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The alien species in the freshwater parts of the Hudson basin came from two major sources: Europe and the American Interior Basin (i.e., the Great Lakes and Mississippi drainages) (Fig. 21.2). Alien plants in the Hudson basin are chiefly from Europe, while animals originated largely from the Interior Basin (Fig. 21.2). Alien species were brought into the basin by several methods (Table 21.2). European species came into the basin in the solid ballast and ballast water of ships, as agricultural escapes and weeds, and as deliberate introductions. In contrast, Interior Basin species moved into the Hudson mainly through the Erie Canal, and secondarily through deliberate introductions.

Patterns of species invasions reflect the extent to which human activities break natural barriers to dispersal. Plant invasions were most frequent in the nineteenth century because solid ballast and agriculture brought many alien plants over the previously insurmountable barrier of the Atlantic Ocean. Plant invasions slowed in the twentieth century, in part because solid ballast was replaced by ballast water. At the same time, the switch to ballast water opened a gate for freshwater animals that could not cross the ocean in solid ballast but could survive the oceanic voyage in ballast tanks, and invasion rates of aquatic animals rose. Likewise, the opening of the Erie Canal in 1825 breached a major barrier to eastward dispersal of aquatic animals, many of which then invaded the Hudson Basin. The Hudson Basin does not have many alien plants from the Interior Basin because the Alleghenian Divide between the Interior Basin and the Hudson Basin was never a significant barrier to natural dispersal of plants.

Although we know that alien species live in the brackish waters of the lower Hudson estuary, we do not have a comprehensive analysis of invasions like that done by Mills etal. (1996,1997). Nonetheless, we can guess about aliens in the lower Hudson based on the probable vectors that brought aliens into this area. Shipping activities probably brought many aliens into the lower Hudson, either as ballast or as fouling organisms on ships' hulls. Because of the long history of ship traffic between Europe and NewYork harbor, shipping probably brought aliens into the lower Hudson beginning in the sixteenth century, and continues to bring in new aliens today. Invaders from early in this period maybe so widely naturalized along the East Coast that they appear to be native [Carlton (1996) called these "cryptogenic species" because their foreign origin is so obscured]. In contrast to the situation in fresh water, canals probably were insignificant as a vector. Thus, the lower Hudson probably contains many (>100) alien species; most of them arrived in or on ships, especially from Europe; the lower Hudson was already well invaded before 1800 by aliens that are so well established that they may appear to be natives; and plants and invertebrates, rather than fish, constitute the bulk of invaders.

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