Invasive species are non-indigenous organisms introduced into an ecosystem that is not their native habitat either by accident or intentionally.
While some alien species may have little impact within their new habitat, others can become invasive and pose a serious threat to marine biodiversity, coastal economies, local cultures and livelihoods, and human health.
The threat of alien invasive species continues to grow as global trade, travel, and tourism allow species to be transported over increased distances to areas that were not previously accessible to them. Areas subjected to the worst pollution, intensive fisheries and/or bottom trawling, and major shipping routes are likely to be the most seriously impacted by the invasion of non-native species.
The approximately 3–5 billion tons of ballast water from large ships transferred throughout the world each year by large ships (Raaymakers, 2002) is believed to be the main vector for the spread of invasive aquatic species today, with an estimated 7,000 species transported each day (Carlton, 2001). Large numbers of alien species are also transported as ‘hitchikers’ attached to the hulls of ships or on floating objects such as marine trash.
How Was It Measured?
A spatial map from (Micheli et al. 2013) were used in this analysis. This map is based on the actual distribution of a subset of invasive species in the Mediterranean compiled by The Mediterranean Science Commission (CIESM Atlas of Exotic Species, www.ciesm.org) on the distribution of exotic crustaceans, fish, molluscs and macroalgae in the Mediterranean. CIESM provided low resolution, per-species images showing the area impacted by each invasive species.