The Intermountain Region comprises a huge arena where major avifaunas more highly developed beyond the Great Basin come into contact. For this reason the distribution and composition of avifaunas in riparian and pinyon-juniper woodlands and in coniferous forests in this region are understood most clearly if considered as part of a broader system of patterns evident in western North America. Riparian woodland samples point to a complex meeting ground in the Intermountain Region of two major avifaunas of equivalent size, but from opposite distributional backgrounds. These northern and southern avifaunas do not mix among the samples represented except in western Nevada where two species of ultimate southern origin have penetrated a basically northern avifauna. Approaching the Great Basin, from the two centers of abundance of riparian species in the Snake and Colorado River drainages, species richness drops. Habitat depletion and, to a lesser extent, insularity play roles in this impoverishment. In the most depauperate riparian avifaunas, six species commonly coexist, each in a different family. Comparison of the pinyon zone avifaunas of two groups of mountain ranges, 90 km apart along the California-Nevada border, demonstrates a striking trade off among species of northern and southern biogeographic histories. The northern or Boreal forms, many of which are numerous in the Sierra Nevada, have had easy access to the favorable, cool, and relatively moist pinyon forests in the adjacent spur ranges. In contrast, the species of southern or Austral derivation prefer the warm and very arid pinyon woodland a short distance to the south. Few species are confined to either northern or southern sites, overlap in species composition is great, and equivalent species richness is achieved. However, despite these similarities, strong geographic differences in abundance of most species in each pinyon avifauna and the occurrence of at least 12 specific and subspecific range boundaries suggest the interposition between northern and southern pinyon areas of a substantial, but as yet poorly characterized, climatic barrier. Boreal species richness declines abruptly from high values in the southern Cascades and Sierra Nevada to low levels in a zone of impoverishment across western and central Nevada. From near 116° W Longitude in eastern Nevada, coincident with the appearance of fir and/or bristlecone pine forests, species numbers climb gradually until the main Rocky Mountains are reached. There, species richness compares favorably with that in the Sierra Nevada. The proportion of species favoring riparian woodlands over coniferous forests is higher on the island mountaintops of the Great Basin than in the Boreal "continents" of the Sierra Nevada-Cascades and Rocky Mountains. The western edge of the Great Basin richly demonstrated examples of stages in avian speciation. A full range of interactions is represented, from intergradation of poorly characterized races, through abrupt zones of hybridization between strongly marked subspecies of different racial complexes or of semispecies, to sympatry and infrequent interbreeding of closely-related, full species in recent secondary contact. These various zones of population interaction coincide strikingly with sharp floristic and climatic gradients. A major avifaunal break occurs between coastal or Sierra Nevadan forms that inhabit oak-chaparral and/or coniferous forests and closely-related interior forms that prefer pinyon-juniper or aspen-willow associations. In keeping with the special requirements of each species, contact zones and areas of disjunction show general, rather than precise, coincidence in the western Great Basin. There is no Great Basin Boreal Avifauna; the most distinctive interior forms occur across the entire span form eastern California to Colorado. The low desert trough along the east side of the Sierra Nevada that divides major mountain systems in the western portion of the Intermountain Region is not the principal barrier dividing coastal Sierra Nevada from interior avifaunas. Instead, the major avifaunal transition occurs in a belt of variable width just east of the crest of the Cascade Mountains and Sierra Nevada, where the precipitation shadow and continental climate begin to exert crucial influence.
Johnson, Ned K.
"Patterns of avian geography and speciation in the Intermountain Region,"
Great Basin Naturalist Memoirs: Vol. 2
, Article 9.
Available at: http://scholarsarchive.byu.edu/gbnm/vol2/iss1/9