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171 1 he Southwestent Naturadst to several years after colonization, although long-term coexistent y ,i kA occur. On a geographic scale, topminnows have declined only in the un>r of overlap of these two species (Meffe et al., 1983). In northwestern Mexicn, where G. affinis has only recently colonized a few localities, P. occidentattu remains abundant (Hendrickson et al., 1981). HYPOTHESES.-Although several topminnow populations have beret extirpated by habitat destruction (Miller, 1961; Minckley and Deacon, ISN, Minckley, 1973; McNatt, 1979; Meffe et al., 1983), many others have btrly lost with no apparent environmental change other than introduction (? mosquitofish. It is unlikely that subtle physical or chemical fluctuation, contributed to decline of this species, as the fish can tolerate a wide ranr;r of conditions (Schoenherr, 1974; Meffe et al., 1983). These observation, suggest that interaction with G. affinis is a major factor in local extirpation of P. occidentalis. Specific mechanisms of replacement are not known, although several hypotheses were advanced (Miller, 1961; Minckley, 1975- Schoenherr, 1974, 1981). The more plausible of these are summaiirrd below, and I present experimental evidence for predation as,a major (an,r of topminnow losses. Parasites and Disease.-Introduced organisms may harbor parasites of diseases that can decimate native species (Elton, 1958, and refer(-u(n therein), and transfer of parasites to native fishes by introduced fish vector? has been documented in some instances (Hoffman, 1970; Bauer anti Hoffman, 1976; Deacon, 1979). In the present case no new parasites Of diseases of P. occidentalis are evident and the fish appears healthy w•herr sympatric with introduced species. Mpoame (1981) found only the ricalaurcle Camallanus sp. infecting P. occidentalis, with no evidence that parasites to introduced fishes affected any native Arizona species. Therefore, hypothvwl suggesting decline of Sonoran topminnows through introduction c>) parasites or diseases are not supported. Hybridization.-There is no evidence that P. occidentalis and G. affirrtt hybridize. Hybrids were not observed in the field or laboratory (Schoenherr. 1974; Meffe, pers. observ.), and conspecific mate choice fidelity was high in laboratory observations (Schoenherr, 1981). Competition.-Because topminnows and mosquitofish are morphology tally similar (Minckley, 1973), and competition is generally greatest among similar species (Darwin, 1859; Gause, 1934; Miller, 1967), one might predict intense interspecific resource competition; however, none can be demonstrated between P. occidentalis and G. affinis (Schoenherr, 1981; Meffe et al., 1983). The mosquitofish is largely a piscivore/insectivoir (Harrington and Harrington, 1961; Myers, 1965), while topminnows ford primarily on detritus and vegetation, with insect larvae and amphip od crustaceans important in the diet of some populations (Minckley, 1975; Deacon and Minckley, 1974; Schoenherr, 1974; Constantz, 1976; Gerking and Plantz, 1980). Food choice also seems little influenced by sympau) (Schoenherr, 1981). Additionally, trophic morphologies are quite different. Mosquitofish have strong, conical teeth, firmly attached to the jaw, and gut length about 0.69 times standard length (SL), whereas Sonoran .1", r1w 1985 srcuc-„ . ............... ._.,,..............-------- vol. Sll e ina 1 „ninnows have elongate, spatulate teeth, rnore weakly attached to the 7)L ow, and a gut length 1.5 to 2.0 times SL (Schoenherr, 1981; Meffe et al., i+g13). These trophic morphologies are characteristic of a carnivore and an urbivore/detritivore, respectively (Al-Hussaini, 1949; Barrington, 1957; lrgler et al., 1977). Furthermore, there is no evidence that food is limiting n highly productive desert habitats (Schreiber and Minckley, 1981), and ttowth rates also appear unaffected in sympatry (Schoenherr, 1981). (irmpetition hypotheses therefore appear unsupported. Predation.-Predation was suggested as a major factor in decline of i.,noran topminnows (Minckley, 1973; Minckley et al., 1977; Schoenherr, 1481; Meffe et al., 1983), yet the hypothesis was not adequately tested. r;antbusia affinis is a known piscivore and its introduction was correlated .01 population reductions of more than 20 species of fishes (Schoenherr, :981). It eats P. occidentalis in the field and laboratory (Minckley, 1973; vhoenherr, 1981; Meffe et al., 1983) and is potentially capable of exerting ;sedation pressure on the species. I tested the hypothesis that predation on pneniles causes local extinction of Sonoran topminnows. Physiological Stress.-Schoenherr (1981), through observations of these Iwo fishes in aquaria and an experimental pond, and with concordant ,observations by P. Winkler (in Schoenherr, 1981), concluded that ,nosquitofish physiologically stress topminnow adults by constant Aggression. This resulted in reduced fecundity, cessation of feeding, and increased mortality of P. occidentalis when sympatric with G. affinis. I ,ntcd Schoenherr's hypothesis through examination of those parameters in i controlled laboratory experiment, and at a field site with natural rxlxrimental and control groups. %I,TEBtAIS AND METHODS.-Laboratory Coexistence Study.-I examined impacts of predation 1, G. affinis on population size of P. occidentalis under non-competitive conditions. The null t,l.,thesis is that there is no difference in topminnow population growth rates, and hence ,owunent, when in allopatry or sympatry with mosquitofish. smipatric and allopatric populations of each species were established in 75.0 and 37.5 1 .imria, respectively. Six replicates of each were in simple habitats (sand substrate only) and 6 n,,,mplex habitats (sand, rocks, and submergent vegetation 1Vesicularia sp., Hypnaceal). Eight ' -,, Ies and four males of either or both species were stocked on 16 July 1981, at a density of ';R fish%m' (natural densities vary widely from a few individuals to several hundred fish per m'). I yuninnows were taken from a natural spring-seep in southern Arizona (Meffe et al., 1982) and n ?uµnitofish were from an artificial pond at Arizona State University (ASU). lisp were fed powdered trout chow twice daily, with frozen brine shrimp substituted three .:ens weekly. Fish were fed to excess, thus eliminating food competition. Populations were ennrd at regular intervals for a total of 63 weeks and (lead fish were removed and preserved n 10% formalin. Adult fernales were dissected as they died, and mature ova and embryos •?idit}) counted. field coexistence study.-The above design was repeated in a natural habitat, a small spring .n near Bylas, Arizona (habitat descriptions are in Meffe, 1983x, and Meffe and Marsh, 1983), -1&00 ing the same null hypothesis. Open-bottomed cages of 15 mm-mesh hardware cloth lined -ah nylon window screening were sunk into the substrate on 24 June 1981. Three allopatric i :IN) r m') and three sympauic (2,400 em') covered cages were used. Seven female and three male •w,minnows were stoked in the former, and seven males and three females of both species were curd in the latter. All fish were collected in situ, and stocked at densities of I fish/120 an' .alate area (water depth = 3-10 on). Only natural food was available. Individuals in cacti cage -i, tounted 5, 13, and 24 days after stocking.