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unpublished data).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