Proceedings of the Second Mediterranean Symposium on Marine
Proceedings of the Second Mediterranean Symposium on Marine
Proceedings of the Second Mediterranean Symposium on Marine
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PROCEEDINGS OF THE SECOND MEDITERRANEAN SYMPOSIUM ON MARINE VEGETATION (ATHENS, 12-13 DECEMBER 2003)<br />
44<br />
In <str<strong>on</strong>g>the</str<strong>on</strong>g> same way, Caulerpa prolifera (Chlorobi<strong>on</strong>ta, Viridobi<strong>on</strong>ta, Plantae), which moves<br />
northwards from <str<strong>on</strong>g>the</str<strong>on</strong>g> central and sou<str<strong>on</strong>g>the</str<strong>on</strong>g>rn <str<strong>on</strong>g>Mediterranean</str<strong>on</strong>g> during warm climatic episodes,<br />
<str<strong>on</strong>g>the</str<strong>on</strong>g>n southwards during cold episodes, is not an introduced species in <str<strong>on</strong>g>the</str<strong>on</strong>g> north-western<br />
Med-iterranean (e.g. <str<strong>on</strong>g>the</str<strong>on</strong>g> Gulf <str<strong>on</strong>g>of</str<strong>on</strong>g> Marseilles), where it occurs nowadays (Marc Verlaque,<br />
unpublished data).<br />
Fig. 7. Change over time (every 20 years, 1880 through 2000) in <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
cumulative number <str<strong>on</strong>g>of</str<strong>on</strong>g> likely introduced macrophytes in <str<strong>on</strong>g>the</str<strong>on</strong>g> <str<strong>on</strong>g>Mediterranean</str<strong>on</strong>g><br />
Sea. From Boudouresque (1999a) and Boudouresque and<br />
Verlaque (2002), up-dated.<br />
Since <str<strong>on</strong>g>the</str<strong>on</strong>g> early 20 th century, <str<strong>on</strong>g>the</str<strong>on</strong>g> number <str<strong>on</strong>g>of</str<strong>on</strong>g> species probably<br />
introduced into <str<strong>on</strong>g>the</str<strong>on</strong>g> <str<strong>on</strong>g>Mediterranean</str<strong>on</strong>g> has more or less<br />
doubled every 20 years (Fig. 7; Boudouresque, 1999a;<br />
Boudouresque and Verlaque, 2002). To date, a hundred <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
macrophytes have probably been introduced into <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
<str<strong>on</strong>g>Mediterranean</str<strong>on</strong>g>. They represent 6.5% <str<strong>on</strong>g>of</str<strong>on</strong>g> its known flora. For<br />
comparative purposes, <str<strong>on</strong>g>the</str<strong>on</strong>g>re are <strong>on</strong>ly 37 al<strong>on</strong>g <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
European and nor<str<strong>on</strong>g>the</str<strong>on</strong>g>rn Africa Atlantic coasts (from Canary<br />
Islands to Scandinavia and <str<strong>on</strong>g>the</str<strong>on</strong>g> Baltic Sea). The <str<strong>on</strong>g>Mediterranean</str<strong>on</strong>g><br />
Sea is <str<strong>on</strong>g>the</str<strong>on</strong>g>refore a major hot spot for introduced species.<br />
The main vectors <str<strong>on</strong>g>of</str<strong>on</strong>g> introducti<strong>on</strong> are shellfish aquaculture, fouling <strong>on</strong> ship hulls and<br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> Suez Canal (Bou-dou-resque, 1999a, 1999b; Boudouresque and Verlaque,<br />
2002). After habitat destructi<strong>on</strong>, introduced species are <str<strong>on</strong>g>the</str<strong>on</strong>g> sec<strong>on</strong>d greatest cause <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
species endangerment and decline worldwide (Schmitz and Simberl<str<strong>on</strong>g>of</str<strong>on</strong>g>f, 1997).<br />
However, each introduced species c<strong>on</strong>stitutes a special case (Carlt<strong>on</strong> and Geller,<br />
1993). According to species, <str<strong>on</strong>g>the</str<strong>on</strong>g> following has been observed (Boudouresque,<br />
1999a): (i) Zero to slight impact. (ii) More or less drastic change in <str<strong>on</strong>g>the</str<strong>on</strong>g> number<br />
and/or abundance <str<strong>on</strong>g>of</str<strong>on</strong>g> native species (e.g. in macrophyte communities invaded by<br />
Caulerpa taxifolia). (iii) Displacement <str<strong>on</strong>g>of</str<strong>on</strong>g> species occupying a close ecological niche.<br />
(iv) Change in <str<strong>on</strong>g>the</str<strong>on</strong>g> functi<strong>on</strong>ing <str<strong>on</strong>g>of</str<strong>on</strong>g> native ecosystems, due to an introduced species<br />
which acts as a keys-pecies (Fig. 8-10). (v) Displacement <str<strong>on</strong>g>of</str<strong>on</strong>g> native ecosystems, due<br />
to <str<strong>on</strong>g>the</str<strong>on</strong>g> setting up <str<strong>on</strong>g>of</str<strong>on</strong>g> a totally new ecosystem. This is <str<strong>on</strong>g>the</str<strong>on</strong>g> case with Caulerpa taxifolia,<br />
which c<strong>on</strong>stitutes a meadow displacing several native ecosystems (e.g. <str<strong>on</strong>g>the</str<strong>on</strong>g> Cystoseira<br />
brachycarpa forest, <str<strong>on</strong>g>the</str<strong>on</strong>g> Padina pav<strong>on</strong>ica and Halopteris scoparia community, <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
Posid<strong>on</strong>ia oceanica dead matte community, several sand bottom communities and<br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> coralligenous community) and homogenizing habitats and <str<strong>on</strong>g>the</str<strong>on</strong>g> landscape<br />
(Fig. 11). Within a given habitat, alpha species diversity can increase (e.g. sand<br />
bottoms and <str<strong>on</strong>g>the</str<strong>on</strong>g> Cymodocea nodosa meadow) or decrease (e.g. <str<strong>on</strong>g>the</str<strong>on</strong>g> Posid<strong>on</strong>ia<br />
oceanica meadow and Cystoseira forests) (Verlaque and Fritayre, 1994; Verlaque<br />
and Boudouresque, 1995; Harmelin-Vivien et al., 1999). However, at regi<strong>on</strong> scale,<br />
gamma species diversity always decreases (Fig. 11). Synergy between several