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the first time, others expanding their distribution from the neighbouring sub regions
where they are already established. (Zenetos et al., 2012).
Some authors proposing that the Mediterranean Sea is heading towards
‘tropicalisation’ (Bianchi, 2007). The use of this term might appear exaggerated in
view of the data currently available, but a ‘meridionalisation’ of the Mediterranean
(a definite augmentation of the proportion of thermophilic species in the
Mediterranean biota) seems a more realistic description of changes to come
(Lejeusne et al., 2010). Climatic models (Parry, 2000) further predict that the
Mediterranean basin will be one of the regions most affected by the ongoing
warming trend and by an increase in extreme events.
Climatic fluctuations exert an overriding role on the marine biota (Cushing
and Dickson, 1976; Southward and Boalch, 1994; Wilkinson and Buddemeier, 1994;
Southward et al., 1995; Bianchi, 1997). Biodiversity is affected by a combination of:
(i) a direct effect on the organisms (temperature causes changes in survival,
reproductive success, dispersal pattern and behaviour); (ii) effects mediated by biotic
interactions (conferral of competitive advantage to one of a pair of overlapping
species); and (iii) indirect effects through ocean currents. Sanford (1999) showed
that small changes in climate may generate large changes in marine communities
through regulation of keystone predation. Petchey et al. (1999) demonstrated that
environmental warming alters food-web structure and function of aquatic
ecosystems. There is some evidence that Mediterranean biodiversity patterns are
presently facing changes that can be related to increasing seawater temperature
(Francour et al., 1994).
A direct consequence of warming is a simultaneous increase in the
abundance of thermotolerant species and the disappearance reduction of ‘cold’
stenothermal species. Such changes occur as shifts in distribution ranges and/or
population dynamics, and were detected as early as the 1980s (Francour et al.,
1994). Although seawater warming probably affects the entire Mediterranean (Rixen
et al., 2005; Moron, 2003), range shifts have mainly been reported in north western
Mediterranean taxa; this is either due to the higher proportion of cold stenotherm
species in the north western Mediterranean or to observation bias, or a mixture of
both. Considering the north western Mediterranean only, one of the coldest areas in
the Mediterranean, tens of significant range expansions of species of warm water
affinity have been recorded, two-thirds of which correspond to mobile species
(UNEP-MAP-RAC/SPA, 2008).
Mapping the surface isotherms of the Mediterranean Sea, averaged over a
century of records and therefore representing the climatology of the basin (Brasseur
et al., 1996), shows that the isotherm of 15°C for February (the coldest month in the
year) crosses the Straits of Sicily, splits the Ionian Sea into a north-western and a
south-eastern part, and finally separates the Peloponnese from the Aegean Sea (Fig.
5). According to Bianchi (2007) the February 15°C surface isotherm follows quite
closely all the biogeographic boundaries between the western and eastern
Mediterranean, possibly explaining the similarity between the Aegean Sea biota to
that of the western Mediterranean (both basins laying mostly to the north of the
February 15°C surface isotherm) than to that of the Levant Sea (which remains to the
south of that isotherm).