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<br />At that time the highest concentration of juvenile prey would be availa6
<br />and the greatest intensity of predation would occur. Afterward, availabilltt i
<br />of juveniles would be reduced and few would be detected in gut analyse
<br />Because no G. affinis were available from any locality directly altre
<br />colonization, and intense cropping was recorded in all experiments, gu
<br />content analyses were deemed unlikely to provide further insight.
<br />Second, there may generally be a low probability of finding juvenile list
<br />in guts of these predators even under the best of circumstances. In the
<br />laboratory, fish were recognizable in gut contents for less than four hour
<br />after ingestion. If a single predation event occurred during each 16 hot;
<br />daylength, an investigator would detect at most 4/16 or 25% of pin
<br />captures by gut analyses. Also, if predators greatly outnumber prey, as it
<br />several populations where G. affinis swarm by the thousands, thec
<br />probability of detecting predation on young topminnows would be low.
<br />Third, even if a predation rate is calculated, impact on the pit,
<br />population is difficult to assess. A low predation rate may still have a large
<br />impact if the prey species has relatively low fecundity, as is the case heir,
<br />or if predators are abundant. For example, Schoenherr (1974) studied a
<br />population of G. affinis soon after it colonized a P. occidentalis pond arum
<br />reported that about 6% of mosquitofish diet (by volume) consisted tl
<br />juvenile fish. Newborn topminnows average 1.336 µg dry weight, and full
<br />grown female mosquitofish are approximately 0.55 g. Assuming y
<br />mosquitofish ingests four times its weight per year (a conservative estimate
<br />[S. D. Gerking, pers. comm.]), a single female would consume about 10Q
<br />topminnow juveniles per year. Since topminnow brood sizes range from
<br />about 5 to 15 individuals, with 2 to 5 broods per year (Constantz, 1971.
<br />Schoenherr, 1974), a single female mosquitofish could easily consume all
<br />offspring of a single topminnow in one season at this relatively low
<br />predation rate. Even "rare" predation on topminnows could decimate then
<br />populations.
<br />Other examples of fishes being limited by piscivorous fishes were
<br />presented (summarized in Stroud and Clepper, 1979), but complete
<br />extirpation appears to be most prevalent where a new predator is introduced
<br />(e.g., Zaret and Paine, 1973). Rapid elimination of a prey species in these
<br />situations may reflect an evolution in communities with few or different
<br />natural predators. Consequently, there may be an absence of morphological
<br />or behavioral "anti-predator traits" in native species appropriate in
<br />avoiding new types of piscivores (Miller, 1961; Smith, 1978; Hobson, 1979.
<br />Stein, 1979).
<br />"Naivete" of other prey taxa, due to evolution in predator-pool
<br />environments, was suggested as relevant to their declines (Simpson, 1950.
<br />1953; Eckhardt, 1972), and may be a major factor in predation of natisr
<br />western fishes. As a group, these organisms evolved in habitats with few
<br />predaceous fishes. For example, in at! -irea covering 325,000 kmz of the
<br />southwest, including parts of the Chihuahuan, Sonoran, Mojave, and Great
<br />Basin deserts, there are 37 native inland fishes (Lee et al., 1980), of which
<br />only four (10.8%) are known piscivores. One (Flops affinis) is primarilt
<br />marine, venturing periodically only into the lower Colorado River, and
<br />t.tther (Ptychocheilus lucius) is restricted as adults to deeper waters (>1 m)
<br />larger rivers (Minckley, 1973). Only the facultatively piscivorous chubs
<br />,;a robusta and G. intermedia interacted with spring or small-stream
<br />reties on a regular basis, and even they are usually restricted to deeper
<br />iters (Minckley, 1973). In contrast, of at least 57 introduced fishes
<br />Alished in this region, k6 (45.6%) are piscivorous, including such
<br />carious egg, juvenile, or adult predators as largemouth (Mieropterus
<br />; ,noides) and smallmouth (M. dolomieui) bass, several catfishes (.Ictalurus
<br />p), carp (Cyprinus carpio), and mosquitofish (Minckley, 1973). This
<br />,tease in predator load (2, to at least 28 species) is compounded by the
<br />,I that several introduced predators are small enough to gain access to
<br />:-allow backwaters and marshes which formerly provided refuge from
<br />,: er piscivores (Minckley, 1983). This shift to predator-influenced
<br />tnmunities occurred instantaneously on an evolutionary time scale, with
<br />de opportunity for native species to evolve appropriate behavioral
<br />,Ix>nses. As a result, small, native western fishes may fall easy prey to this
<br />rw suite of predators, as illustrated by the extinct Monkey Spring pupfish,
<br />,prinodon sp. This undescribed species from southern Arizona was
<br />uminated from its only known locality in just a few months by
<br />,,,grmouth bass predation (Minckley, 1973).
<br />This paper forms part of a dissertation submitted to Arizona State University, in partial
<br />:,Ilmcnt of the requirements for the Ph.D. degree. I thank my committee, J. P. Collins, S. G.
<br />axxr, t4'. L. Minckley, D. 1. Rasmussen, and R. L. Rutowski for input on various aspects of
<br />u,eartb and writing; G. IL Constantz, S. D. Gerking, P. C. Marsh, and R. R. Miller also
<br />oiled critical reviews of the manuscript. J. E. Cheek, C. B. Haddix, N. J. Meffe, and J. Roth
<br />tided -assistance in the laboratory or the field, and N. B. Grimm conducted water analyses.
<br />-7,uks to F. Sharp and the San Carlos Apache Nation for access to private land, acrd N. J. Meffe
<br />aping the manuscript. The research was supported by a contract to Arizona State University
<br />m the U.S. Fish and Wildlife Service, Albuquerque, New Mexico, in cooperation with the
<br />t As Mountain Forest and Range Experiment Station, Tempe, and by the Graduate College and
<br />•;?mnent of Zoology, ASU. Collections were made under Federal Permit PRT2-649, in
<br />4 junction with permits from the Arizona Game and Fish Department.
<br />LITERATURE CITED
<br />liUSSMNt, A, H. 1949. On the functional morphology of the alimentary tract of some fishes
<br />in relation to differences in their feeding habits. Anatomy and histology. Quart. Jour.
<br />Micr. Sci. 90:109-139.
<br />L-NINGTON, E. J. W. 1957. 'I'Ite alimentary canal and digestion. Pp. 109-161, in Fish
<br />physiology, vol. 1, W. S. Hoar and D. J. Randall, eds. Academic Press, New York.
<br />r... ill, O. N. ANn G. L. HOFFMAN. 1976. Helmmth range extension by translocatlon of
<br />fish. Pp. 163-172, in Wildlife Diseases, L. A. Page, ed. Plenum Publishing Corp, New
<br />York.
<br />.,rANTZ, G. D. 1974. Reproductive effort in Poeriliopsis occidentalis. Southwest. Nat.,
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<br />1975. Behavorial ecology of mating in the male Gila topminnow, Poeriliopsis
<br />occidentalis (Cyprinodonorform es: Poecihidae). Ecology 56:966-973.
<br />1976. Life history strategy of the Gila topminnow, Poeriliopsis occidentalis: a field
<br />evaluation of theory on the evolution of life histories. Ph.D. Dissertation, Arizona State
<br />Univ., 'Iempc.
<br />1979. Life history patterns of a live-bearing fish in contrasting environ-
<br />ments. Oecologia 40:189-201.
<br />1980. Energetics of viviparity in the Gila topminnow (Pis(es: Poe6hidae). (,opera
<br />1980:876-878.
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