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Fouling

Transport of organisms on the hulls of ships

Solid ballast

Transport of organisms with the solid ballast of ships

Ballast water

Transport of organisms with the ballast water of ships

Canals

Movement of organisms through canals (but not on or in ships)

The high invasion rates in the Hudson are not unusual; estuaries and other aquatic habitats near centers of human activity typically contain large and increasing numbers of aliens (Table 21.3). However, the invasion history of the Hudson differs from that of the Great Lakes and San Francisco Bay (Fig. 21.3). Invasion rates in the Hudson were relatively high in the nineteenth century, but rose less sharply in the late twentieth century than rates in the Great Lakes and San Francisco Bay. These differences presumably reflect differences in human commerce in the three areas, with the Hudson subject to high inputs of solid ballast and immigrants from the Erie Canal in the nineteenth century, while

Interior

1 plant

Basin (40) 39 animals

1 plant

Basin (40) 39 animals

Europe (60)

Figure 21.2. Sources of alien species now established in the fresh waters of the Hudson River basin, based on data of Mills et al. (1996, 1997). Arrow widths are proportional to the number of aliens arriving from each source region, which is also given in parentheses after the name of the source region.

Europe (60)

Southern and eastern United States (8)

Figure 21.2. Sources of alien species now established in the fresh waters of the Hudson River basin, based on data of Mills et al. (1996, 1997). Arrow widths are proportional to the number of aliens arriving from each source region, which is also given in parentheses after the name of the source region.

Table 21.3. Numbers of alien species known from some American waters. Because data on microscopic organisms are inadequate, all of the values in this table are underestimated to an unknown degree

Body of water

Number of known aliens

Current rate at which new aliens are established (species/yr)

Notes

Hudson River basin

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