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It is difficult to predict when an alien species will become that its invasiveness has been proven elsewhere in the world
invasive, as it does not always happen. Generally, the (Boudouresque and Verlaque, 2002). In this guide, we will
invasion process consists of several major stages, from the use the term invasive species in accordance with the
transport of a species into new habitats to its establishment previous description to include all alien species with
and eventual spread (Fig. 1). Each of these stages is limited proven invasive potential, meaning that they are able to
by a set of barriers that will determine whether or not the establish viable populations, can expand into previously
species will move on to the next stage in the invasion uncolonized areas in the Mediterranean Sea and are
process and finally become an invasive species (Blackburn capable of damaging the environment, the economy, or
et al., 2011). An alien species must progressively overcome human health.
a series of geographical, survival, reproductive and
dispersal barriers before it can finally expand into a new Marine invasive species are regarded as one of the main
environment. During this final stage of invasion, the species causes of biodiversity loss in the Mediterranean (Galil, 2007;
can ultimately also be affected by ‘boom and bust’ cycles Coll et al., 2010), potentially modifying all aspects of marine
and pass through periods of sudden population decline or and other aquatic ecosystems. They represent a growing
growth, as observed in several marine alien species in the problem due to the unprecedented rate of their introduction
early years of invasion. (Zenetos et al., 2010) and the unexpected and harmful
impacts that they have on the environment, economy and
Because of this, it is generally assumed that the best factor human health (Galil, 2008). This is a general phenomenon
for distinguishing an invasive species from other aliens is that extends to all regions of the Mediterranean (Galil, 2007,
2009; Zenetos et al., 2010). That is why invasive species are
considered ‘focal species’ and should be monitored in all
regions (Pomeroy et al., 2004).
More than 5% of the marine species in the Mediterranean
are now considered non-native species (Zenetos et al.,
2012; Fig. 2). According to the latest regional reviews,
13.5% of those species are classed as being invasive in
nature, with macrophytes (macroalgae and seagrasses) the
dominant group in the western Mediterranean and Adriatic
Sea, and polychaetes, crustaceans, molluscs and fishes in
the eastern and central Mediterranean (Galil, 2009; Zenetos
et al., 2010; Zenetos et al., 2012). The vast majority of alien
species occur in the eastern Mediterranean; some are
located exclusively in the south-eastern basin, others are
Fig. 2. Number of marine alien species per major groups in the restricted to the western basin, while others have colonized
Mediterranean Sea. From Zenetos et al., 2012. the entire Mediterranean.

Aplysia dactylomela. Photo: E. Azzurro

Monitoring Marine Invasive Species in Mediterranean MPAs: A strategy and practical guide for managers 09