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International Journal of Marine Science 2013, Vol.3, No.20, 158-165
http://ijms.sophiapublisher.com
163
Our hypothesis is that “invasive” species become
invasive when the plant community is damaged. When
the causes of damage cease, the typical components of
the phytocoenosis tend to recover. Reconstitution of
the community may occur by a specific succession if
settlement and growth of autochthonous species are
able to occur before the sea and other forces destroy
edaphic characteristics.
Few studies have focused on self-restoration of
biocoenoses after destructive events, and none after an
invasion by
C. racemosa
. One experiment on recovery
of the macroalgal community after manual removal of
C. racemosa
proved completely ineffective (Piazzi
and Ceccherelli, 2006), probably because the causes
of impoverishment of the original community
persisted. Recently, Tsiamisis et al (2013) documented
the reconstitution of benthic macroalgal communities
and the return of oligotrophic conditions after cessation
of a source of eutrophication, an urban sewage outlet.
Reconstitution of original biocoenoses is an expression
of ecosystem resilience. We sustain that resilience can
also be expressed towards aggressive invasive species,
provided strong perturbation (human impacts or
climatic variations) does not persist. The aggressiveness
of a species is clearly linked to environmental
conditions and species rightly described as “invasive”
develop well in a wide range of conditions. However,
there may also be blooms of species not considered
invasive. For example,
Alsidium corallinum
C. Ag., a
typical marine species, developed in an unexpected
way in Orbetello lagoon in 2007. Initially barely
detectable, it reached a biomass of thousands of tons
in a single year. This was due to transient conditions
favourable for its growth (Lenzi et al., 2012). Naturally,
such cases are more likely when an ecosystem is not
in equilibrium and does not express stable community
structure. In other words, use of the term “invasive”
for an allochthonous species, such as
C. racemosa
v.
cylindracea
, should be reconsidered. Invasiveness
should rather be regarded as a potential attribute,
probably common to many other species. This is
sustained by the fact that more than 10 years ago,
Ceccherelli et al (2000) found that fast spread of
C.
racemosa
in a
Posidonia
meadow was related to the
availability of free substrate and that the species could
not penetrate a dense meadow. More recently, Casu et
al (2009) demonstrated an important trophic role of the
species for zoobenthic organisms on rocky substrates.
Thus the invasive potential and danger of
C. racemosa
v.
cylindracea
is expressed when environmental
conditions permit, as in the case of biocoenoses
impoverished for different reasons.
3 Materials and Methods
The study area lies off Santa Liberata beach, on the
right side of the outlet channel connecting Orbetello
Lagoon to the sea (Figure 1). Close to the artificial
channel, an emerging
P. oceanica
barrier-reef defines
a back-reef area up to 2.5 m deep and about 5000 m
2
in area (LB, Figure 1). In the past, this area hosted a
mixed meadow of the seagrasses
Cymodocea nodosa
(Ucria) Ascherson,
Nanozostera noltii
(Horneman)
Tomlinson
et
Posluzny, and the chlorophycea
Caulerpa prolifera
(Forsskål) J.V. Lamouroux (Lenzi,
1987). Since 2003, increasing rarefaction and
regression of the mixed meadow has been observed in
LB by aerial photography. Field observations conducted
between July and October 2005 and 2006 (Lenzi et al.,
2007) showed rapid progressive invasion by
Caulerpa
racemosa
.
The area was again viewed by scuba early in August
2011, in order to determine the current vegetation
assemblages. The two 5×5 m
2
sampling stations (1.5 m
deep) established in the back-reef area during the
2005-2006 research, were used again in the field
research of 2011 (LB-a and LB-b; Figure 1).
Samples were taken from these plots to determine
species and plant biomass and a survey was carried
out to determine specific cover. In the survey, 25
photographs of the phytobenthos were taken in each
plot in a 20×20 cm frame, positioning the frame
according to a fixed scheme. For each picture taken, a
sample of macroalgae was collected for species
determination.
In each image, species cover (R
i
) was
determined by a phytosociological method (Boudouresque,
1971) with the aid of a grid. The average cover of
each species (RM
i
) was calculated for the plots. Cover
scores 1, 2, 3 and 4 for <5%, 5%~25%, 25%~50%
and >50% cover, respectively, were assigned to
phytobenthos species. The result was compared with
those of late July 2005 and 2006 (Lenzi et al., 2007),
carried out by the same method. For comparison of
the lists of species presence and cover for 2005, 2006
and 2011, we used explorative correspondence analysis
with the software package ade4
(Chessel et al., 2004;
Dray and Dufour, 2007; Dray et al., 2007).