Great Basin Naturalist Memoirs


Eighty-three species of fishes belonging to 26 genera live in the area bounded by the Sierra Nevada, Grand Canyon, Rocky Mountains, and Snake River Plain. The waters inhabited by these fishes are part of the Great Basin, Colorado River, Snake River, upper Pit River and upper Klamath River drainages. The adaptations and distribution patterns of these fishes have been shaped by extensional faulting and volcanic activity in the Great Basin, uplift of the surrounding ranges and plateaus, and cyclic fluctuations of the Pleistocene pluvials and interpluvials. Fossil evidence indicates that in the Pliocene many of the lineages had established distributions broadly inclusive of the present-day patterns, and the subsequent trends have been extinction and some species differentiation.
Analysis of the fauna is based on designation of 48 barrier-bounded, faunally homogeneous drainage units and quantitative evaluation of patterns among species distributions and faunal similarities of drainages. Cluster analysis of species based on correlations among their patterns revealed the existence of a basic northern intermountain fluvial fauna consisting of Cottus bairdi, Prosopium williamsoni, Catostomus platyrhynchus, Rhinichthys cataractae, Richardsonius balteatus, and their vicariants. These fishes have similar ecology and dispersal patterns. They are ecologically associated with Salmo clarki (upstream) and Rhinichthys osculus (downstream), but these two species have broader distributions, probably because of more frequent colonization via stream capture by the former and extinction resistance by the latter. Rhinichthys osculus is the most widespread intermountain fish, being found in 32 of the 48 drainage units; Gila bicolor is next most widespread, being found in 21 units. Fifty-one of the 83 species are found in only one drainage unit; 18 of these are endemic to that unit (33 are more widespread outside the study area).
Species distributions are broader in the north, and northern and peripheral units have more species. The species area curve for the Great Basin shows a steep slope (z = .59), with especially low species density in small drainages, indicating high extinction and low colonization. Postpluvial aridity, especially in the south, is the major cause of extinction and a major cause of isolation. Principal components and cluster analysis of drainage units, based in shared species, show a high correspondence between faunal similarity and geographic proximity (and weakness of barriers) and also reveal the effects of extinction in erasure of patterns. The Wasatch Range, Sierra Nevada, and southern divide boundary of the Snake River Plain have been strong barriers, leading to intensive faunal differentiation. Strong barriers and concomitant differentiation also exist in eastern Nevada, near the original center of Basin and Range tectonism. Two dozen examples of vicariant species are found to be associated with stronger-than-average barriers.
The colonization rate and extinction rate have both been accelerated in postsettlement time by introductions and habitat destruction. Most drainages and populations have been affected. Five species and many local populations have become extinct. Eighteen species and many more populations are vulnerable or threatened. Reversal of the trend will require sound ecosystem management of watersheds and restriction of exotic introductions.