Page 17 - Klaoudatos_Kapiris_2016

environmental change, and hence species occurrence, with few equals in the world.
Another reason is the present-day variety of climatic and hydrologic situations to be
found within the Mediterranean, leading to the occurrence in this sea of both
temperate and subtropical biota (Sara, 1985).
The Mediterranean marine biota has historically been divided into three
major biogeographical provinces (the western and eastern basins, and the Adriatic
Sea), each subdivided along latitudinal patterns (Fredj, 1972). Species of subtropical
origin are therefore more abundant in southern parts of these provinces whereas
temperate species dominate the northern parts. According to Bianchi and Morri
(2000) there are 10 distinct biogeographic sectors within the Mediterranean,
namely: (A) Alboran Sea; (B) Algeria and southern Spain; (C) Balearic Sea to
Tyrrhenian Sea; (D) Gulf of Lyon and Ligurian Sea; (E) North Adriatic; (F) Central
Adriatic; (G) South Adriatic; (H) North Aegean; (I) Ionian Sea and South Aegean; (J)
Gulf of Gabes to Levant Sea.
The marine biota of the Mediterranean are comprised by several
biogeographic categories: (i) temperate Atlantic-Mediterranean species; (ii)
cosmopolitan species; (iii) endemic elements; (iv) subtropical Atlantic species; (v)
boreal Atlantic species; (vi) Red Sea migrants (especially into the Levant Sea); (vii)
eastern Atlantic migrants (especially into the Alboran Sea). (Bianchi and Morri,
2000).
Most of the introduced biota in the Mediterranean originates from the Red
Sea (i.e. Lessepsian migrants; about 67%), with an additional set of species (about
7%) from other tropical areas. The bulk of the species introduced into the
Mediterranean are of tropical origin. They have long been confined to the
easternmost Levantine shores, but the warming of the Mediterranean favours their
spread (Occhipinti-Ambrogi, 2007). They are now rapidly progressing westwards and
northwards, through the entire eastern basin, with some now reaching the Adriatic
and the western basin. Although it is difficult to disentangle natural range expansion
through time from climate-induced effects, particularly in regions lacking systematic
field monitoring and temperature records, it seems that the last 20 years have seen
an accelerated rate of westward migration of Lessepsian species (Galil and Zenetos,
2002).
The eastern Mediterranean is an important source of thermophilic species
expanding their ranges in the north-western Mediterranean and the Adriatic. In
addition to native Mediterranean species, a particular case of distribution range
expansion is that of introduced species. An introduced species is defined here as a
species that (a) colonises a new area, (b) displays a geographical discontinuity with
its native area, (c) has human activities as a primary vector of range extension, and
(d) can reproduce in situ without human assistance. It is termed invasive when it acts
as key or engineer species within the recipient ecosystem (Lejeusne et al., 2010). The
Mediterranean is one of the areas worldwide most severely hit, with about 600
introduced species (Boudouresque et al., 2005; Galil, 2008). Their number has more
or less doubled every 20 years since the beginning of the 20th century (Galil, 2008;
Ribera and Boudouresque, 1995), but this increase does not seem to be related to
climate change.
According to the most recent data (Zenetos et al., 2012) the total number of
non-indigenous species present in the Mediterranean is 986 (775 in the eastern