While many of the alien species become part of the
background flora and fauna,others become invasive,
reaching densities of 1000 s m2
,and come to dominate
the native flora and fauna. Virtually every coastal
habitat in the San Francisco Bay area is now dominated
by one or more alien species. Three of the six most
common benthic marine species in Port Phillip Bay in
1996 were alien species,a statistic that does not include
two recent and rapidly proliferating alien species,one of
which—the North Pacific seastar, Asterias amurensis—
has increased to over 100 million individuals covering
1500 km2 that have a greater biomass than that of all
fished species in the Bay (Fig. 1). In the 15 years since its
discovery off Monaco,the invasive green algae, Caulerpa
taxifolia,has come to cover 97% of available
surfaces between Toulon and Genes (France,Monaco
and Italy) has already been spread to the Adriatic Sea,
and is projected to eventually spread over most of the
Mediterranean [10].