Many vegetated systems in temperate regions throughout the world may face dramatic ecosystems shifts to unvegetated states. So, flourishing kelp forests may become desolated bare rocky substrates void of the rich floras and faunas usually associated to forests.
In the Mediterranean there are no kelp forests, but rocky bottoms can be covered by macroalgae (e.g. Cystoseira spp) that host juvenile and adult stages of many invertebrate and fish species. The persistence of these macroalgal beds, that greatly contribute to maintain coastal biodiversity and productivity, is the result of a delicate equilibrium among environmental conditions, interactions among species and human disturbance.
In recent years it has been demonstrated that in Mediterranean rocky reefs where fishing is particularly intense fish predators may decrease in abundance and size. Their decreased predation impact on preys, including sea urchins (the most important grazers in such systems), may reflect in a dramatic increase of prey abundance.
The ultimate consequence of the fishing impact may thus be an increased grazing impact of sea urchins that causes the transition from macroalgal beds to the so-called ‘barrens’, that is bare rocks partially covered by encrusting coralline algae. They are a sort of rocky deserts.
How to cope with this? Management of complex natural systems is not easy because many processes may interact. Formation of barrens, for instance, can be also a consequence of other processes like water warming or sedimentation. However, fishing restrictions and fish predator recovery may be useful to reverse this phenomenon.
From this perspective, there is increasing evidence that the creation of effective Marine Protected Areas (and not paper parks) can allow fish predators to recover along with their potential in controlling sea urchin populations and their effects on macroalgae.
At the MPA of Torre Guaceto (SE Italy), in the same rocky reefs where in the past (before the establishment of the MPA) there were barrens, there are at present (after about 7-8 years of effective protection) more fish predators, less sea urchins and lower barren covers than outside the MPA, which is the proof that properly managed MPAs can allow the entire ecosystems to recover.
Paolo Guidetti
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For more information:
Guidetti P. 2006. Marine reserves reestablish lost predatory interactions and cause community changes in rocky reefs. Ecological Applications 16(3):963–976.
