David L. Strayer abstract Like most estuaries, the Hudson has been heavily invaded by alien species (those moved out of their native ranges by human activities). The Hudson and its tributaries now contain more than 100 alien species, which are abundant in every habitat in the basin. Invasions of the freshwater part of the basin (aliens in brackish waters of the basin have not been systematically studied) were underway by the early nineteenth century, and continue at a rate of ~7 new species per decade. Most of these species came from Europe or the Great Lakes through shipping activities, the Erie Canal, or deliberate introductions. Case studies of selected aliens (the zebra mussel, black bass, water-chestnut, Atlantic rangia, and Asian shore crab) show that alien species have had large and varied ecological impacts on all parts of the Hudson estuary. The impacts of alien species on the Hudson are likely to increase in the future as new aliens become established. We may be able to reduce future undesirable impacts of aliens on the Hudson by controlling or eradicating undesirable species, in the few cases where this is feasible; aggressively reducing the numbers of alien species that are inadvertently transported by human activities; and adopting more stringent criteria for allowing the intentional introduction of species.
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