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96 Gianmarco Ingrosso et al.

habitats, where gradients due to hydrodynamic forcing and light variation
are much stronger. This idea of reduced abiotic variation with depth leads
to the hypothesis that β-diversity should also decrease from shallower to
deeper habitats because of the reduced importance of habitat filtering, a
key mechanism maintaining β-diversity. Unfortunately, this hypothesis
has not been tested yet, but the outcome would be particularly interesting,
in whatever direction. Support for the hypothesis would indicate that habitat
filtering is reduced in low subtidal environments and, possibly, that connec-
tivity is an important assembling process. Rejection of the hypothesis would
also be appealing, suggesting that dispersal limitation and species interactions
become stronger with increasing depth to compensate for the weakening of
habitat filtering.
Harsh abiotic conditions in shallow waters, in terms of intense light inten-
sity, temperature and water movement, especially in the intertidal, can favour
assemblages that tolerate such extreme situations, so forming low-diversity
communities where individual species are prominent. As depth increases,
especially on hard substrates, clonal species become dominant and biotic inter-
actions, especially competition for space, become the main drivers of the com-
position of assemblages where clonal species are dominant ( Jackson, 1977).
Understanding the relative contribution of these processes will be
increasingly important, since abiotic variation is expected to increase with
climate change, as we have already seen with recent heat waves reaching
low subtidal environments (Garrabou et al., 2009). These extreme events
can alter the mechanisms that maintain biodiversity on bioconstructions,
and β-diversity seems particularly suited to detect these changes.

4. MEASURES
4.1 Major Threats to Mediterranean Bioconstructions
The intensive human exploitation of the Mediterranean Sea has many pro-
found negative effects on marine biota. Industrial, urban and agricultural
pollution, coastal development, climate change, increases in sedimentation,
trawling, anchoring and introduction of alien species represent the main
threats for Mediterranean marine benthic communities (Ballesteros, 2006;
Coll et al., 2010; Piazzi et al., 2012). Although with variable spatial and tem-
poral extent, and variable intensity, almost all coastal biodiversity of the
Mediterranean is affected by these threats (UNEP-MAP-RAC/SPA,
2003). Below is a synthesis of the main threats affecting bioconstructions.