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While these alterations may not eliminate a robust and widespread species, they are <br />likely to cause its decline and make the native population more susceptible to the effects <br />of habitat alteration or isolation. Similarly, habitat alteration may reduce the range and <br />abundance of a native species, and make the native species more susceptible to <br />predation or competition from introduced species, especially where the introduced <br />species is favored by habitat changes. Population decline results in small population <br />size, and small populations are prone to problems such as demographic stochasticity, <br />genetic deterioration, social dysfunction, and extrinsic forces (reviewed by Raup 1991). <br />Habitat fragmentation also could reduce the size of isolated populations. Extinction is <br />more likely to occur in small populations that have fallen below the size of minimum <br />viability (Soule' 1983; Raup 1991). <br />The case for conservation becomes most urgent as the population size of a <br />species becomes very small. Random events such as a chemical spill that would have <br />a relatively minor impact on a large and widely distributed population, could have <br />catastrophic and perhaps permanent effects on a smaller, restricted population. In the <br />interest of preserving the species and maintaining biodiversity, extraordinary measures <br />may be required to prevent extinction. For the big river fauna of the Missouri River that <br />are threatened by biotic factors, control of introduced species becomes imperative. The <br />task is not necessarily a simple one: control of introduced species has been called the <br />"nasty necessity [because of] misconceptions about the nature and magnitude of the <br />problem, fears of the negative public reactions... and intimidation by the inefficient labor - <br />intensive nature of current eradication technologies" (Temple 1990). In hindsight, the <br />best policy is not to introduce nonnative species in the first place. <br />CONCLUSIONS <br />At least nine native fishes have experienced significant declines in abundance in <br />all of some portion of the river system. These declines have been overwhelmingly <br />attributed to physical and chemical alterations of habitat, and by comparison, the <br />introduced fish problem has been largely ignored. <br />Excluding common carp, early introductions into the mainstream Missouri River <br />did not generally result in the establishment of new fish populations (Pflieger 1997). <br />However, as stream habitats were altered by water resources development, nonnative <br />fishes were more successful in these modified environments. In the newly- created <br />reservoirs, turbid riverine conditions were replaced by clear lacustrine -like environments <br />and stocked with hardy, predaceous, highly aggressive, and/or competitive fishes. <br />Some introduced fishes were "pre- adapted" to the modified environments and thrived in <br />them. But these same conditions were alien to the native riverine fishes, which <br />universally declined. Remaining riverine habitats are more suitable to the native fishes, <br />and some have persisted with "viable" populations. Numbers of predaceous fishes in the <br />remaining riverine habitats have steadily increased due to direct stockings, and <br />escapement from stocked reservoir and tailwater reaches. <br />29 <br />