HEADLINE INDICATOR Trends in invasive alien species
Invasive alien species can have devastating impacts on native biota, causing extinctions and affecting natural and cultivated ecosystems. Since the 17th century, invasive alien species have contributed to nearly 40% of all animal extinctions for which the cause is known. In the Fynbos biome of South Africa, 80% of the threatened species are endangered because of invading alien species.
A proportion of invasive alien species are important pests or pathogens that can cause enormous economic costs. The annual environmental losses caused by introduced pests in the United States, United Kingdom, Australia, South Africa, India and Brazil have been calculated at over US$ 100 billion. Invasive alien species can transform the structure and species composition of ecosystems by repressing or excluding native species. Because invasive species are often one of a whole suite of factors affecting particular sites or ecosystems, it is not always easy to determine the proportion of the impact that can be attributed to them. In the recent past, the rate and risk associated with alien species introductions have increased significantly because human population growth and human activities altering the environment have escalated rapidly, combined with the higher likelihood of species being spread as a result of increased travel, trade and tourism.
A major source of marine introductions of alien species is hull fouling and the release of ballast water from ships, although other vectors, such as aquaculture and aquarium releases, are also important and less well regulated than ballast water. In the marine ecosystem, the movement of non-native species has been well studied. Of the 150 species that have recently arrived in the Great Lakes, 75% originated from the Baltic Sea. Similarly, migration flow from the Red Sea to the Mediterranean through the Suez Canal continues unabated with nearly 300 species of these Lessepsian migrants, including decapod crustaceans, molluscs and fishes, having entered the Mediterranean since 1891.
Equally long-term data available from five Nordic countries (Iceland, Denmark, Norway, Sweden and Finland) that have recorded the cumulative number of alien species in freshwater, marine and terrestrial environments since 1900 demonstrate the continuing arrival of new immigrants of plants, vertebrates and invertebrates (Figure 2.17).
Invasive alien species are a global problem requiring responses at all levels. Many countries have established systems to prevent and control invasive alien species and, as part of risk assessments, to predict the likelihood of alien species becoming invasive and the potential ecological and economic cost they may incur. To effectively communicate the challenges posed by invasive alien species there is a need to develop a methodology for integrating information quantifying the threat and its impacts on biodiversity into a coherent indicator.