca and Carall) and outside (Falaguer, Molinet, and Punta Salines) the reserve. The red line represents the limits of the Marine Reserve, where all fishing is prohibited.
	Figure 2. Paracentrotus lividus density over time.Number of Paracentrotus lividus (>1 cm diameter) per 10 m2 (mean ± SE) over time at each site in MIMR on a) boulder substrates and (b) vertical walls. Solid symbols represent sites within the reserve (R); open symbols represent sites in the nearby unprotected area (NR). Note the different scales on both types of habitat.

	Figure 4. Arbacia lixula density over time.The number of Arbacia lixula (>1 cm diameter) per 10 m2 (mean ± SE) over time at each site in the MIMR on a) boulder substrates and (b) vertical walls. Solid symbols represent sites within the reserve (R); open symbols represent sites in the nearby unprotected area (NR). Note the different scales of both types of habitat.