<P> A special case of introduction is the reintroduction of a species that has become locally endangered or extinct, done in the interests of conservation . Examples of successful reintroductions include wolves to Yellowstone National Park in the U.S., and the red kite to parts of England and Scotland . Introductions or translocations of species have also been proposed in the interest of genetic conservation, which advocates the introduction of new individuals into genetically depauperate populations of endangered or threatened species . </P> <P> The above examples highlight the intent of humans to introduce species as a means of incurring some benefit . While these benefits have in some cases been realized, introductions have also resulted in unforeseen costs, particularly when introduced species take on characteristics of invasive species . </P> <P> Unintentional introductions occur when species are transported by human vectors . Increasing rates of human travel are providing accelerating opportunities for species to be accidentally transported into areas in which they are not considered native . For example, three species of rat (the black, Norway and Polynesian) have spread to most of the world as hitchhikers on ships . There are also numerous examples of marine organisms being transported in ballast water, one being the zebra mussel . Over 200 species have been introduced to the San Francisco Bay in this manner making it the most heavily invaded estuary in the world . There is also the accidental release of the Africanized honey bees (AHB), known colloquially as "killer bees" or Africanized bee to Brazil in 1957 and the Asian carps to the United States . The insect commonly known as the brown marmorated stink bug (Halyomorpha halys) was introduced accidentally in Pennsylvania . Another form of unintentional introductions is when an intentionally introduced plant carries a parasite or herbivore with it . Some become invasive, for example the oleander aphid, accidentally introduced with the ornamental plant, oleander . </P> <P> Most accidentally or intentionally introduced species do not become invasive as the ones mentioned above . For instance Some 179 coccinellid species have been introduced to the U.S. and Canada; about 27 of these non-native species have become established, and only a handful can be considered invasive, including the intentionally introduced Harmonia axyridis, multicolored Asian lady beetle . However the small percentage of introduced species that become invasive can produce profound ecological changes . In North America Harmonia axyridis has become the most abundant lady beetle and probably accounts for more observations than all the native lady beetles put together . </P>

Most exotic species are introduced into a new environment accidentally as a result of