Faculty of 1000 evaluations, dissents and comments for [Coma R et al. Sea urchins predation facilitates coral invasion in a marine reserve. PLoS One.2011; 6(7):e22017; doi: 10.1371/journal.pone.0022017]. Faculty of 1000, 04 Aug 2011. F1000.com/12425957

Ferdinando Boero, Universita' del Salento, Italy. F1000 Ecology

Evaluation details:

Must Read [8]
New Finding

Sections:
Population Ecology,Community Ecology & Biodiversity,Ecosystem Ecology,Conservation & Restoration Ecology,Global Change Ecology,Marine & Freshwater Ecology

Comments:
The Mediterranean Sea is rapidly changing; the biota of today are very different from those of just a few decades ago. In this article, we see the report of a regime shift in a Mediterranean Marine Protected Area formerly dominated by algal forests and now ‘invaded’ by a zooxanthellate coral. The change is mediated by sea urchins that graze on the algae, paving the way for the success of the corals that, indeed, might also be favored by global warming. Just a few years ago, while witnessing the dramatic changes affecting the Mediterranean biota with the establishment of a plethora of tropical species, I thought that, in a near future, coral reefs might come back to the basin where they were present before the Messinian crisis, as shown by the fossil record. And now, apparently, here they are! This brilliant study shows that the success of the corals is mediated by the grazing of sea urchins. According to what we consider as a ‘good’ state of the environment, sea urchins are perceived either as beneficial or detrimental to biodiversity. In coral reef environments, sea urchins promote coral growth, preventing the success of algae over the corals (and this is seen as ‘good’), whereas in temperate environments sea urchin grazing leads to the ‘bad’ sea urchin barrens. In the first case, sea urchins are keystone predators and prevent the monopolization of the substrate by the algae so allowing coral growth, whereas in the case of the barrens they are biodiversity depressors since they cause real deserts. A classical case of keystone predation is the sea otter that feeds on the sea urchins, so allowing the growth of kelp in California.

Whatever our judgement about the role of the sea urchins in question (are they good or bad?), this study shows that the biodiversity of the Mediterranean Sea is going through dramatic changes. By the way, the sea urchin that is eradicating algae is indigenous, whereas the coral is non-indigenous. The alternative to the coral, as in other sea urchin-dominated areas of the Mediterranean Sea, is the desert (unless some predator removes the grazers).

I wonder how we would have judged this shift in dominance if, in the place of the non-indigenous coral (Oculina patagonica), an indigenous one (such as Cladocora caespitosa) would have flourished.

Maybe a phycologist (a specialist of algae) would have found it a disaster, whereas a coral reef biologist would have considered it as a timely evolution of the community, a positive response to global warming. Evidently, there is a lot of relativity in the way we evaluate changes, as two colleagues and I argued almost a decade ago in a paper on species roles {1}.

References:
{1} Piraino et al. Mar Biol 2002, 140:1067-74 [DOI:10.1007/s00227-001-0769-2].