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corresponding cascade down to the phytoplankton as is so often in the case in lentic <br />systems (Carpenter et at. 1987, Johannes et at. 1989, McQueen 1990, Shapiro and <br />Wright 1984, but see McQueen et at. 1989). It is somewhat surprising that there was <br />no effect of invertebrate predators on oligochaetes since both predatory and <br />omnivorous chironomids (such as T n s and Chironomus, as well as corixids of the <br />genus Tricorixa, are known predators of Oligochaetes. One explanation for this could <br />be the observation that oligochaetes often break into two pieces when in the grasp of <br />a predator (Loden 1974). Given the regenerative abilities of oligochaetes, this would <br />suggest that oligochaete biomass may decrease in the presence of predators while <br />their overall density remained the same (Wisniewski 1978). <br />Diversity <br />As shown in Table 5, chironomids appear to be more diverse in control and <br />perforated areas. These results, coupled with the fact that all of the uncommon <br />genera found were taken from the control and perforated treatments, suggest that fish <br />predation helps to maintain the diversity of chironomids in backwaters. This is <br />consistent with other studies that have shown that predation lessens the impact of <br />competition and even prevents the local extinction of some benthic invertebrates <br />(Connell 1961, Paine 1966, Menge and Sutherland 1976). <br />Implications <br />The Colorado squawfish, North America's largest minnow (cyprinidae), now <br />occupies less than 25% of its historical range and is listed as endangered by the U.S. <br />Fish and Wildlife Service (Karp and Tyus 1990). Though several factors have been <br />13