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<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.
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