Marc Verlaque

Is Global Warming Involved in the Success of Seaweed Introductions in the Mediterranean Sea

At least 106 species of seaweeds, i.e., multicellular photosynthetic organisms belonging to Rhodobionta and Viridiplantae (kingdom Plantae) and Phaeophyceae (kingdom Stramenopiles), have been introduced into the Mediterranean Sea. Since the beginning of the twentieth century, their number has more or less doubled every 20 years. Available data do not support the assumption that climate warming enhances biological invasions in the Mediterranean, at least in the case of the seaweeds. What matters most is the nature and the strength of the vector. From the 1970s, aquaculture took over from the Suez Canal as the main vector, resulting in a change in the donor region (Northern Pacific rather than Red Sea) and in the biogeographical origin of newly introduced species (cold-water rather than tropical). The alleged ‘aggressiveness’ of tropical introduced species, such as Caulerpa taxifolia and C. racemosa var. cylindracea, is because they are often seen as tropical origin, when they are actually native to temperate seas. Finally, the warming should advantage thermophilic introduced species, and at the same time, it could disadvantage cold water species. The overall amount of new introduced species and the overall dominance of introduced species might therefore be unchanged.