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IH? <br />A 14 <br />1,2 <br />1.0 <br /> <br />I <br />W 06 <br />0.4 <br />0.2 <br />B 35 <br />30 <br />25 <br />r <br />1- 20 <br />J <br />LLJ w 15 <br />L <br />10 <br />5 <br />t tie suuouvestrr?i tvaturaost <br />•, <br />0 °i <br />0op <br />ptO' 0 <br />0 <br />, • <br />/?0 <br />0 0 <br />0 <br />eo sb -0 d - <br />`b • o <br />26 28 30 <br />0 <br />VOL 3u, iwl ^.. s 198' -1--. .11 .. ...... ...... ... 1,,...1..,......,....""" <br />1. A n A It was subset uently re-stocked with the original P <br />• <br />• <br />• o <br />i <br />v <br />•,tr o <br />s <br />i 0 <br />• <br />0 <br />,•-ir _ _ - <br />• o <br />32 34 36 <br />• <br />• <br />0 <br />:-"or <br />o • <br />• • <br />0 Zo <br />fQ 0? 0 o ° <br />26 28 30 32 <br />0 <br />0 <br />34 36 <br />STANDARD LENGTH (mm) <br />Fit;. 5.-Results of field stress analysis. Symbols as in Figure 3. <br />ptu tut c nttrnyct 1 <br />,,odentalis (Meffe, 1983a). Before removal of G. affinis, juvenile <br />topminnows numbered in the tens of individuals in each of two spring <br />rrurces. Three weeks after re-stocking there were hundreds of juvenile <br />,ptninnows at densities higher than I had ever observed, stiggesting a <br />;),r6ously g land Ebling (167), paraphrased byf rnberloff (1981), proposed <br />wur criteria for establishing that predation is responsible for the absence of <br />t species from an area. All four are met by this interaction: <br />1) "One must know, by prevention of predation, that physical factors are <br />not limiting". This is obviously true in this case, since P occidentalis <br />,,ttuerly occupied habitats where G. affinis now prevails. Several of these <br />habitats have not been physically altered other than through colonization <br />t, mosquitofish (Meffe et al., 1983). <br />2) "The geographical patterns of the predator and the putative prey must <br />tK complementary". These poeciliids, with allopatric natural ranges, tneet <br />;his criterion (Rosen and Bailey, 1963). <br />3) "One must demonstrate, through laboratory, or preferably, field <br />observations, the occurrence of predation". Both sets of observations have <br />trpeatedly been made (Minckley, )973; Meffe et at., 1983; Minckley, pers. <br />Comm; and present study). <br />t) "Conduct transplant experiments in which the predator has been <br />4served to eat the putative prey". This criterion has been satisfied <br />numerous times, both in the present laboratory an field experiments, and <br />is introduction of mosquitofish into natural habitats throughout Arizona, <br />%ith subsequent predation on, and extinction of, topminnows. <br />:Although predation is deemed important, rates and outcomes are <br />-ksiously affected by other parameters such as alternative foods available for <br />t.. affinis, juvenile and adult densities, and particularly, environmental <br />hricrogeneity. Coexistence of predator and prey due to increased habitat <br />M111plexity was proposed theoretically (Rolf, 1974a,b; Hilborn, 1975; Levin, <br />1976; Hastings, 1977) and demonstrated empirically in fishes (Cooper and <br />(towder, 1979; Fraser and Cerri, 1982; Crowder and Cooper, 1982). I do not <br />iuve data that address heterogeneity, but the most rapid elimination of <br />topminnows occurred in the smallest, simplest habitats, such as field cages, <br />Llx>ratory aquaria, a 0.004 ha experimental pond (Schoeriherr, 1974), and <br />lager artificial ponds (Meffe et al., 1983). Long-term coexistence (up to 18 <br />.rays) occurs in large, spatially and temporally more complex systems. The <br />Santa Cruz River, which has several . tributary springs that serve as <br />irimpulation centers, and Sharp Spring, with 18 separate sub-units (pools) <br />-kmnected by a complex marsh network, and with periodic floods, both <br />fuve supported coexisting populations for long periods (Minckley et al., <br />1977; Meffe et al., 1982, 1983; Meffe, 1983b). <br />A common approach to studying predation is to analyze gut contents and <br />utter population effects of cropping. No such analyses were conducted here, <br />lot several reasons. First, to observe full impact of predation on topntinnow <br />i,ogwlations, it would be necessary to examine gut contents soon after <br />initial colonization by mosquitofish, probably within the first few weeks.