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Sea urchin populations in the Mediterranean

May 24, 2012
Universidad de Barcelona
Scientists have new insight into which processes regulate long-term populations of sea urchins. New research includes an analysis of extraordinary episodes such as the violent storm that hit the Catalan coast on 26 December 2008, which caused the disappearance of around 80% of the population of sea urchins in the archipelago of the Medes and the Montgrí coast.

Underwater monitoring work carried out in the Medes Islands.
Credit: Bernat Hereu, UB

A paper published in the journal PLoS ONE, whose first author is lecturer Bernat Hereu, from the Department of Ecology of the UB, shows the main results of twenty years of scientific studies monitoring sea urchins (Paracentrotus lividus and Arbacia lixula) in the Medes Islands. These two species are the most common sea urchins in the Mediterranean area.

The work gives insight into which processes regulate long-term populations of sea urchins, and it includes extraordinary episodes such as the violent storm that hit the Catalan coast on 26 December 2008, which caused the disappearance of around 80% of the population of sea urchins in the archipelago of the Medes and the Montgrí coast. The paper is also signed by experts Cristina Linares and Mikel Zabala (Department of Ecology, UB); Enric Sala (Centre for Advanced Studies of Blanes, CSIC); Joaquim Garrabou and Antoni Garcia Rubies (Institute of Marine Sciences, CSIC), and David Díaz (Spanish Institute of Oceanography).

Can fish control sea urchin populations?

There are still many unanswered questions on the factors that affect the population dynamics of sea urchins in the marine environment. These benthic organisms, whose predators are fish such as white sea bream and gilthead sea bream, feed on algae and play a key role in the coastal ecosystems. As Bernat Hereu explains, "studies conducted over long temporal scales allow us to know the long-term evolution of natural systems and detect processes that, otherwise, would remain unnoticed. These kinds of studies are rare, they are difficult to be maintained, and they require considerable scientific, logistic and economic effort, but they have great scientific interest."

The research, carried out with the collaboration of the Montgrí, Medes Islands and Baix Ter Natural Park, reviews issues discussed by ecologists, such as, whether a linear relationship in the food chain exists between the population of fish, sea urchins and algae. "It is the first time -- Hereu points out -- that we could see how such processes operate simultaneously in natural populations in the Mediterranean, in protected and unprotected areas. By studying the predator-prey relationships at different temporal scales in the Medes Islands, we have discovered that fish can control to some extent sea urchin populations, but the correlation is not direct. Other environmental factors can also modify or lessen such processes: for example, the behaviour of sea urchins, the survival of larvae, the topography of the habitat, etc."

Looking for refuges on rocky sea bottoms

Does the marine reserve factor have any impact on sea urchin populations? The study published in PLoS ONE reviews the temporal evolution of sea urchin populations, in protected and unprotected areas, in different habitats, characterised by limestone boulder bottoms and vertical walls. In theory, in protected areas where fish abound, sea urchins population decreases. In practice, the reserve effect is not noticed in populations on the sea bottoms. "What we have seen -- Hereu states -- is that if there are many fish, sea urchins hide, and they are less accessible to predators. This explains why, in protected areas, despite the presence of predators, there are large populations of sea urchins that live hidden to avoid them." However, as they remain hidden, they do not move much and they do not eat so many algae. On vertical walls, in contrast, according to Hereu, "sea urchins cannot find refuge. Here differences between protected and unprotected areas are found. There is also a positive association between juvenile and adult sea urchin populations in protected areas. Outside the marine reserve, this effect is not seen due to the lower control predators have on recruiting sea urchins."

The great storm of 2008 as a turning point

On 26 December 2008, a violent storm on the Catalan coast left its mark on marine ecosystems in the Medes. According to experts, more than 80% of the adult population of sea urchins disappeared from the rocky sea bottoms, and nowadays the populations are still recovering. "Having data over long temporal scales helped us to assess the ecological impact of this phenomenon. The most affected communities were sea urchins and algae from the stone blocks on the sea bottoms, which were found displaced as a result of the storm," states Hereu. The monitoring work allows knowing how the populations of sea urchins have evolved after the storm, whether they are more susceptible to predation, etc. "In any case, -- the lecturer goes on -- , the recovery of the populations after the storm has been much quicker outside the marine reserve, and we work with the hypothesis that it may be the result of the effect of fishing and the lack of predators, whereas in the marine reserve fish seem to be able to control them."

Marine reserves: protecting the richness of the natural environment

The research work on the biological communities in the archipelago of the Medes had the support of the Catalan Ministry of Environment and Housing until 2008. Since then, experts from the Department of Ecology of the UB continue the task of carrying out temporal scales on the evolution of the ecological state of marine ecosystems. The team from the UB took part in another study, also published in PLoS ONE, under the direction of Enric Sala (CSIC), on effective protection of biodiversity in protected areas in the Mediterranean. According to this study, marine reserves are only effective if they are completely protected, that is, if fishing is totally prohibited and if there is an effective control system to enforce such prohibition. The archipelago of the Medes, where fishing was prohibited in 1983, is considered to be the only Mediterranean marine reserve where all trophic levels of marine ecosystems have been monitored over a long period of time. It is also the place where the only case of recovery of Cystoseira algae communities is found, which used to be abundant in the Mediterranean, although nowadays they only survive in very well-preserved areas.

As Bernat Hereu warns, "marine reserves are clearly very beneficial for society. However, all the effort made over the years in order to preserve the ecosystems could be lost all of a sudden unless we keep protecting natural areas. Preservation does not have immediate positive effects but we must be constant in order not to lose everything achieved over the years."

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