Gambusia as a competitor with native species
Interspecific competition for resources may extend to predation, by gambusia, of eggs and larvae of endemic fishes and amphibians. Milton & Arthington (1982) and Courtenay & Meffe (1989) listed reports that implicated gambusia in the decline of various native fishes. In Australia, gambusia was suggested to be an imminent threat to red finned blue eye (Scaturiginichthys vermeilipinnis, Pseudomugilidae) and Edgbaston goby (Chlamydogobius squamigenus, Gobiidae) (Unmack & Brumley, 1991; Unmack, 1992; Wager, 1994, 1995; Wager & Unmack, in prep). They also negatively effect southern blue eye (Pseudomugil signifer) populations (Howe et al., 1997) and tadpoles (Morgan & Buttemer, 1997; Webb & Joss, 1997). Glover (1989) reported gambusia caused a decrease in desert goby (Chlamydogobius eremius) and spangled perch (Leiopotherapon unicolor, Terapontidae) populations inhabiting Clayton Bore in South Australia. Speculation that gambusia preyed on the eggs and larvae of rainbowfish (Melanotaeniidae) in the wild (Arthington & Lloyd, 1989; Arthington, 1991) was confirmed over summer 1997/98 in a field study in the upper Orara River, near Karangi, New South Wales (Ivantsoff & Aarn, 1999). In New Zealand, Barrier & Hicks (1994) showed that although gambusia was harassed by the larger black mudfish (Neochanna diversus, Galaxiidae), gambusia ate their larvae.
Many examples from North America demonstrate the negative effects of gambusia. Due in large part to predation, gambusia have eliminated Gila topminnow (Poecilliopsis o. occidentalis) from almost it's entire range. Populations only persist where gambusia are absent or in a few springs where other as yet unknown ecological characters allow them to coexist (Minckley et al. 1991). The other subspecies, the Yaqui topminnow (P. o. sonoriensis) is also in great danger as gambusia are only just starting to invade and spread throughout the Yaqui River system. Gambusia have a major impact on some pupfish (Cyprinodon spp.) populations. While no extinctions due to this have been recorded, coexisting populutions tend to be quite depressed in abundance. Evidence collected in part by Unmack (unpub. data) from Ash Meadows, Nevada suggests that when gambusia are decreased in abundance by physical removal, significantly higher numbers of pupfish occur within a year. Gambusia have also been demonstrated to cause extinction of California newt (Taricha torosa populations (Gamradt & Kats, 1996). Much to Diamond's amazement gambusia are freely given out to anyone who wants them in southern California. To directly quote Diamond (1996);