Demography of Animal Forests: The Example of Mediterranean Gorgonians
Gorgonian corals may form dense canopies, resembling miniature forests. Similarly to terrestrial forests, gorgonian canopies can increase local complexity and biodiversity. The demographic study of gorgonian populations can supply valuable tools to our understanding of their complex, long-lasting life cycles. In this chapter we report on the demographic history of two Mediterranean gorgonian corals provided of opposite reproductive strategies: Paramuricea clavata and Corallium rubrum. The two study cases show how a demographic approach can be applied to study the effects of disturbances (mass mortality in P. clavata and harvesting in C. rubrum) on long-lived species. The population of P. clavata, object of our study, dwells at the edge of the summer thermocline in the NW Mediterranean Sea. It has been strongly affected by two high mortality events in 1999 and 2003, associated with an anomalous high temperature of the water column. Long-term data (12 years) allowed the analysis of the population structure before and after the mortality events which killed 78 % of the colonies. Nowadays the population is recovering, exhibiting a fivefold increase in recruitment density despite the reproductive output reduced to 7.25 %. Our observations support the hypothesis of an over abundant reproductive output of the species and a strict density-dependence control of recruitment operated by larger colonies in crowded, stable P. clavata populations.
C. rubrum is a long-lived, slow-growing, and low reproductive gorgonian, whose populations living in the shallower part of the species bathymetric distribution range (between 20 and 50 m depth) are mainly composed by crowded colonies having a small size/early age at first reproduction and high recruitment rates. Deep populations (deeper than 50 m), mainly composed by large colonies, are the main target of commercial fishing nowadays. The minimum harvestable colony size is 7 mm of colony basal diameter, according to General Fisheries Commission for the Mediterranean (GFCM) Scientific Advisory Committee (SAC). Report of the transversal workshop on red coral Ajaccio (Corsica) (5–7 Oct 2011) stated that sexual maturity is reached at an age of about 30–35 years. The assessment of population structure in size/age classes of reproductive and survival rates allowed to project population structure over time.
KeywordsOctocorals Mesophotic corals Paramuricea clavata Corallium rubrum Demography Population dynamics Mediterranean Sea
We would like to thank J.M. Gili, S. Rossi, G. Tsounis, I. Vielmini, the Italian Research Group on Red Coral, the researchers and technicians of ISPRA, ENEA S. Teresa (La Spezia), the Portofino and Cap de Creus MAP authorities, the Astrea R/V crew, T. Garcia, R. Rinaldi, and A. Ferrucci for their invaluable and friendly help in collecting deep-dwelling red coral colonies. These studies have been funded by the UNIPI-CISC (Italy-Spain) research project, by the Italian PRIN project 2009–2011, the Italian project on deep-dwelling red coral populations, MedSea, and the COREM European Projects. L. Bramanti’s work has been funded by a Marie Curie Intra-European fellowship. The researches leading to these results has received funding from the European Community’s Seventh Framework Programme under grant agreement 265103 (Project MedSeA).
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