People have always been on the move, which helps explain how our species spread over the planet. They took with them species such as wheat, barley, rye, cattle, pigs, horses, sheep, and goats, but in the early years were limited by the available means of transport. Once steam-powered ships came into common use, the floodgates opened and over 50 million Europeans emigrated between 1820 and 1930, taking numerous plants and animals with them and often overwhelming the native flora and fauna. This tremendous economic performance has been built on an increasingly-homogenized foundation of information, finance, culture, and ecosystems. Many invasive species seem to do best in urban and urban-fringe environments where long histories of human disturbance have created vacant niches and abundant bare ground.
Cities tend to be the focal points of the global economy and the entry points for many invasives.
They also tend to be largely anthropogenic habitats, often of very great species richness. Despite some arguably positive effects on biodiversity at the local level, however, overwhelming evidence indicates the profoundly negative effects of introductions on species and genetic diversity at both the local and global level. The unique has been replaced by the commonplace. In the cases where the direct cause of extinction is identifiable, introduced species head the list. For example, introduced mammals are responsible for all but one of the nine known extinctions of endemic vertebrate species or sub-species from the islands of northwest Mexico.