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<br />uJ2'184 <br /> <br />regarding insular ecology (Wilcox 1980, Smith 1978, Molles 1980, Stanford and Ward <br />1986b). <br /> <br />The island (insular) model originally proposed by MacArthur and Wilson (1967) <br />has been expanded by various workers to include any system that is insulated by <br />barriers so inhospitable that movement and dispersals of organisms into or out of the <br />system is greatly restricted (reviewed by Wilcox 1980, Frankel and Soule 1981). <br />Studies of large insular systems have shown that faunal collapse of vertebrate <br />communities (i.e., loss of most of the vertebrate species) may occur when isolated <br />ecosystems (isolates) are invaded by more diverse faunas (Wilcox 1980, Frankel and <br />Soule 1981). This collapse occurs for various reasons, but lack of habitat diversity and <br />lack of defenses to new predators are main reasons (e.g. see Frankel and Soule 1981). <br />Smaller isolates, such as reserves, have been shown to be universally sensitive to <br />faunal collapse (Wilcox 1980). The increasing fragmentation of the Colorado River <br />system is presumably very similar to these smaller isolates, and pressures on the <br />limited fauna do not have to be very great to cause serious impacts to them. In this <br />context, the impact of nonnative fishes must be considered as a great menace to <br />isolates of the once larger native fish community. <br /> <br />Nonnative Species of Concern <br /> <br />All of the fishes introduced into the Colorado River basin are suspected of <br />adversely affecting the native mainstream fishes in some fashion. Salmonid fishes <br />potentially have the least effect because they seldom come in contact with the younger <br />stages of the native fishes in the upper Colorado River. As an example, it has been the <br />experience of the senior author (HMT) that only the adults of the endangered species <br />are collected sympatrically with salmonids in the Yampa and Green rivers. However, <br />there are cases in which unusual circumstances result in predation by brown and <br />rainbow trouts on one or more of the endangered species. Valdez and Ryel (1995) <br />estimated that brown trout consumed 230,000 humpback chub annually in the Grand <br />Canyon, and that rainbow trout consumed 27,375 annually. In addition, these authors <br />reported that the trouts also compete with the chub. <br /> <br />Warmwater gamefish are thought to have the greatest adverse effect on <br />endangered native fishes. This is consistent with the ANSTF (1994) report which listed <br />centrarchids (e.g., largemouth bass, green sunfish, bluegill, black crappie, and <br />smallmouth bass) and ictalurids (e.g., channel catfish and bullheads) as frequent <br />contributors to the demise of native fishes nationwide. All of these species have been <br />identified as causing problems in the Colorado River system (e.g., Hawkins and Nesler <br />1991, Lentsch et al. 1995). <br /> <br />In a survey of regional fisheries biologists, Hawkins and Nesler (1991) identified <br />28 nonnative fish species in the Colorado River Basin that were threats to the <br /> <br />12 <br />