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606 SCOPPETIONE study was narrowed to focus primarily on how nonnative adults affected native forms. Predation on young has been identified as a mechanism by which nonnative fish replace na- tives (Meffe 1985). The results of my study indi- cate that predation on young Moapa dace by adult shortfin mollies might have contributed to the population's numerical decline. Under' experi- mental conditions, shortfin mollies were formi- dable predators of fish larvae. I have seen them cannibalize their own young in aquaria and in me- socosms with ample forage, where they consumed Moapa White River springfish larvae as well (un- published data). However, analysis oftheir stom- ach contents suggested that they tend toward her- bivoryin the wild. Demonstration that they prey upon Moapa dace larvae in the wild is made dif- ficult by the current rarity of Moapa dace. Also, the sample of shortfin molly guts examined was low, and it was taken in a month (November) when Moapa dace reproduction is low (Scoppet- tone et al. 1992). To enhance the probability of determining their predatory potential on Moapa dace larvae, shortfin molly adults should be col- lected in the spring or summer, when Moapa dace recruitment is greatest. If shortfin mollies indeed prey upon larvae in the wild, they probably would affect the Moapa dace population more than the Moapa White Riv- er springfish population. Microhabitat overlap in- dices for adult shortfin mollies were 44% with Moapa dace larvae but only 19% with Moapa White River springfish larvae. Moreover, 87% of Moapa dace larvae. occurred in habitat used by shortfin mollies, compared with only 30% of springfish larvae. Relative vulnerability of larvae may also explain why Moapa dace declined after the introduction of shortfin molly, even though the species had not been notably affected by the arrival of western mosquitofish, a known predator of fish larvae (Minckley 1973; Moyle 1976; Meffe 1985), 25 years earlier. Larval Moapa dace showed only 25% habitat overlap with adult western mos- quitofish, and 36% of them occurred in habitat used by adult mosquitofish. My data indicate that adult Moapa dace and Moapa White River springfish probably prey very little on each other's larvae. Moapa dace adults had trivial overlap in microhabitat use with springfish larvae, and springfish adults indicated only 18% overlap with dace larvae. Furthermore, Moapa White River springfish did not appear to feed aggressively on fish larvae in aquaria. Macrohabitat analysis added further evidence that adult shortfin mollies were a substantial force in reducing the Moapa dace population. The rel- atively high density of shortfin mollies in Moapa dace nursery tributaries may have intensified their predatory threat to larval dace by further enhanc- ing the probability of encounter rates. Meffe (1984) showed that another poeciliid, Gila topmi~now Poeciliopsis occidentalis, increased its intensity of cannibalism when its population density in- creased. Western mOSQuitofish had relatively low densities in my study area, about 10% of the num- ber of shortfin mollies. Although the highest number of Moapa dace adults occurred in the tributary with the lowest density of shortfin mollies, no conclusions can be drawn from this observation. Moapa dace are transients; most of the approximately 2,900 re- maining adults were in the four tributaries at the time of the survey, but most adults typically occur in the upper Muddy River (Scoppettone et al. 1992). Although there seems to be enough separation in habitat use to ensure only a small amount of interaction between Moapa dace and shortfin molly or western mosquito fish, one cannot discount the possibility that a niche shift or interactive segre- gation has occurred because of intense competi- tion. Such shifts have been observed for other spe- cies (Werner and Hall 1979; Brown and Moyle 1991). The weight of evidence I have collected so far gives little support for such a shift, but it is difficult to prove such shifts without experimental introductions. Nonnative poeciliids have caused the decline, extirpation, or extinction of several native fish populations (Cross 1976; Courtenay et al. 1985; Meffe 1985; Courtenay and Meffe 1989). When predation on larvae is the mechanism affecting native forms, analysis of relative overlap in mi- crohabitat between the potential predator and prey may explain why one species is more affected by the introduced form than another. The approach used in this study may be useful in determining why a species is driven to extinction in one habitat while it persists in another, as has been noted for the Gila topminnow (Meffe 1985). Also, this study adds information on a subject that has received much discussion but little study, the mechanisms by which native fishes are affected by nonnatives. Acknowledgments H. Burge, P. Tuttle, P. Rissler, M. Parker, and N. Kanim assisted in field work and data collec- tion. M. Parker analyzed guts for food items con-