Part 4

Following is a list of the mammals taken in the course of approximately 600 trap nights in the lower parts of the chaparral belt. All of the traps were set on slopes in San Antonio Canyon below 4000 feet elevation. The list gives a general indication of the relative numbers of rodents inhabiting one chaparral habitat: the arid greasewood-covered south slopes of the lower chaparral belt.

TABLE 4.--YIELD OF 600 TRAP-NIGHTS IN GREASEWOOD CHAPARRAL.

====================================================================== | Number | Per cent | | of total --------------------------------------------------+--------+---------- Perognathus californicus dispar | 4 | 10.0 Dipodomys agilis agilis | 4 | 10.0 Peromyscus californicus insignis | 25 | 62.5 Neotoma fuscipes macrotis | 7 | 17.5 --------------------------------------------------+--------+----------

Heteromyids are evidently absent from the upper parts of the chaparral association, but cricetid rodents are common there beneath heavy clumps of lilac and in the talus beneath oaks and bay trees. The following list gives the mammals taken in the course of about 200 trap nights in the granite talus one half mile northwest of the mouth of Icehouse Canyon, at 5200 feet elevation.

TABLE 5.--YIELD OF 200 TRAP-NIGHTS IN THE UPPER PART OF THE CHAPARRAL ASSOCIATION.

====================================================================== | Number | Per cent | | of total --------------------------------------------------+--------+---------- Eutamias merriami merriami | 3 | 6.3 Peromyscus boylii rowleyi | 38 | 79.2 Neotoma lepida intermedia | 2 | 4.2 Neotoma fuscipes macrotis | 5 | 10.4 --------------------------------------------------+--------+----------

The gray fox is the dominant carnivore of the chaparral association and forages widely in all habitats.

Yellow Pine Forest Association

MAJOR PLANTS

Pinus ponderosa P. lambertiana Libocedrus decurrens Abies concolor Quercus Kelloggii Ribes nevadense Ribes Roezlii Arctostaphylos sp. Ceanothus cordulatus

The crest of the range, from the upper limit of the chaparral association at roughly 6000 feet to the limited areas of boreal flora above 8500 feet elevation, is covered by yellow pine forests. On the desert slope of the range the coniferous forests which extend down to about 6000 feet represent the best development of this association, while the coniferous forests on the coastal side of the drainage divide are often more or less diluted by chaparral elements. For example, yellow pines on the Pacific face of Blue Ridge at 7000 feet elevation often grow in association with scrub oak and mountain-mahogany.

Few mammals are resident in the typical yellow pine forest as characterized by dense coniferous timber and little herbaceous or brushy growth. Here most of the species recorded actually find optimal conditions in an adjacent habitat. The forest probably harbors surplus individuals from adjacent preferred habitats, or, as in the case of chipmunks and ground squirrels, the forest often serves as forage ground while nearby brushy areas are utilized for breeding and shelter. The abundance of birds in the timber contrasts strikingly with the paucity of mammals there. The lack of a seed-producing understory, and the open duff-covered stretches of ground on which rodents would be extremely vulnerable to predation, probably in part account for the scarcity of rodents.

Within the general area encompassed by the yellow pine forest there are two major habitats, namely coniferous forest and chaparral. The species of plants comprising the chaparral of the Transition Life-zone are different from those comprising the chaparral of the Upper Sonoran Life-zone on the Pacific slope. In the chaparral of the Transition Life-zone, basin sagebrush and snowbrush grow in extensive patches in clearings in the timber. Dense thickets of choke cherry cover many damp hollows, and these thickets harbor the houses of Neotoma fuscipes. The food and shelter afforded by these chaparral areas importantly influence the local distribution of rodents: for example, Dipodomys agilis and Perognathus californicus in the yellow pine area are found only in association with chaparral, being completely absent from wooded areas.

The severe winter weather in this association must force many of the mammals into periods of inactivity. Probably during the long periods in the winter when snow covers the ground the heteromyids and sciurids remain below ground.

Pinyon-Juniper Woodland Association

MAJOR PLANTS

Pinus monophylla Juniperus californica Quercus dumosa var. turbinella Purshia glandulosa Fremontia californica Cercocarpus ledifolius Yucca Whipplei

In the San Gabriel Mountains this association is limited to the desert slope and reaches its lower limit at the bases of the foothills and extends up to the lower edge of the yellow pine forests. The altitudinal extent of the pinyon-juniper association is from roughly 4000 to 6000 feet elevation.

Several habitats are evident within the pinyon-juniper belt. On north slopes in the upper part of this association, scattered stands of pinyon pines are found with dense patches of scrub oak intervening, while on other such slopes a dense chaparral is present, consisting primarily of scrub oak, mountain-mahogany, and California slippery-elm. In this type of chaparral several hundred trap nights yielded only two rodent species: Neotoma fuscipes simplex and Peromyscus truei montipinoris. There are few pinyons on the south slopes, especially in the lower parts of the association; many of these slopes are clothed with an open growth of manzanita and yucca, while northern exposures there support mostly scrub oak. Many of the flats of the pinyon belt are grown to basin sagebrush.

Following is a list of the mammals taken in about 400 trap nights at one locality in the pinyon-juniper association. The area supported a mixed growth of pinyon, scrub oak, mountain-mahogany, and antelope-brush, together with smaller brushy plants, and was at the head of Grandview Canyon, at an altitude of roughly 5000 feet.

TABLE 6.--YIELD OF 400 TRAP-NIGHTS IN THE PINYON-JUNIPER ASSOCIATION.

===================================================================== | Number | Per cent | | of total -------------------------------------------------+--------+---------- Perognathus fallax pallidus | 3 | 11.5 Dipodomys agilis fuscus | 9 | 34.6 Peromyscus truei montipinoris | 10 | 38.5 Neotoma fuscipes simplex | 4 | 15.4 -------------------------------------------------+--------+----------

Although Munz and Keck (1949:101) considered the pinyon-juniper belt as one association, on the desert slope of the San Gabriels pinyons and junipers do not generally grow on common ground; but rather the juniper belt represents a well defined habitat occurring between the pinyon covered slopes and the flats that support Joshua trees. Because the mammalian populations of the pinyon belt and the juniper belt are somewhat different, the mammals of these areas are most conveniently taken up separately.

In the juniper belt the juniper tree is of marked ecologic significance; the distribution of Peromyscus truei and Neotoma fuscipes is determined here by the presence of junipers. At certain times of year the fruit of this plant is eaten by coyotes, kangaroo rats, and wood rats.

The list below indicates the results of approximately 500 trap nights in the juniper belt near Mescal Canyon, between 4000 and 5000 feet elevation.

TABLE 7.--YIELD OF 500 TRAP-NIGHTS IN THE JUNIPER BELT.

====================================================================== | Number | Per cent | | of total --------------------------------------------------+--------+---------- Perognathus fallax pallidus | 16 | 16.7 Dipodomys merriami merriami | 3 | 3.1 Dipodomys panamintinus mohavensis | 36 | 37.5 Peromyscus truei montipinoris | 22 | 22.9 Peromyscus maniculatus sonoriensis | 12 | 12.5 Neotoma lepida lepida | 2 | 2.1 Neotoma fuscipes simplex | 2 | 2.1 Onychomys torridus pulcher | 3 | 3.1 --------------------------------------------------+--------+----------

PLATE 1