The paradox of invasion

It is paradoxical that exotic species invade and displace native species th at are well adapted to local environments. Yet, even those exotics that eve ntually become abundant and widespread, often do so only after having faile d to establish following multiple earlier introductions. The first pattern, while not generally discussed in this context, is usually explained by exo tic species pre-adaptations for human-altered environments and by a release from enemies. It can be understood further by examining the superior quali ty of colonists from large species-rich regions and the historical continge ncy of evolution. The second pattern is generally explained by invoking dem ographic and environmental stochasticity; however, it can be understood fur ther by examining the role of environmental variation over space and by met apopulation dynamics. These processes provide a context in which these patt erns of invasion are not paradoxical, but instead, expected.