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Mediterranean Bioconstructions Along the Italian Coast 95

corridors, will increase dispersal and this effect should result in reduced spa-
tial turnover in species composition, enhancing the homogeneity of species
assemblages. Thus, β-diversity may be used as an indirect measure of species
connectivity (Thrush et al., 2010). Low values of β-diversity (i.e. high
similarity of species composition) among sites featured by common environ-
mental conditions can be a consequence of a high connectivity among
them through propagule dispersal. However, high values of β-diversity
(corresponding to low similarity in species composition) can provide evi-
dence of isolation only after the exclusion of the influence of postsettlement
processes. In fact, dissimilar assemblages could actually be well connected
through larval and propagule dispersal and the low similarity could be the
result of postsettlement mortality events, which may be different across sites.
Instead, if the differences are really due to low connectivity, compositional
differences should be evident at the initial steps of settlement and/or recruit-
ment in experimentally manipulated plots. Thus, β-diversity is suitable for
identifying connections between populations and communities, although
a high species turnover does not necessarily imply a lack of connectivity.
All this implies the need to combine and integrate β-diversity analyses with
other tools and approaches.
β-Diversity has received little attention in biogenic assemblages. Balata
et al. (2007) showed that increased sedimentation reduced β-diversity on
western Mediterranean biogenic reefs, muffling natural differences in species
composition between vertical and horizontal substrata. This result supports
the importance of β-diversity as a measure that can readily capture changes
in biogenic assemblages. The analysis of β-diversity may reveal the pro-
cesses that shape assemblages on bioconstructions, especially in low subtidal
environments (30–50m depth, as in coralligenous formations). These for-
mations thrive in environments that are generally less variable than shallower

Fig. 9—Cont’d (B) Visualization of a 30-day larval dispersal based on AVISO current
data. (C) Potential coralligenous connectivity in the different subregions. The node
dimension is proportional to the coralligenous area and the line width reflects the
intensity of connections. Acronyms: Central Adriatic (C-Adr), Corsica Sea (CorSea),
Eastern-Central Tyrrhenian (EC-Tyr), Eastern-Southern Tyrrhenian (ES-Tyr), Ligurian
Sea (LigSea), Northern Adriatic (N-Adr), Northern Ionian (N-Ion), Northern Tyrrhenian
(N-Tyr), Sardinia Channel (SarCha), Sardinia Sea (SarSea), Southern Adriatic (S-Adr),
Southern Ionian (S-Ion), Strait of Sicily (StrSic), Western-Central Tyrrhenian (WC-Tyr),
Western–Southern Tyrrhenian (WS-Tyr). Panel (A): From Martin, C.S., Giannoulaki, M.,
De Leo, F., Scardi, M., Salomidi, M., Knittweis, L., et al., 2014. Coralligenous and maërl hab-
itats: predictive modelling to identify their spatial distributions across the Mediterranean
Sea. Sci. Rep. 4, 5073. https://doi.org/10.1038/srep05073.