Proceedings of the Second Mediterranean Symposium on Marine

PROCEEDINGS OF THE SECOND MEDITERRANEAN SYMPOSIUM ON MARINE VEGETATION (ATHENS, 12-13 DECEMBER 2003)
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BIODIVERSITY
Biological diversity (biodiversity) means <strong>the</strong> variety among living organisms from all
sources including, inter alia, terrestrial, marine and o<strong>the</strong>r aquatic ecosystems and <strong>the</strong>
complexes <strong>of</strong> which <strong>the</strong>y are part. This includes diversity within species, diversity
between species, between ecosystems, between landscapes and functional diversity
(Ramade, 1994). Ecosystem diversity is <strong>of</strong>ten referred to as "ecodiversity".
As far as species diversity is concerned, point species diversity refers to <strong>the</strong> number <strong>of</strong>
species within a sample, alpha species diversity to <strong>the</strong> number <strong>of</strong> species within a given
habitat (e.g. <strong>the</strong> Posidonia oceanica meadow) in a given region (e.g. Liguria), beta
species diversity <strong>the</strong> species turnover between adjacent habitats <strong>of</strong> a given region,
gamma species diversity to <strong>the</strong> number <strong>of</strong> species within all habitats <strong>of</strong> a given region,
and epsilon species diversity <strong>the</strong> num-ber <strong>of</strong> species within a biogeographical province
(e.g. <strong>the</strong> <strong>Mediterranean</strong> Sea) (Wilson and Shmida, 1984; Gray, 2000, 2001).
Human impact may increase alpha diversity <strong>of</strong> low species diversity habitats or even <strong>of</strong>
some taxonomical groups in habitats with high species diversity (e.g. Dimech et al.,
2002), while gamma diversity decreases. This is <strong>the</strong> reason why some naïve authors
emphasize that pollution (e.g. aquaculture farms) and introduced species (e.g. Caulerpa
taxifolia) constitute valuable diversity enhancers (Fig. 5).
Fig. 5. Above: an example <strong>of</strong> alpha species diversity within 5 benthic habitats (A through E) in a given region.
Gamma species diversity is not <strong>the</strong> sum <strong>of</strong> <strong>the</strong> 5 alpha diversities, since some species are common to <strong>the</strong>
five habitats (in black). Below: human impact (e.g. pollution or <strong>the</strong> introduced species Caulerpa taxifolia) has
more or less homogenized <strong>the</strong> communities (habitat F). The disturbance may increase (e.g. B, a sand bottom
habitat) or decrease (e.g. E, a rocky bottom habitat) alpha species diversity. However, as far as gamma
species diversity is concerned (right), it dramatically decreases.