Most people know that humans have deeply affected the Earth's ecosystems by pollution, changes in land use, and overharvesting of wild plants and animals. It is much less well known that humans have caused large, widespread, and irreversible changes to ecosystems around the globe by introducing alien species (e.g., Office of Technology Assessment, 1993;Vitouseketal., 1996; Cox, 1999). (Alien species are those moved outside of their native ranges by human activities which subsequently establish self-sustainingpopulations. They are also known as exotic, introduced, or nonindige-nous species). Humans have introduced many alien species to the Hudson River that have profoundly changed the nature of the river's ecosystems. This chapter briefly discusses the numbers and origins of alien species in the Hudson, reviews the biology and impacts of selected alien species in the Hudson, shows that species introductions are one of the most important ways that humans have affected the Hudson ecosystem, and makes recommendations for improving the management of alien species.

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