Biological invasions by alien (c.f. non-native, non-indigenous, foreign, exotic) species are recognised as a significant component of global environmental change, often resulting in a significant loss in the economic value, biological diversity and function of invaded ecosystems (Wittenberg and Cock 2001). Numerous alien species, many introduced only in the last 200 years ago, have become successfully established over large areas of Europe (Hulme 2007). Future global biodiversity scenarios highlight potentially dramatic increases in biological invasions in European ecosystems (Sala et al. 2000). Interacting effects through rising atmospheric CO2 concentrations, warmer temperatures, greater nitrogen deposition, altered disturbance regimes and increased habitat fragmentation may facilitate further invasions (Vilà et al. 2006). Early warning and prevention of the harmful impact of alien species on ecosystems is a fundamental requirement of the European Biodiversity Strategy and the EU Action Plan to 2010 and Beyond (European Commission 2006) yet, in the absence of reliable regional analyses, the European states have been unable to tackle this issue strategically (Miller et al. 2006; Hulme et al. 2007). \ud In the United States, the cost of biological invasions has been estimated to total $97 billion hitherto for 79 major bioinvasions (Pimentel et al. 2001). Although only limited monetary data are available at present for Europe, there is a similar indication that biological invasions have imposed losses on the economy. The strongest evidence is for alien pest and weeds that impact upon the agriculture, forestry, aquaculture and other sectors (Williamson 2002). Examples of direct economic impacts include the damage caused by Japanese knotweed Fallopia japonica to flood defences and the impact of bark stripping by grey squirrels Sciurus carolinensis on forestry production. The western corn rootworm Diabrotica virgifera was accidentally introduced in the 1990s into Serbia and is an important pest of maize and leads to yield losses. Preliminary studies on the potential of establishment of the western corn rootworm show that this pest is likely to survive and develop wherever maize is grown in Europe. Leaving aside introduced pests and diseases affecting agriculture, alien parasites such as Gyrodactylus salaris (an ectoparasite of Atlantic salmon) and Anguillicola crassus (swimbladder nematode of eels) have led to dramatic decreases in fisheries sector incomes in several Nordic countries. The American oyster drill Urosalpinx cinerea is an important gastropod pest of the cultured oyster industry as it feeds preferably on oyster spat and is recorded as consuming more than half the oyster spat in certain European estuaries (Cole 1942). The muskrat Ondatra zibethicus and coypu Myocastor coypus, both introduced by the European fur industry, damage river banks through digging and have increased the risk and severity of floods in many central and southern European countries. Notorious invasive alien weeds are of major economic significance, e.g., Mexican tea Chenopodium ambrosioides, knotgrass Paspalum paspaloides, Canadian horsew-eed Conyza canadensis, Bermuda buttercup Oxalis pes-caprae. While other alien plants act as hosts of plant pathogens e.g., rescuegrass Bromus catharticus as host for barley yellow dwarf virus and wheat stem rust. Invasive alien species can also affect human health e.g., phytophotodermatitis through contact with giant hog-weed Heracleum mantegazzianum, asthma and hay-fever arising from the pollen of annual ragweed Ambrosia artemisiifolia, poisoning of humans through consumption of toxic fruit e.g., American pokeweed Phytolacca americana, silverleaf nightshade Solanum elaeagnifolium, or leptospirosis spread by the brown rat Rattus norvegicus. \u
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