Taxonomy, time and geographic patterns. Chapter 2

A total of 1590 species of arthropods alien to Europe have already established on the continent, including 226 more or less cosmopolitan species of uncertain origin (cryptogenic). Th ese alien species are dispersed across 33 taxonomic orders, including crustaceans, chilopods, diplopods, pauropods, Symphyla, mites,arachnids, and insects. However, insects largely dominate, accounting for more than 87% of the species,far in excess of mites (6.4%). Th ree of the insect orders, namely Coleoptera, Hemiptera and Hymenoptera,overall account for nearly 65 % of the total. Th e alien fauna seems to be highly diverse with a total of 257 families involved, of which 30 have no native representatives. However, just 11 families contribute more than 30 species, mainly aphids, scales and hymenopteran chalcids. For a number of families, thearrival of alien species has signifi cantly modifi ed the composition of the fauna in Europe. Examples are given. Th e number of new records of aliens per year has increased exponentially since the 16th century, buta signifi cant acceleration was observed since the second half of the 20th century, with an average of 19.6 alien species newly reported per year in Europe between 2000 and 2008. Th is acceleration appears to be mainly related to the arrival of phytophagous species, probably with the plant trade, whereas the contributionof detritivores, parasitoids and predators has decreased. Some taxa have not shown any acceleration in the rate of arrivals. Asia has supplied the largest number of alien arthropods occurring in Europe (26.7%), followed by North America (21.9%) but large diff erences in the region of origin are apparent between taxa. Once established, most alien species have not spread throughout Europe, at least yet, with 43.6 %of the species only present in one or two countries, and less than 1% present in more than 40 countries.Large diff erences also exist between European countries in the total number of alien arthropods recordedper country. Italy (700 species) and France (690 species), followed by Great Britain (533 species), host many more species than other countries. Th e number of alien species per country is signifi cantly correlated with socioeconomic and demographic variables.