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104 Gianmarco Ingrosso et al.

visits by divers in coralligenous communities produce important effects over
certain large or fragile suspension feeders that inhabit coralligenous assem-
blages (Ballesteros, 2006; Casoli et al., 2016; Coma et al., 2004; Garrabou
et al., 1998; Guarnieri et al., 2012; Linares et al., 2005; Sala et al., 1996).
The proportion of damage varies significantly with the frequency of SCUBA
diving and sometimes results in damage of bioconstructors like red algae,
bryozoan and coral colonies. Mechanical impacts by divers and snorkelers
cause serious damages to A. calycularis colonies (Di Franco et al., 2009;
Milazzo et al., 2002). In areas where its dense reefs attract tourists, damages
to colonies were frequently caused by accidental contacts with nonexpert
divers (Terro ´n-Sigler et al., 2016a).
Also for Sabellaria reefs, the main anthropogenic impacts related to the
physical disturbance are linked to frequency of human visitation. Along
Italian coasts, one of the main threats to these biogenic reefs is linked to
trampling (Plicanti et al., 2016). Often, the impact is further exacerbated
by the direct removal of Sabellaria worms, which are used as bait for recre-
ational fisheries. The vermetid reef-building species D. cristatum is similarly
sensitive to trampling, which often causes the death of individuals and the
detachment of settled larvae. Thus, given their very low dispersal ability
(Calvo et al., 1998), they are unlikely to recover from local depletion and
habitat degradation (Galil, 2013). Moreover, the increase of water move-
ments linked to maritime traffic also impacts on vermetid reefs (Graziano
et al., 2007).

4.1.7 Biological Threats
The rates of invasion of nonindigenous species (NIS) have increased in the
last 2 centuries, and particularly in the last 50 years (Ruiz et al., 2000), as a
consequence of globalization and industrialization. Coastal marine habitats
are particularly exposed to the impact related to this threat (Williams and
Grosholz, 2008) due to the concentration of activities that promote invasion
(e.g. shipping, aquaculture, fisheries and aquarium trade). Invasive species
can displace native species and alter food-web dynamics, changing habitat
structure by altering species diversity (Ruiz et al., 1999). Knowledge on
the effects of NIS on Mediterranean coralligenous communities is scant.
To date, three algal species are threatening these communities: Womersleyella
setacea (Hollenberg) R.E. Norris, Caulerpa cylindracea Sonder and Caulerpa
taxifolia (M. Vahl) C. Agardh. They colonize only relatively shallow-water
coralligenous reefs (to 60m) (Ballesteros, 2006), where they cover the basal
layer of encrusting corallines and increase sedimentation rates, impairing