Abstract
The alpine community of the Sierra Nevada, California, U.S.A., presents a unique set of biogeographic questions in that numerous cosmopolitan species and species present in other North American cordillera are absent in this range. The white-tailed ptarmigan (Lagopus leucurus (Richardson) is a primary alpine species that was historically absent in the Sierra Nevada until seventy-two birds were introduced in 1971-72 from the Rocky Mountains, Colorado, U.S.A. This study investigates whether ptarmigan could have once inhabited the Sierra Nevada and later become extinct in the range or if the species never successfully colonized the Sierra Nevada. The strongest argument for the former absence of this species in the Sierra Nevada is that barriers east and north prevented the birds from colonizing the range. Pollen records and fossil evidence indicate that the Great Basin presented this terrestrial alpine species with many dispersal barriers between the Rocky Mountains and the Sierra Nevada. From the Mid-Pleistocene to the present, barriers in the Great Basin have existed in forms ranging from extensive woodlands and great pluvial lakes to deserts. From the Pleistocene to the present, the principle barriers to ptarmigan movements from the north were the Columbia River and its gorge and the relatively low altitude of the South Cascades which provide a paucity of suitable alpine habitat. Currently, the Sierra Nevadan alpine is suitable for ptarmigan survival as determined by the breeding success of the introduced population which does not differ significantly from natural populations in the Rocky Mountains. Yet, deep spring snowpack in the Sierra Nevada negatively affects the birds' breeding success$(P<0.05)$. If, as hypothesized, Pleistocene snows were consistently heavier in the Sierra Nevada than today, the establishment of a population of ptarmigan may not have been successful. In the Holocene, temperatures, exceeding present conditions in the Sierra Nevada may have subjected the birds to critical heat stress since the species is intolerant of high temperatures. Thus, the Sierra Nwevada may have been unsuitable for ptarmigan survival in the past in addition to being inaccessible.