Alien macrophytes in the Mediterranean Sea: an overview

Abstract

The Mediterranean Sea, one of the most complex marine ecosystems, is inhabited by a rich and diverse biota which is disproportionate to its dimensions. Such high species richness makes the Mediterranean Sea a true hotspot of biodiversity. However, the Mediterranean Sea is amongst the most impacted regional sea areas, due to increasing levels of threats, mainly driven by human activities such as climate change and the introduction of non-indigenous species (NIS, i.e. organisms introduced outside of their natural, past or present, range and outside of their natural dispersal potential). NIS may in time become invasive (i.e. invasive alien species (IAS)) with severe impacts on biodiversity and ecosystem services. Due to the dramatically accelerating rate of such introductions (nearly 1000 marine NIS so far) the Mediterranean Sea may be considered as a true hotspot of marine biological invasions in terms both of the number of species and rate of introduction. In the marine realm, species richness not always confer an “invasion resistance”. For instance, biological invasions may severely affect Marine Protected Areas, whose major aim is biodiversity conservation, due to their proximity to marinas or tourism activities. The Mediterranean Sea harbours the largest number of non-indigenous macrophytes (around 120-130 taxa) and their number has steadily increased over time. The main vectors of introduction of non-indigenous macrophytes into the Mediterranean Sea are shellfish aquaculture, shipping and the Suez Canal, while the main donor region is the Indo-Pacific Ocean. Most of Indo-Pacific invaders are still confined to the Eastern Mediterranean Basin whereas some have spread to the Western Basin, and with the forecast increase of temperature the westward spread is expected to continue. In the Mediterranean Sea, the Rhodophyta Womersleyella setacea (Hollenberg) R. E. Norris and the Chlorophyta Caulerpa cylindracea Sonder, successfully colonizing valuable habitats (e.g. coralligenous outcrops and vermetid reefs), are among the most harmful IAS. Due to the significant threats biological invasions pose, NIS are targeted in the more recent legislative instruments, such as the Strategy on Invasive Species, the Marine Strategy Framework Directive and the Biodiversity Strategy. Accurate and reliable data on the distribution, pathways and impacts of NIS, contained in several inventories and databases which share their data over the web, are essential for developing effective policies for prevention and control. However, to stem the tide of IAS introduction, a coordination and cooperation among all the States bordering the Mediterranean Sea is needed.