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The effects of invasive species

The introduction of invasive alien species is a major threat succeed in the new environment. Several of these species
to ecosystem biodiversity, structure and function. They may periodically become a major problem, clogging waterways,
displace native species, reduce community biodiversity, fouling nets, and changing nutrient regimes in areas around
change species composition and abundance across fisheries, desalination facilities and aquaculture systems. In
habitats, modify habitat structure and produce cascading MPAs, the spread of invasive macroalgae such as Caulerpa
effects or trophic web shifts that could result in major sp., Lophocladia lallemandii or Womersleyella setacea
negative impacts on the ecosystem (EEA, Technical report Fig.4 & 5) might also reduce the attractiveness of the marine
No 16/2012). Nevertheless, their effects on the biodiversity landscape for scuba divers and cause a decline in marine
and habitats of the Mediterranean cannot be generalized, community diversity. Threatened or endangered species in
as alien species can cause very diverse effects at different those areas could also be at risk because of predation,
locations or different times, sometimes with a strong parasitism and competition with these alien invaders.
invasive component and sometimes not.
Caulerpa racemosa var. cylindracea, an invasive alga
Marine invasions can also have economic and human endemic to south-western Australia, has spread rapidly
health implications. In Europe, the economic impacts of throughout the Mediterranean, from Cyprus and Turkey to
non-native terrestrial and aquatic species have been Spain and all around the larger islands — including in MPAs
estimated to be at least EUR 12.5 billion per year, and (Fig. 4). It can form a dense canopy that overgrows native
probably amount to over EUR 20 billion (Kettunen et al., algae and significantly decreases their diversity and cover.
2009). Of that, the negative impact of aquatic invasive It increases siltation, reduces shoot density and biomass of
species alone has been estimated to cost the region at least some native seagrass meadows and prompts significant
EUR 2.2 billion per year. Nonetheless, there is limited changes in the benthic macrofauna.
comprehensive evidence for most economic impacts of
invasive marine species. Examples of any potential benefits For instance, in Port-Cros National Park and Scandola
provided by some of these invasive species or benefits Regional Park, the invasion by Caulerpa racemosa and
associated with the prevention and control programmes are Womersleyella setacea has been observed to affect the
even scarcer. Scientific research has only just started to survival rate and growth of juvenile colonies of the
glimpse the extent of some of these impacts in the gorgonian Paramuricea clavata (Cebrian et al., 2012) and the
Mediterranean and, for most of these introductions, the reproductive capacity of sponge communities (de Caralt
effects are completely unknown. and Cebrian, 2013). Alien filamentous, turf-forming algae
such as Acrothamnion preissii and Womersleyella setacea
Non-native macroalgae (seaweeds) are particularly likely to may establish an almost monospecific stratum suffocating
become invasive in coastal environments; they can easily the underlying communities and reducing species number
monopolize the available space, reduce biodiversity and and diversity in the affected area by trapping sediments.
change the whole ecosystem structure. Currently, the
Mediterranean has the largest number of introduced marine
plants in the world. More than 60 macroalgae have already
been introduced and 8 or 9 of them have been proved to
cause serious invasions (Piazzi and Balatta, 2009;
Boudouresque and Verlaque, 2003). For most of them,
however, and with the exception of the well-known Caulerpa
racemosa var. cylindracea, their invasive nature does not
seem to be a general phenomenon everywhere and it is
possible that they may not become invasive in every area
they colonize. This may be due to a variety of factors, such
as the presence of a predator species or less-than-ideal
habitat conditions.
Some common features of these macroalgae, such as their
vegetative reproductive capacity (a single propagule can
start a new colony), their production of toxic metabolites
that deter grazers or their perennial status, make them more
competitive than the native macroalgae species, increasing
the probability that they will become invasive if they Asparagopsis armata. Photo: B. Weitzmann.

Monitoring Marine Invasive Species in Mediterranean MPAs: A strategy and practical guide for managers 11