V I I . DESERT FISHES
453
sisted. In Arizona (and most of the other desert regions), there was little
evident extinction or marked changes in population sizes indicated through
the period of arroyo cutting at the turn of the century (Section II,A)—or
even with the early habitat alterations such as damming and diversion of
streams (Minckley, 1971).
In about the 1930's, however, introduced species began to form a substantial proportion of the fauna, and populations of native forms began
to decline, in some instances to extinction. The Gila topminnow and desert
pupfish are essentially gone, presumably through interactions with mosquitofish. More recently, the red shiner appears to have essentially replaced
the spikedace and loach minnow throughout substantial parts of their
ranges (Minckley and Deacon, 1968; Minckley, 1971). The recent (1970)
collection of spikedace in the Verde River, where it was thought extirpated,
may indicate a resurgence of its population after a period of adjustment.
In Nevada habitats, several instances of reduction of native fishes have
been closely studied following establishment of alien species. Population
decline of the Pahrump killifish following establishment of goldfish at
Manse Spring was discussed by Deacon et al. (1964) and by Minckley
and Deacon (1968). Subsequent data also indicate a reduction in the average sizes of individuals in the native population, and in the median number
of eggs per female. Following an initial reduction in numbers, the killifish
population recovered to a somewhat lower than original level, but lengths
of individual fish and numbers of ova remained low. Since rather drastic
alterations of the habitat occurred almost simultaneously with introduction
of goldfish, it is impossible to segregate the influence of competitive interactions from those of habitat alteration.
Deacon et al. (1964) and Hubbs and Deacon (1964) presented corroborative information on other cyprinodontids from several Nevada
springs. General reductions in size of individual native fishes occurred in
apparent response to establishment of exotics; additional data collected
since that time confirm this trend. However, in most instances a variable
amount of physical change in addition to introduction of alien fishes also
was associated with depletion of natives.
In most of these situations, if extinction of the native population does
not occur, the interactions result in rapid, apparent, niche segregation. For
example, mosquitofish, shortfin mollies {Poecilia mexicana), and sailfin
mollies (P. latipinna) tend to frequent surface and marginal habitats, forcing the native Crenichthys baileyi and Cyprinodon nevadensis subspp. into
deeper areas. In springs with extreme gradients of temperature or dissolved
oxygen, native fishes utilize their unique physiological tolerances and segregate spatially from the introduced forms by moving up-gradient into areas
inhospitable for the aliens.
453
sisted. In Arizona (and most of the other desert regions), there was little
evident extinction or marked changes in population sizes indicated through
the period of arroyo cutting at the turn of the century (Section II,A)—or
even with the early habitat alterations such as damming and diversion of
streams (Minckley, 1971).
In about the 1930's, however, introduced species began to form a substantial proportion of the fauna, and populations of native forms began
to decline, in some instances to extinction. The Gila topminnow and desert
pupfish are essentially gone, presumably through interactions with mosquitofish. More recently, the red shiner appears to have essentially replaced
the spikedace and loach minnow throughout substantial parts of their
ranges (Minckley and Deacon, 1968; Minckley, 1971). The recent (1970)
collection of spikedace in the Verde River, where it was thought extirpated,
may indicate a resurgence of its population after a period of adjustment.
In Nevada habitats, several instances of reduction of native fishes have
been closely studied following establishment of alien species. Population
decline of the Pahrump killifish following establishment of goldfish at
Manse Spring was discussed by Deacon et al. (1964) and by Minckley
and Deacon (1968). Subsequent data also indicate a reduction in the average sizes of individuals in the native population, and in the median number
of eggs per female. Following an initial reduction in numbers, the killifish
population recovered to a somewhat lower than original level, but lengths
of individual fish and numbers of ova remained low. Since rather drastic
alterations of the habitat occurred almost simultaneously with introduction
of goldfish, it is impossible to segregate the influence of competitive interactions from those of habitat alteration.
Deacon et al. (1964) and Hubbs and Deacon (1964) presented corroborative information on other cyprinodontids from several Nevada
springs. General reductions in size of individual native fishes occurred in
apparent response to establishment of exotics; additional data collected
since that time confirm this trend. However, in most instances a variable
amount of physical change in addition to introduction of alien fishes also
was associated with depletion of natives.
In most of these situations, if extinction of the native population does
not occur, the interactions result in rapid, apparent, niche segregation. For
example, mosquitofish, shortfin mollies {Poecilia mexicana), and sailfin
mollies (P. latipinna) tend to frequent surface and marginal habitats, forcing the native Crenichthys baileyi and Cyprinodon nevadensis subspp. into
deeper areas. In springs with extreme gradients of temperature or dissolved
oxygen, native fishes utilize their unique physiological tolerances and segregate spatially from the introduced forms by moving up-gradient into areas
inhospitable for the aliens.