Invasion of the Mediterranean

The Mediterranean sea is home to 17,000 species, of which around 20% occur nowhere else. Millions of tourists visit the Mediterranean Sea each year, but what no one knows is that underwater we are witnessing the largest invasion currently underway on Earth. Almost 1,000 alien species, including fish, crustaceans, and algae have become established from other seas largely through human activities. These invasive species disrupt the balance of this unique marine ecosystem. For example, Siganus luridus and Siganus rivulatus fish from the Indian Ocean have invaded the eastern Mediterranean, where they overgraze on forests of brown algae in the shallows, stripping them bare to the rocks, with devastating effects on the native animal species within this ecosystem. Elsewhere, communities of native algae, corals, and invertebrates die because they are starved of oxygen, light, and food beneath the fast-growing invasive alga Caulerpa cylindracea, which forms mats of up to 15 cm thick.

An international team of researchers analyzed data from a new information system developed by the European Commission to show how the introduction of alien species has changed the native biodiversity within the Mediterranean:

Recently, the European Alien Species Information Network (EASIN) increased the accessibility to alien species spatial information by creating a network of interoperable web services through which data in distributed sources is accessed. Integrated distribution maps of single species or species aggregations can be easily produced with EASIN's freely available mapping tools.

Here, we utilize information on alien species distribution from EASIN to investigate the distribution patterns of marine alien species in the Mediterranean Sea, in relation to the main pathways of introduction. We investigate how specific human activities (opening of the Suez Canal, shipping, aquaculture) may shape the patterns of alien species distribution and consequently the overall biodiversity patterns in the Mediterranean Sea. We also compare the distributions of alien species with those of native ones to investigate differences in their patterns, and thus induced changes in pre-existing distribution patterns of native biodiversity.

The researchers found that around 60 species, mainly algae, have been introduced accidentally through aquaculture, especially off the coast of Venice and southwestern France. More than 400 species of alien fish and invertebrates in the Mediterranean have come by way of the Suez Canal which is only 145 years old. The rate of their arrival is increasing: over 80% first arrived less than 50 years ago.

The colleagues believe that "stowaway" species have profited from increasing shipping activities over this period, but the invasion has also been helped by global warming. Especially the waters between southern Turkey, Syria, Lebanon, Israel, Cyprus, and Egypt have become markedly warmer over the past 20 years, well suited for species from the Red Sea, the Arabian Sea, and the Indian Ocean. In this region of the Mediterranean, conveniently located near the exit of the Suez Canal, up to 40% of the marine fauna is now of alien origin.

The biodiversity of the Mediterranean Sea is changing, and further research is needed to better understand how the new biodiversity patterns shaped by human activities will affect the Mediterranean food webs, ecosystem functioning, and the provision of ecosystem services.