EXOTIC AND INVASIVE SPECIES INFESTATIONS – THE NEW PIRATES OF THE WORLD’S OCEANS
All across the planet, the number and severity of outbreaks and infestations of invasive species (i.e. species purposefully or accidentally introduced in non-native environments) is grow- ing, and invasions of marine habitats are now occurring at an alarming rate (Ruiz et al . 1997). Exotic and invasive species have been identified by scientists and policymakers as a major threat to marine ecosystems, with dramatic effects on biodiversity, biological productivity, habitat structure and fisheries (Carlton 1999, Lotze et al . 2006). The combined number of invasive marine plant and invertebrates in Europe and North-America has increased from some 25 around 1900 to over 175 in 2000, and is still rising, particularly concurrent with the intensifica- tion of fishing and bottom trawling after 1950. Although no habitat is immune to invasions (Lodge 1993), some habitats are more invaded than others. This can be explained in two, not mutually exclusive, conceptually different ways. The first is that the number of established exotic species is a di- rect function of the number introduced. Thereby, habitats that are more influenced by introduction vectors than others will harbour more exotic species (Williamson 1996). The second explanation is that some habitats are more readily invaded than others due to physical or biological factors that facilitate or pre- vent the success of exotic species (Elton 1958). One factor that may contribute to the success of exotic species is when the re- cipient ecosystem is heavily destabilized (Vermeij 1991) by hu- man disturbance (e.g. pollution, overfishing etc.). In the Black Sea, overfishing and eutrophication triggered a trophic cascade leading to a massive bloom of the invasive comb jelly ( Mnemi- opsis leidyi ) (Daskalov et al . 2007). In this study the depletion of
marine predators was detected as the first ‘regime shift’. There are several reports from around the world demonstrating a de- cline in the abundance of marine predators caused by intensive fishing (trawling etc.) (e.g. Stevens et al . 2000, Graham et al . 2001), probably resulting in habitats that are more susceptible to exotic species. Most introductions of exotic and invasive species result from anthropogenic dispersal (Ruiz et al . 1997). The relative impor- tance of different mechanisms of dispersal varies spatially and temporally, but the worldwide movement of ships seems to be the largest single introduction vector (ballast water and ship fouling) (Ruiz et al . 1997, Gollasch 2006). Indeed, the patterns of dispersal are strongly concurrent withmajor shipping routes, while the establishment globally appears to be strongly concur- rent with intensity of fisheries, bottom trawling, pollution and other stressors. Hence, while some species may become inva- sive or exotic species may become infestations, it is clear that this pattern is so strongly concurrent with other man-made pressures to the oceans that their dispersal and establishment as pests appear to be caused by severe man-made disruptions of the marine ecosystems. It may be true that exotic and invasive species have not caused extinction of native marine species (Briggs 2007), but there are examples of invasive species totally changing the relative abundance of species within a community (Daskalov et al . 2007). Thus, the invasions of exotic and invasive species to marine habitats becoming a subject of global environmental concern seem legitimate.