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The successful invasion of a biological community appears to be the result of
the relationship between native species richness and alien species ability to colonize
new habitats (Bulleri et al., 2008). This concept implies that habitats with high levels
of diversity are difficult to invade. In contrast, species-poor communities, or stressed
ecosystems, are arguably more prone to invasion, primarily due to lack of biotic
resistance (Occhipinti-Ambrogi and Savini, 2003).
Alien species can be viewed as drivers and passengers of change in biological
communities (MacDougall and Turkington, 2005; Didham et al., 2005). Many invasive
species exert strong impacts on invaded communities and ecosystems (Vila et al.,
2009) and transform ecosystem properties (Richardson et al., 2000), which inevitably
leads to changes in biological communities. Acclimated exotic species may replace
indigenous species, by altering trophic webs and interspecific relationships inducing
profound modifications in the original ecosystems (Dukes and Mooney, 1999).
Resident species can become increasingly poorly adapted to the local environment,
which will then provide opportunities for newcomers that are better adapted and,
thus, more competitive under the new conditions.
Combinations of the invasion of alien species and climate change have
resulted in the reorganization of marine ecosystems, as shown for example in the
Atlantic waters off the coast of the USA (Stachowicz et al., 2002), Europe (Boelens et
al., 2005) and in the Mediterranean Sea (Occhipinti-Ambrogi, 2007). In a changing
world, it will be increasingly difficult to evaluate the impacts of alien species and it is
likely that the increasing presence of ‘new’ species and the decline of ‘old’ ones will
change succession patterns and ecosystem functioning (Harrington et al., 1999;
McNeely, 2001).
On the other hand, beneficial aspects of introductions are claimed, since
intentionally introduced species have significantly contributed to aquaculture
production (FAO DIAS, 1998), as well as fisheries and angling (Minchin and
Rosenthal, 2002). Unintentionally introduced species, such as the Indo-Pacific
species which entered the Mediterranean through the Suez Canal, have become
locally of commercial importance (Golani and Ben Tuvia, 1995); the Mediterranean
mussel Mytilus galloprovincialis, accidentally introduced to the West coast of South
Africa in the mid 1970s, was deliberately introduced to the South coast for
mariculture purposes, despite the fact that it had become invasive, outcompeting
local mussels (Branch and Steffani, 2004).
However according to Galil (2007) the optimistic view on the effects of
invasion by Tortonese (1973) was wrong. It seems that the establishment of alien
biota, and the concurrent adverse changes in the native communities, are part of a
catastrophic anthropogenic ecosystem shift in the Mediterranean Sea.
Introduced crabs constitute some of the best examples of introduced marine
and estuarine species that have had significant impacts on coastal habitats and
economies. Among the best studied is the European green crab Carcinus maenas, for
which ecological and economic impacts have been demonstrated on several coasts.
Green crabs contributed substantially to the demise of the commercial soft-shell
clam fishery in the northeastern United States during the 1940s and 1950s. There
have also been long-term changes in the benthic communities in bays and estuaries
in central California because of green crab predation on small native crabs and