Even alpine lakes at high elevations in national parks of the West have not been spared the introduction of exotic species. In the early twentieth century, brown trout from Europe, brook trout and Atlantic salmon from eastern North America, rainbow trout from several regions, golden trout from the southwestern United States, arctic char, rainbow trout from several locations, and several races of cutthroat trout, especially the Yellowstone cutthroat, were widely introduced into waters where they were not native, including naturally fishless alpine lakes (Donald, 1987; Bahls, 1992).

Although there were few studies of aquatic communities done in conjunction with the stocking, the few contemporary studies (Anderson, 1980) and later paleoecological studies (Miskimmin and Schindler, 1993; Lamontagne and Schindler, 1994) showed the impoverishment of invertebrate communities by stocked fish. In some cases, the eliminated species did not return (Paul and Schindler, 1994). Similar effects, dating to pre-World War I times, have been identified by paleolimnological studies (Pechlaner, 1984). Mosquitofish introduced in 1924 to control biting insects and other tropical fish released by hobbyists into the hot springs of Banff National Park eliminated the rare Banff longnose dace. The native bull trout is now endangered or eliminated from much of the West by interbreeding with introduced brook trout. The hybrids are sterile.

forage base involves two other species that invaded the Great Lakes, the alewife and the rainbow smelt, each of which has had large-scale effects on the lakes' food webs. The mechanism for the alewife's introduction to the Great Lakes is unclear (Scott and Crossman, 1973). Rainbow smelt were successfully introduced into Crystal Lake, a small lake connected to Lake Michigan by a stream, in 1912 after several failed attempts at direct establishment in the Great Lakes (Evans and Loftus, 1987). This population eventually expanded throughout the Great Lakes. Once established, rainbow smelt and alewife ultimately caused a substantial reduction in populations of native fish species (such as bloaters, lake whitefish, and cisco) through the combined effects of competition and predation (Crowder, 1980; Evans and Loftus, 1987; Crossman, 1991; McClain, 1991). Rainbow smelt are currently spreading through inland lakes throughout the Midwest with as yet unknown but likely important consequences for native species and food webs (Evans and Loftus, 1987).

Important invasions of aquatic ecosystems have not been limited to animal species. Several important cases, dating back many decades, involve the spread of plant species. The North American aquatic plant Elodea canadensis was introduced into Europe in the early 1800s and had

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