Coma R et al. (2011), Sea urchins predation facilitates coral invasio...

ECB-ART-42117

PLoS One 2011 Jan 01;67:e22017. doi: 10.1371/journal.pone.0022017.

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Sea urchins predation facilitates coral invasion in a marine reserve.

Coma R , Serrano E , Linares C , Ribes M , Díaz D , Ballesteros E .

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Macroalgae is the dominant trophic group on Mediterranean infralittoral rocky bottoms, whereas zooxanthellate corals are extremely rare. However, in recent years, the invasive coral Oculina patagonica appears to be increasing its abundance through unknown means. Here we examine the pattern of variation of this species at a marine reserve between 2002 and 2010 and contribute to the understanding of the mechanisms that allow its current increase. Because indirect interactions between species can play a relevant role in the establishment of species, a parallel assessment of the sea urchin Paracentrotus lividus, the main herbivorous invertebrate in this habitat and thus a key species, was conducted. O. patagonica has shown a 3-fold increase in abundance over the last 8 years and has become the most abundant invertebrate in the shallow waters of the marine reserve, matching some dominant erect macroalgae in abundance. High recruitment played an important role in this increasing coral abundance. The results from this study provide compelling evidence that the increase in sea urchin abundance may be one of the main drivers of the observed increase in coral abundance. Sea urchins overgraze macroalgae and create barren patches in the space-limited macroalgal community that subsequently facilitate coral recruitment. This study indicates that trophic interactions contributed to the success of an invasive coral in the Mediterranean because sea urchins grazing activity indirectly facilitated expansion of the coral. Current coral abundance at the marine reserve has ended the monopolization of algae in rocky infralittoral assemblages, an event that could greatly modify both the underwater seascape and the sources of primary production in the ecosystem.

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Genes referenced: impact LOC100887844 ncaph

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	Figure 1. Study sites.(a) Location of Cape of Palos (south-east Spain) in the NW Mediterranean. (b) Location of the Marine Reserve of Cape of Palos-Islas Hormigas. (c) Location of 4 study sites at the Cape of Palos-Islas Hormigas Marine Reserve: Bajo de Dentro, Bajo de Fuera, La Hormiga and El Hormigón.
	Figure 2. Trends exhibited by the density and the coverage of Oculina patagonica over time at La Hormiga and El Hormigón.Pearson product moment correlations between coral density and time and between coral coverage and time are indicated.
	Figure 3. Density of sea urchins (ind m−2; mean ± SE) over time.Only sea urchins with >2 cm in test diameter were counted. a) Paracentrotus lividus. b) Arbacia lixula. c) both sea urchins species together.
	Figure 4. Biomass (g dry weight m−2; mean ± SE) of the sea urchin Paracentrotus lividus at La Hormiga and El Hormigón between 2006 and 2010.
	Figure 5. Pearson product moment correlation between the density of both sea urchin species (P. lividus and A. lixula) and abundance of the coral Oculina patagonica at both studied locations .[La Hormiga a) density and c) cover; El Hormigón. b) density and d) cover].
	Figure 6. Density of Oculina patagonica colonies in 2002 and 2010 at the four studied sites.
	Figure 7. Contrast between the observed proportion of small colonies (up to 100 cm2) on open spaces and that expected from the consideration of the abundance of the different colony size classes and their random distribution on open spaces in 2005.2006. 2007 and 2010.
	Figure 8. Schematic representation of the observed interactions.The two major assemblages in Mediterranean rocky infralittoral ecosystems are represented at the left side: erect algal forest (a) and coralline barrens (c). Variations in sea urchins density and their grazing impact is the main driver of the shift from algal forests to coralline barrens and vice versa. Intermediate densities of sea urchins create and maintain open spaces in the space-limited algal forest (b). These open spaces are usually filled up again by erect algae in a dynamic process of creation and removal of open spaces. However, under the presence of the invasive coral Oculina patagonica (d), these open spaces facilitate coral recuitment (e) and increase the abundance of the coral to the extent of matching that of some dominant erect macroalgal species. Therefore, under the presence of Oculina patagonica and high to medium sea urchin grazing, two new assemblages flourish: an algal forest-coral assemblage (f) and a coral-coralline barren assemblage (g), depending on the abundance and grazing impact of sea urchins.

References [+] :

Arévalo, Changes in the composition and structure of Mediterranean rocky-shore communities following a gradient of nutrient enrichment: descriptive study and test of proposed methods to assess water quality regarding macroalgae. 2007, Pubmed