Distribution and trends in abundance of non indigenous species in the Mediterranean
Both the number and rate of non-indigenous introductions to the Mediterranean have been increasing in recent years (UNEP/ MAP 2009). Currently, about a thousand non-indigenous aquatic species have been identified in the Mediterranean Sea, with a new species being introduced roughly every ten days. About 500 of these species are well-established; many others are one-off observations (UNEP/MAP 2012). In addition, there are also ter- restrial non-indigenous species that have been introduced into the coastal zone of the Mediterranean but these species have so far not been systematically monitored.
Benthic, or seabed-living, animals are the most plentiful non-in- digenous species in the Mediterranean. Most are molluscs, crus- taceans, or sea worms (UNEP/MAP 2009). More non-indigenous species are found in the Eastern Mediterranean than in the West- ern Mediterranean.
Major vectors of introduction
Non-indigenous species enter the Mediterranean through three broad avenues: • Natural invasion through waterways such as the Suez Canal or Straits of Gibraltar;
• Transportation by ships through clinging or fouling on ship hulls, ballast water; and,
• Intentional and unintentional introduction by aquaculture activities, including commercial species, bait, and species for the aquarium trade (EEA and UNEP 1999).
Maritime transportation and aquaculture are the main ways non-indigenous species enter the Western Basin of the Mediter- ranean. Migration through the Suez Canal is responsible for most non-indigenous species in the Eastern Basin.
An estimated 47 % of non-indigenous species now in the Medi- terranean entered through the Suez Canal, with 28 % trans- ported by vessels and 10 % introduced through aquaculture (UNEP/MAP 2009). Species introductions from marine transport have increased because of the growth in shipping oil from the Middle East and consumer goods from Southeast Asia (UNEP/
Non-indigenous species, also known as alien species, are organisms that have entered ecosystems outside of their previously known ranges and that may survive and subsequently reproduce. They can be classified as unestablished, established, invasive (rapidly in- creasing numbers and range), or noxious (posing a risk). Source: Occhipinti-Ambrogi and Galil 2004
MAP 2009). The waters around Israel and Turkey have the high- est number of non-indigenous species, mainly because of their proximity to the Suez Canal (UNEP/MAP 2009).
Impact of nonindigenous particularly invasive species
The vulnerability of an ecosystem to non-indigenous species de- pends upon the health of that system (UNEP/MAP 2012). A pol- luted environment, for example, is more vulnerable than a pris- tine one. Physical damage from dredging, bottom trawling and other forms of habitat destruction also make an environment more vulnerable to the pressures brought by non-indigenous species (UNEP/MAP 2009).
Ecological impact The effect non-indigenous species have on native biodiversity is poorly understood in most cases (UNEP/MAP 2012). Although no recorded extinctions of native Mediterranean species have been
Some documented ecological impacts of invasive non- indigenous species:
• Predation on native species affecting marine food chains • Invasive non-indigenous species of fish – parrotfish (Thalassoma pavo), yellowmouth barracuda (Sphyrae- na viridensis), and bluefish (Pomatomus saltator), for example – prey on commercial fish species.
• Competition with native species • Invasive non-indigenous algae of the genus Caulerpa displaced native sea grass (Posidonia spp.) meadows.
• In Israel three native species – a starfish (Asterina gib- bosa), a prawn (Melicertus kerathurus), and a jellyfish (Rhizostoma pulmo) – decreased in abundance at the same time as three non-indigenous species – also a starfish (A. Burtoni), a prawn (Maruspenaues japoni- cas), and a jellyfish (Rhopilema pulma) – increased in abundance.
• Changes to native communities • One invasive non-indigenous seaweed (Caulerpa taxi- folia) can create dense mats that affect benthic com- munities and reduce spawning and feeding grounds for fish.
• Another, related non-indigenous species (C. racemosa) can grow over other species of seaweed and has been linked to a decrease in sponges.
Sources: UNEP/MAP 2012; EEA and UNEP 2006 56 STATE OF THE MEDITERRANEAN MARINE AND COASTAL ENVIRONMENT