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Alien Species: General remarks. Pathways of introduction and vectors of dispersal
According to Zenetos et al. (2012) the definition of an alien, non-indigenous, exotic,
non-native or allochthonous species is defined by its presence in the wild, through
introduction outside its natural range and beyond its natural dispersal potential. In
the last decade, the establishment in marine ecosystems of invasive alien species
(i.e., non-indigenous species having an adverse effect on biological diversity,
ecosystem functioning, socioeconomic values and/or human health in invaded
regions: Olenin et al., 2011) has rapidly become a central environmental issue (Ruiz
et al., 2000; Grosholz, 2002; Occhipinti- Ambrogi, 2007; Galil et al., 2009; Walther et
al., 2009; Occhipinti- Ambrogi and Galil, 2010).
Marine alien species are a component of global change in all marine coastal
ecosystems. The Mediterranean Sea is today one of the areas worldwide most
severely affected by biological invasions, in terms of detected number of alien
species and rate of introduction (Raitsos et al., 2010; Occhipinti-Ambrogi et al., 2011;
Zenetos et al., 2012).
The most `typical' Mediterranean flora and fauna obviously occur in the
central parts of this sea, and especially in the western basin. The Alboran Sea,
located immediately east of Gibraltar, exhibits stronger Atlantic affinities, due to the
continued penetration of Atlantic flora and fauna with the incoming flux of water
(Harmelin and d'Hont, 1993). The Alboran basin, at the entrance to the
Mediterranean, acts as a buffer reducing gene flow (Lejeusne et al., 2010). On the
contrary, the Levant Sea is experiencing an important influx of Red Sea species after
the opening of the Suez Canal a phenomenon known as `Lessepsian migration' in
recognition of Ferdinand de Lesseps, the French diplomat who promoted the cut of
the Canal (Galil, 1993). Lessepsian species now acclimated in the Mediterranean
include algae, a sea grass, various invertebrates and fish (Golani, 1998); they are so
abundant that the south-eastern Mediterranean Sea has been proposed as a
separate biogeographic province (Por,1999). The rate of Lessepsian migration has
been increasing particularly in the last decade. This is partly attributed to the
continued enlargement of the Suez Canal. According to Rilov and Galil (2009) this is
the main cause of the apparent acceleration in the rate of Lessepsian invasion over
the last five decades.
A significant number of Indo-Pacific species reaches the Western
Mediterranean, which is enriched by ship-transferred species of Pacific origin mostly
among macrophytes (Hilgen and Langereis, 1993). The Strait of Gibraltar is
essentially different from the Suez Canal as a potential pathway for alien species. It
constitutes an ancient waterway, believed to have originated 5.33 million years ago
(Hilgen and Langereis, 1993), compared to the 142 years of the Suez Canal.
Therefore, the current distribution of Atlantic species, tropical or not, with part of
their range in the Mediterranean is the result of a natural process over a long time;
these species do not in any case qualify as aliens, even if their discovery in the
Alboran Sea comes later than their first description in the Atlantic. Additionally, the
Atlantic coast of Morocco is swept by a prevalently southward oceanic circulation
that prevents many potential newcomers to approach the Strait of Gibraltar.
More than half (54%) of the marine non indigenous species in the
Mediterranean Sea were probably introduced by corridors (mainly Suez) (Fig 3).
Shipping is the second most common pathway of introduction, followed by