Probably the primary stimulus which leads people to visit our national parks is the change that is experienced from familiar surroundings to those which are emphatically different. This change involves 'air' (that is, climate), and 'scene' (topography and vegetation). An entirely new set of conditions is encountered, and the new reactions set up mean recreation in the physiological sense—the exercise of faculties, both mental and physical, in kind or degree, that are more or less dormant during the ordinary routine of the year's program. Quick transportation between the lowlands of the San Joaquin Valley and the upper altitudes of the Sierras carries the traveler in either direction from one set of surroundings into a totally different one where he is thrilled because of the great changes which he encounters.

Let us now discuss, then, these differences in environment and their correlation with the continuous or discontinuous occurrence of vertebrate animals in the region. The section of the Sierra Nevada selected for faunal study is of such extent transversally to the Sierran axis that it takes in almost as great extremes of conditions as are to be encountered anywhere in California. Analysis of the changes to be observed as a person traverses the section from the west will soon show that he has witnessed not one single change, evenly and progressively from one set of conditions to just one other set, but that, having reached the highest altitudes, he has witnessed several steps. There has not been a uniform and continual gradient but he has passed through several belts, parallel roughly to the axis of the Sierra Nevada, each characterized by a considerable degree of uniformity as regards the plant and animal life.

A total of 231 kinds of birds are now (December 31, 1920) authentically known from the Yosemite section; there are 97 kinds of mammals, 22 kinds of snakes and lizards, and 12 kinds of frogs, toads, and salamanders. This makes a grand total, for the vertebrate fauna outside of fishes, of 362 forms. This seeming richness in number of kinds, be it emphasized, is apparent only when one takes into account the full extent of the Yosemite section. As a matter of fact, but a small proportion of the total number of species occur together at any one level. And here is the remarkable thing: They are more or less assorted and delimited in occurrence so that they help to constitute the belts, or 'life zones,' just referred to.

Fig. 1. Cross-section of the Sierra Nevada through the Yosemite region showing some mammals which are either restricted to or find their maximum abundance in single life-zones. (click on image for an enlargement in a new window)

We may express the facts in another way. The large number of kinds of animals present in the entire Yosemite section is due to the great range of physical conditions (temperature, moisture, soil, light, and perhaps others) with the accompanying diversity of vegetational features. Man is able to traverse the whole gamut of these conditions, even with benefit to himself by reason of the stimulus change produces, adjusting his mode of dress and behavior to them and carrying his food with him. But animals and plants are more or less directly in contact with the conditions around them; they are, as a rule, far less adaptable; and they are vitally affected by differences in temperature, in moisture, in food supply, and so on. The interesting thing is that in many species the degree of sensitiveness is so great that they can maintain existence only within a relatively narrow range of the critical conditions.

Such underlying reasons as those just suggested help to explain what impresses the traveler in ascending the west slope of the Sierras, namely, the correlation, roughly, with respect both to animals and plants, of zonation with altitude and, therefore, temperature. And it is because of this inter-correlation that the student is led to the conclusion that it is the factor of temperature which has most to do with the causation of life zones.

Reference to our map and cross-section diagram (pls. 61, 62) will show the application, to the Yosemite section, of the system of recognizing these belts of animal and plant life as some naturalists have worked them out and named them. Each life zone is a belt of relatively uniform constitution with respect to species. At the same time, we must emphasize that there is rarely an abrupt line of demarcation between any two adjoining zones. There is, as a rule, along the meeting ground more or less mixing or overlapping of the specific elements. This is especially true where the slope is very gentle, broad, and all facing in one direction. The steeper the slope, or the more abrupt the change of exposure (say from west to north), the sharper will be the boundary between the two adjacent zones.

Fig. 2. Cross-section of the Sierra Nevada through the Yosemite region showing some birds which, in the breeding season, are limited to single life-zones. (click on image for an enlargement in a new window)

To enter here into a further discussion of the life-zone concept is not necessary. We will simply refer the inquiring reader to some of the literature relating to the subject¹ and confine the present treatise to the particular state of affairs found in the Yosemite region. Since all animal life is more or less directly dependent upon plant life for its existence, the zoologist who seeks to explain the distribution of animals must concern himself attentively also with the botany of the region he is studying. A very useful essay on the distribution of plant life on the upper western slope of the Yosemite section is contained in Professor and Mrs. Hall's Yosemite Flora²; and further valuable data on the distribution of plants in the Sierras will be found in a report by Dr. Smiley.³

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¹C. Hart Merriam, Life Zones and Crop Zones of the United States (U. S. Dept. Agric., Div. Biol. Surv., Bull. no. 10, 1898), 79 pp., 1 colored map. C. Hart Merriam, Results of a Biological Survey of Mount Shasta, California (U. S. Dept. Agric., Div. Biol. Surv., N. Am. Fauna, no. 16, 1899), 179 pp., 5 pls., 46 figs. in text. H. M. Hall, A Botanical Survey of San Jacinto Mountain (Univ. Calif. Publ. Bot., vol. 1, 1902), pp. 1-140, pls. 1-14. J. Grinnell, An Account of the Mammals and Birds of the Lower Colorado Valley, with Especial Reference to the Distributional Problems Presented (Univ. Calif. Publ. Zool., vol. 12, 1914), pp. 51-294, pls. 3-13, 9 figs. in text. J. Grinnell, A Distributional List of the Birds of California (Pac. Coast Avifauna, no. 11, 1915), 217 pp., 3 maps. H. M. Hall and J. Grinnell, Life-Zone Indicators in California (Proc. Calif. Acad. Sci., ser. 4, vol. 9, 1919), pp. 37-67.

²H. M. Hall and C. C. Hall, A Yosemite Flora (Paul Elder, San Francisco, 1912), pp. viii + 282, 170 text figs., 11 pls.

³F. J. Smiley, A Report upon the Boreal Flora of the Sierra Nevada of California (Univ. Calif. Publ. Bot., vol. 9, 1921), pp. 1-423, pls. 1-7.

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All of the six life zones in the Yosemite region are represented in full measure on the western slope of the section. There the distance involved in the slope is so great, over seventy miles, that there is plenty of room for the development of a separate representation of species, both plant and animal, in each zone. But on the eastern slope the situation is somewhat different; and we find the zonation there obscure. Indeed, in our field work below the Hudsonian Zone we met with much trouble in diagnosing many of the localities; for instance, whether to call the upper meadows on the Farrington ranch (pl. 19a), Canadian or Transition; the south face of Williams Butte (pl. 19b), Transition or Upper Sonoran.

On the basis of the facts obtained within the eastern boundary of our Yosemite section alone, the situation would be exceedingly difficult, even impossible, of explanation. But when we take into account the east-Sierran region generally, especially toward the southern end of the Sierran ridge in the vicinity of Walker and Tehachapi passes, it becomes fairly easy to see why conditions are as we find them between Mono Lake and Mono and Tioga passes.

Base-level in the Mono Basin is high, averaging 7000 feet in altitude. Furthermore, the distance between Mono Lake and the high Sierran crest, which is 10,000 to 13,000 feet in altitude, is short. In other words, this slope is abrupt; in fact, close to the divide, a declivity. The life zones, in so far as characteristic representatives of them are to be found, are crowded together—telescoped, as it were. There is a well established law that a sequestered faunal area can be too small to support a permanent, distinctive fauna of its own, even though conditions be otherwise wholly propitious. The Sierra Nevada, which by area is of mainly western slope, supports a large mass of 'boreal' plant and animal life; the Great Basin area to the east is the metropolis of a highly developed 'austral' assemblage of species. These two major areas adjoin one another at the steep eastern declivity of the Sierras. On the long western slope where austral adjoins boreal there is not only a well-marked belt of overlapping (comprising the Transition Zone) but in this belt there are numerous species closely restricted to it. On the eastern slope, however, Canadian and Upper Sonoran are jammed so closely together by reason of the steepness that the belt of intermingling of elements is very narrow or at best indistinct; there is scarcely if any room for the existence of restrictedly Transition Zone species.

Although presenting a strongly Great Basin aspect, the Mono basin, doubtless because of its high altitude, does not show a pure representation of austral life. It does contain a number of elements (that is, species) which from a study of their entire ranges we know to belong predominantly to the upper division of the austral, namely Upper Sonoran. But there are also present about as many, or as dominant, boreal elements.

Frankly, we found difficulty in assigning some parts of the Mono portion of the Yosemite section to one life zone rather than to another. This was particularly true of the south, sun-facing slope of Williams Butte (pl. 19b), which is clothed with piñon. This tree to the southward along the Sierras forms a belt which through Walker Pass is continuous with the digger pine belt of the west slope of the Sierras; and as a rule we can safely diagnose this belt by reason of this one plant indicator as Upper Sonoran. But on Williams Butte the piñons are mixed with western junipers, Jeffrey pines, mountain mahogany, and certain shrubby plants which are accepted as diagnostic of Transition, Canadian, or even Hudsonian. We found in this anomalous assemblage of plants such 'good' Upper Sonoran birds as bush-tits and Woodhouse jays in close association with mountain chickadees and Clark nutcrackers. This was after the breeding season; and, of course, there was a chance that in the case of the last-named species, at least, the individuals observed had moved down from the higher altitudes but a very few miles to the westward. In the case of small mammals, which are incapable of quickly traversing considerable stretches of territory, we found, on Williams Butte, True white-footed mice, which are typically Upper Sonoran, in the same trap-line with Mono chipmunks, which find their metropolis in the Canadian life zone.

Another tract in the Mono country which was for similar reasons perplexing occupies the lower slope down toward the lake shore from Mono Mills. There, pale-faced kangaroo rats, Stephens soft-haired ground squirrels, and desert jack rabbits were found, species which belong to groups whose habitats lie chiefly within the Upper Sonoran Zone, but here were found in company with animals and plants of more northern, Transition or even Canadian, predilections. The sage-hen, to cite one of these latter, is a 'good' Transition bird.

In the nature of the case, as regards these exceptional localities, we trust that the reader will understand why it is impossible for us to make positive statements with regard to their zonal complexion. Two persons, with some difference in perspective—that is, with a different understanding of the 'importance' of indicators—would very probably weight their findings differently. Our conclusion, as shown on our map and in our life-zone table, namely, to call the western part of the Mono Lake basin, that part included within the Yosemite 'section,' Transition, is therefore presented tentatively. The margin of determination is so small, with regard especially to Williams Butte and the tract immediately south of Mono Lake, that someone else, working the territory more intensively and listing the critical species statistically (by individual composition, which we did not), might find adequate grounds for mapping it as Upper Sonoran.

Returning to the Sierran divide: The Hudsonian Zone is found to be well characterized on the east slope down to an average of about the 9500-foot contour. This zone simply mantles the Sierras, save for the Arctic-Alpine 'islands' which rise above timber line. Below the Hudsonian, good Canadian is represented, with marked resemblance florally to that on the western slope, in the lower part of Bloody Cañon. Moisture conditions are there more exactly as they are on the west flank of the Sierras. Elsewhere, Canadian is rather different in aspect from what it looks like on the western slope, because of the prevailing aridity. Jeffrey pines and mountain mahogany predominate in the place of red firs and aspens. The steepest declivities, close to the Sierran divide, involve a lowering of altitude to about the 8000-foot contour; thence east to Mono Lake the slopes involved in the long, lateral moraines are gentle, and the blending of Canadian through Transition with 'austral' takes place gradually over several miles of territory. Here is where most trouble was experienced in fixing upon a boundary between Canadian and Transition—and for the same reasons as given above with respect to the Transition-Upper-Sonoran boundary. Good Canadian extends east along the cold streams, where it is marked conspicuously by thickets of aspen, well down toward the shores of Mono Lake—to as low as 7000 feet; Transition extends west up toward the foot of the east Sierran face, especially along the south-facing slopes of glacial ridges, to 9000 feet. Thus at Walker Lake one finds the interesting situation of the Canadian Zone occupying the cool, shaded bed of the glacial groove, with Transition on the south, sun-facing wall above it: the usual zonal relationship is reversed. Facts such as this strengthen our belief that the prime physical factor accounting for zonation is not altitude, or moisture, or soil, per se, but temperature.

As is clearly set forth in some of the literature we here cite for perusal by the inquiring reader, the limitation of species on the basis of the life- zone concept is not the only sort of segregation which occurs. Indeed, locally, as in Yosemite Valley proper, often a far more conspicuous manner of delimitation is manifest, the delimitation which takes place on the basis of 'associations.' These minor units involve each a certain type of environment within one zone; furthermore, closely similar or even identical associations may recur, or be continuous in two or more adjacent zones. Not rarely, associational restriction seems to be transcendent over zonal restriction, as in the case of the badger, western chipping sparrow, and rock wren. Appropriate discussion of these cases will be found in the chapters (pp. 92, 452, 550) treating of these species.

Some of the more important associations of animal with plant or substratum conditions that it has proved useful to recognize in the Yosemite section are as follows, classified by zones. The names chosen are those of some predominating feature, usually of the vegetation. (Consult plates 13 to 19, 36a, and figure 21.)

ASSOCIATIONS WITHIN THE LOWER SONORAN ZONE

ASSOCIATIONS WITHIN THE UPPER SONORAN ZONE

ASSOCIATIONS WITHIN THE TRANSITION ZONE

ASSOCIATIONS ON THE ARID EAST SIDE OF THE SIERRA, IN THE GREAT BASIN FAUNAL DIVISION OF THE TRANSITION ZONE

ASSOCIATIONS WITHIN THE CANADIAN ZONE

ASSOCIATIONS WITHIN THE HUDSONIAN ZONE

ASSOCIATIONS WITHIN THE ARCTIC-ALPINE ZONE

Within each general association there is often plainly to be seen still further restriction in the habitat preferences of species. For example, in the major association, "coniferous forest," in its minor division (within the Canadian Zone) known as the red-fir association, we find several species of birds and of mammals, each adhering closely to a yet smaller division of the general environment. The Sierra Creeper keeps to the larger tree trunks, the Short-tailed Mountain Chickadee to the smaller twiggery, the Western Golden-crowned Kinglet to the terminal leafage, and the Hammond Flycatcher to the most prominent twig-ends and the air-spaces between branches and between trees. The Tahoe Chipmunk is largely arboreal, the Allen Chipmunk terrestrial.

In final analysis, no two species well established in a region occupy precisely the same ecologic space; each has its own peculiar places for foraging, and for securing safety for itself and for its eggs or young. These ultimate units of occurrence are called "ecologic niches." If two species of the same ecologic predilections are thrown into the same environment, one or the other will quickly disappear through the drastic process we call competitive replacement. Thus it comes to pass that the amplitude of the general environment—the number and extent of distinct ecologic niches it compasses—determines the richness of the fauna, both as regards number of species, and the number of the individuals to the unit of area representing each species. This principle may be abundantly verified by any student who will carry on active field observations a season or two over even a small part of the Yosemite section.