General Discussion and Conclusion

It is difficult to give a definite answer to the question we asked ('Is global warming involved in the success of seaweed introductions?'). Several distortions may affect the data set we used. (i) Study taxa and study areas largely depend upon the phycologists and their location. (ii) Large introduced species, belonging to taxa whose delineation is not controversial, are easier to detect than tiny species whose taxonomy is confused and accessible to very few specialists. (iii) Cryptogenic introductions are by definition unknown. Taking them into consideration, where it is possible, might conspicuously modify the baseline of our data set, i.e., the panel of anciently introduced species. (iv) Cryptic introductions are not taken into account, though progress in taxonomy will progressively make this possible. (v) The native area (and biogeographical province) of a species is not always accurately known. Either it is naturally present in unknown regions and the native area is underestimated, or it constitutes a cryptogenic introduction in part of its current area, and the native area is therefore overestimated.

Even taking into account these caveats, our data do not support the assumption that climate warming enhances biological invasions in the Mediterranean, at least in the case of the seaweeds. (i) The increase over time in the number of introduced species simply reflects the development of the vectors. In the early and mid-twentieth century, the Red Sea was the main donor region (Fig. 2). Subsequently, the relative strength of this vector declined. It can be hypothesised that most of the species from the Northern Red Sea, suited to survival in Mediterranean habitats and under their present conditions, have already taken the Suez Canal. In the 1970s, oyster culture took over from the Suez Canal as the main vector (Fig. 2). Since the turn of the century, oyster culture seems to be losing ground: either because oyster importation from Northwestern Pacific is officially banned or because most of the Japanese species that were able to thrive in the Mediterranean have been already introduced. In the absence of a new leading vector, the rate of introductions seems to be slowing down (Fig. 1; see also Galil et al., 2007, for Metazoa). Is this a durable trend or just a provisional one, i.e., waiting for the occurrence of the next prevailing vector? (ii) Since the 1980s, i.e., since the undisputable warming of Mediterranean surface water, not only has the relative percentage of new introduced species of tropical origin not increased, but also it has conspicuously declined (Table 3). The reason is that what matters first is the vector (see above). (iii) The alleged 'aggressiveness' of tropical introduced species, such as Caulerpa taxifolia and C. racemosa var. cylindracea, is due to the fact that they are seen as of tropical origin, when they are actually native to temperate seas. Their success in the Mediterranean, a temperate sea, is therefore in no way unexpected. (iv) The warming can advantage thermophilic introduced species. However, at the same time, it can disadvantage cold water species. The overall numbers of new introduced species and the overall dominance of introduced species might therefore be unchanged.

It is interesting to note that the simulation of the effects of climate warming and biological invasions (from 1900 to 2050) on the Mediterranean continental vegetation led to the conclusion that the driving force was the introduced species, whereas warming alone or in combination with introduced species was likely to be negligible in many of the simulated ecosystems (Gritti et al., 2006).

The link between climate warming and biological invasions is therefore poorly supported by the Mediterranean seaweeds. From a quantitative point of view, there are no grounds to believe that warming is responsible for the increase in the number of introduced species, or that species of tropical origin are more 'aggressive' than those of cold-water region origin. From a qualitative point of view (i.e., which species?) together with the spread and dominance of these species, the authors who claim that warming enhances the introduction, spreading and dominance of tropical species, are simply putting Descartes before the horse: if warming becomes more pronounced, which is unfortunately highly probable, there is no doubt that they will end up being proved right.

As far as the politicians, decision-makers and civil servants are concerned, their belief that the current increase in the number of introduced species results from global warming is not supported by the available data. There is no reason for this to change in the near future, and there is therefore no excuse for not implementing the international agreements for limiting and controlling biological invasions.

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