Numbers Origins and Characteristics of Aliens in the Hudson

How many alien species live in the Hudson River? Where did they come from, how did they get here, and when did they arrive? We do not yet have complete answers to these questions, but a study of alien species in the freshwater parts of the Hudson basin (Mills et al., 1996, 1997) provides partial answers. Mills et al. concluded that at least 113 alien plant and animal species have become established in the fresh waters of the Hudson basin (Table 21.1). This estimate is probably far too low, because we do not have good distributional or historical information on microscopic organisms, and so have no way to distinguish native species from aliens. Thus, a large part of the freshwater flora and fauna of the Hudson basin is not native, but was brought here by humans.

Records of species introductions are incomplete, so the time-course of invasions shown in Figure 21.1 is only approximate. Nonetheless, alien species had already arrived in the Hudson basin by the beginning of the nineteenth century, and invasions continue to this day. It appears that the invasion rate has been relatively constant (about 7 species per decade) since the mid-nineteenth century. The kinds of invaders have changed markedly over that period, though, from mostly plants to mostly animals (Fig. 21.1).

Table 21.1. Numbers of native and alien

species in fresh waters of the Hudson River

basin, modified from Mills et al. (1996)


Native Alien

% alien

Aquatic mammals

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