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By Joseph Grinnell,
and Tracy Irwin
Storer, Field Naturalist, Museum of Vertebrate
Zo÷logy, University of California
(Contribution of the Museum of Vertebrate Zo÷logy of the
University of California1)
1For several years past the natural history of the Yosemite region has been the subject of special study by staff members of the California Museum of Vertebrate Zo÷logy. This and the following three chapters are based upon the results of that study. Attention has been concentrated upon the mammals, birds, reptiles, and amphibians occurring within the section across the Sierra Nevada extending from Snelling in the San Joaquin Valley to Mono Lake, east of the mountains. This cross-section is at right angles to the main axis of the Sierras and is approximately ninety miles long and seventeen miles wide. It embraces Yosemite Valley and its environs, the lower canyon of the Merced River, and the country traversed by the Tioga Road; but neither Hetch Hetchy Valley nor the Mariposa Grove of Big Trees is included.
The "Yosemite section," embracing in that term the territory included between the eastern side of the San Joaquin Valley and the basin in which Mono Lake lies, may be considered to afford a fair sample of the fauna and flora of the entire Sierra Nevada. The following brief accounts of the life zones and of some of the vertebrate animals belonging to them may thus prove useful beyond the immediate limits of the Yosemite Park itself.
A total of 226 kinds of birds is now authentically known from the Yosemite section; there are 97 kinds of mammals, ranging in size from bats to bears, 20 kind of snakes and lizards, and 11 kinds of frogs, toads, and salamanders. This makes, all told, a vertebrate fauna, outside of fishes, of 353 forms. This richness in number of kinds is due to the wide range of climatic conditions, with the depending vegetational features, covered in the Yosemite section. Only a small proportion of the total number of species occur together at any one level. The curious and interesting thing is that the changes in faunal constitution across the Sierras are not perfectly gradual but take place at intervals, abruptly. Several belts or "zones" of life result, in each of which conditions are relatively uniform. These belts have been described and named. and it is useful to know their names so as to be able to state the distribution of species in more exact terms than would otherwise be possible. These life zones are correlated roughly with altitude, and from bottom to top are called Lower Sonoran, Upper Sonoran, Transition, Canadian, Hudsonian, and Arctic-Alpine. As will be recognized at once, these zones are in the nature of temperature belts, the warmest at the base of the mountains, the coldest at the crest, on the highest peaks. It will be further seen that they correspond roughly to the transcontinental belts of climate, and that they bear names significant of their location—Sonora (in northern Mexico), Canada, Hudson Bay, etc. To outline very briefly the condition of affairs in the Yosemite section, let us begin at the west.
In following the Yosemite Valley railroad out of Merced, one traverses for the first hour the level floor of the San Joaquin Valley. From the train one sees along the Merced River bottom numerous Fremont cottonwoods and valley oaks, and planted orchards of fig, orange, and olive, all indicative of the Lower Sonoran Life Zone. A day put in at a representative point, such as Snelling, would show the presence there of Mockingbirds, Texas Nighthawks, Blue Grosbeaks, Dwarf Cowbirds, Fresno Pocket Gophers, Merced Kangaroo Rats, Golden Beavers, and other exclusively warm-belt types of animals as well as of plants.
At Merced Falls the railroad enters the first foot-hills of the Sierra Nevada, and concurrently there appears with remarkable abruptness an entirely new set of trees and lesser plants, accompanied by as distinct a set of birds and mammals. The Upper Sonoran Zone has been entered and may always be recognized in distant view by the presence of digger pines, buckeyes, blue oaks, and interior live oaks, and by a host of bushy plants which constitutes the "California chaparral." This zone continues some fifty miles, all the way to El Portal, and up to an altitude of 4000 feet on south-facing slopes. Some of its distinctive species of animals are: California Jay, Northern Brown Towhee, Pallid Wren-tit, Plain Titmouse, California Thrasher, California Bush-tit, San Joaquin Wren, Hutton Vireo, Anna Hummingbird, Western Gnat-catcher, Bell Sparrow, Rufous-crowned Sparrow, Dusky Poor-will, Nuttall Woodpecker, Mariposa Brush Rabbit, Gilbert White-footed Mouse, Parasitic White-footed Mouse, Mariposa Meadow Mouse, Digger Pine Pocket Gopher, Heermann Kangaroo Rat, San Diego Alligator Lizard, and California Striped Racer.
At El Portal, on shaded, north-facing slopes, the visitor for the first time encounters the Transition Zone, which is characterized by the yellow pine. Douglas spruce, golden oak, black oak, and incense cedar. This zone continues east throughout the Yosemite Valley, and rises on the walls of the Valley to about the 6000-foot level. A few Upper Sonoran birds and mammals reach up into the Transition, but for the most part an entirely new set predominates. But few of these are absolutely restricted to this zone; the greater number range farther upward, through the next one or two zones above. The more distinctively Transition vertebrates are: Band-tailed Pigeon, California Purple Finch, Black-throated Gray Warbler, Calaveras Warbler, Western Flycatcher, Black Swift Pigmy Owl, Northern Spotted Owl, Northwestern Long-legged Bat, Boyle White-footed Mouse, Yosemite Pocket Gopher, and Coral King Snake. Some well-known species which range down into Transition from the zones above are: Blue-fronted Jay, Western Robin, Sierra Junco, Sierra Creeper, Short-tailed Mountain Chickadee, American Dipper, Sierra Hermit Thrush, Mountain Weasel, Yosemite Meadow Mouse, and Sierra Nevada Flying Squirrel.
At about the 6000-foot contour on any of the trails leading up out of the Valley, a rather impressive change is to be noted; the golden oak becomes replaced by the dwarf huckleberry oak, the California laurel and maple and black oak disappear, the Jeffrey pine replaces the yellow pine, and red firs and aspens appear. These mark the Canadian Zone. Birds encountered here are: Yosemite Fox Sparrow, Williamson Sapsucker, Sierra Grouse, Townsend Solitaire, Western Ruby-crowned Kinglet, Red-breasted Nuthatch, Casin Purple Finch, California Evening Grosbeak, Lincoln Sparrow, Hammond Flycatcher, and Western Goshawk. Among the mammals are: Navigator Shrew, Pacific Fisher, Allen jumping Mouse, Yellow-haired Porcupine, Sierra Mountain Beaver, Sierra Golden-mantled Ground Squirrel, Tahoe Chipmunk, Allen Chipmunk, Sierra Chickaree, Tenaya Blue-bellied Lizard, Mountain Lizard, and Sierra Alligator Lizard.
Life zones on cross-sectional profile of Yosemite National Park
The Hudsonian Zone is the belt of forest just below timberline. It contains the lodgepole pine, which occurs commonly in the Canadian Zone, and has also trees of its own, namely alpine hemlock, silver pine, and white-bark pine. Birds become scarcer in this zone though mammals remain plentiful; some of the species extend up from the zone below. The California Pine Grosbeak, Mountain Bluebird, White-crowned Sparrow, Alpine Chipmunk, Belding Ground Squirrel, Sierra Marmot, Mountain Lemming Mouse, Gray Bushy-tailed Wood Rat, Yosemite Cony, Sierra White-tailed Jack-rabbit, Pine Marten, Wolverine, and Sierra Least Weasel are rather closely restricted to it.
The Arctic-Alpine is the highest of all the zones and Covers the treeless area from about the 10,500-foot contour to the summits of the loftiest peaks. Only one species of bird is confined to it, the Sierra Nevada Rosy Finch. Some of the Hudsonian mammals enter it locally; for example, Gray Bushy-tailed Wood Rat, Yosemite Cony, and Alpine Chipmunk.
It must be kept in mind that many of the vertebrate animals of the Yosemite section are not so closely restricted as the ones named in the preceding paragraphs. Certain species range regularly through two zones, for example, the Blue-fronted Jay; a few through three zones, as with the Sierra Junco, and in exceptional cases as many as five out of the six zones name are covered, as is done by the Red-shafted Flicker, Sparrow Hawk, and Western Chipping Sparrow. The last named was found by us summering in the orange groves at Snelling and also among the timberline trees in Mono Pass. It was, perhaps, more numerous on the floor of Yosemite Valley than anywhere else.
Other zonal contacts than those just given may be mentioned for the use of persons coming into the Yosemite National Park along the roadways or who may go on foot, on horseback, or by vehicle to other portions of the park. On the Big Oak Flat Road, the Transition Zone is reached in the vicinity of Groveland; Canadian is entered at Tuolumne Grove Big Trees and is left again near Tamarack Flat. On the Coulterville road, Transition is reached at the top of the grade three miles east of Coulterville, and this zone is traversed practically all the way thence into Yosemite. The Tioga Road begins in Transition, reaches Canadian just below Aspen Valley, touches Hudsonian on Snow Flat and enters it again at Lake Tenaya and continues in that zone until reaching Warren Fork of Leevining Creek. Tuolumne Meadows, Lyell Canyon, and Tioga Pass are all in the Hudsonian Zone. The Wawona Road lies just at the upper margin of the Transition Zone for most of its course between Fort Monroe and Mariposa Grove of Big Trees.
The restriction of animals by "zones" applies particularly to the breeding season. Migratory species of both birds and mammals range more or less widely at other times of the year according to food requirements. Close adaptation of a species to a kind of food supply which disappears at the close of the summer season makes necessary search elsewhere for it in the winter time.
The study of the distribution of the animal life on the slopes of the Sierras is a fascinating one, especially when the student attempts to ascertain what the limiting factors may be; for it is certainly not in every instance temperature alone, up or down, which forms the barrier to the species. The intricate interrelations which we seek to understand are to be worked out only by patient and thoughtful study of the animals in their many and diverse environments.
Grinnell, J., 1915. "A Distributional List of the Birds of California." Pacific Coast Avifauna, No. 11, 217 pp., pls. I-III. (Published by Cooper Ornithological Club, Berkeley, Calif.)
Merriam, C. H., 1898. "Life Zones and Crop Zones of the United States." U. S. Department of Agriculture, Division of Biological Survey, Bulletin No. 10, 79 pp., 1 plate.
For further information on life zones and also on the birds, mammals, reptiles, and amphibians, consult, in addition to the titles given, articles in the following series:
University of California Publications in Zo÷logy, 1908-1920. Volumes v., vii., x., xii., xvii., xxi. Many references to the vertebrates of the Yosemite region.
United States Department of Agriculture, Bureau of Biological Survey, 1889-1920. Bulletins, Circulars, and other series contain articles relating to vertebrates found in the Yosemite region.
The Condor, 1889-1920. A magazine of western ornithology. Published by the Cooper Ornithological Club at Berkeley, Calif. Contains much information relating to bird species, which occur in the Yosemite region.
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