The history of colonization of various animal and plant groups indicates a number of patterns. For instance, many European bird species have successfully colonized North America (but not vice versa), and nearly half of all North American insect pests are considered to be of exotic origin (Kim et al. 1993). Similarly, the European continent has been successfully invaded by deliberate introductions by several North American crayfish species. The notable example is Procambarus clarkii, which may now be considered nearly global by its current distribution. The most successful crayfish species to colonise are relatively r-selected species, with short life cycles and high fecundity. Procambarus clarkii is a relatively r-selected species, while the native Astacus astacus, for instance, is a relatively K-selected species, with a long life cycle and low fecundity. A. astacus is, thus, more competitive in mature ecosystems, while P. clarkii is able to colonize disturbed habitats and areas which have been modified by man. Modified aquatic habitats are often more tropical or temperate in terms of their thermal conditions. An additional factor influencing the success of North American crayfish in Europe often involves their simultaneous resistance and ability to transmit the crayfish fungus plague, Aphanomyces astaci, to highly susceptible native crayfish.