Part 5

PLATE 2

PLATE 3

PLATE 4

The biota of the washes that cut through the juniper belt in and below many of the larger canyons differs from that of the surrounding juniper-clad benches. Because the washes are in the same geographic area as the juniper belt they are discussed together. These washes on desert slopes are densely populated by rodents derived from adjacent areas, and support vegetation typical of higher floral belts in association with xerophytic, typically desert, species. In a sense, the washes serve to mix up the mammals of adjacent areas. For example, Onychomys torridus pulcher and Peromyscus eremicus eremicus, which are mammals typical of the desert, were found in Mescal Wash above their usual desert range; and Peromyscus californicus insignis and Peromyscus boylii rowleyi, which are chaparral inhabiting mammals, were found in the wash far removed from their chaparral environment. Washes are evidently effective agents in facilitating the dispersal of certain species of mammals. It is easy to envision a species crossing hostile habitats via dry washes to invade suitable niches in an area which is geographically and ecologically isolated from the original home of the species. Approximately 500 trap nights in Mescal Wash, at 4100 feet elevation, in the lower edge of the juniper belt, yielded the following mammals:

TABLE 8.--YIELD OF 500 TRAP-NIGHTS IN MESCAL WASH (DESERT SLOPE).

========================================================== | Number | Per cent | | of total --------------------------------------+--------+---------- Perognathus fallax pallidus | 5 | 4.5 Dipodomys panamintinus mohavensis | 43 | 38.7 Peromyscus californicus insignis | 3 | 2.7 Peromyscus truei montipinoris | 1 | .9 Peromyscus boylii rowleyi | 2 | 1.8 Peromyscus eremicus eremicus | 28 | 25.0 Peromyscus maniculatus sonoriensis | 23 | 20.5 Onychomys torridus pulcher | 4 | 3.5 Neotoma lepida lepida | 3 | 2.7 --------------------------------------+--------+----------

Dipodomys panamintinus mohavensis, Neotoma fuscipes simplex, and Peromyscus truei montipinoris are probably the most characteristic mammals of the pinyon-juniper association.

Sagebrush Scrub Association

MAJOR PLANTS

Bromus sp. Artemisia tridentata Chrysothamnus nauseosus Purshia glandulosa

This association is found on only the crest and desert slope of the range between 5000 and 8000 feet elevation. There it characteristically occupies flats and clearings in the yellow pine forest and pinyon-juniper woodland. The dominant plant of the association is basin sagebrush, and in many places this plant forms mixed growths with snowbrush and Haplopappus. The low brush of this association is formed by closely spaced bushes with grasses growing between.

Because of its limited occurrence in the San Gabriel Mountains, this association there has relatively little effect on mammalian distribution. Locally, nevertheless, the presence of this association governs the distribution of certain mammals. For example, on Blue Ridge, islands of sagebrush amid the conifers provide suitable habitat for Dipodomys agilis perplexus and Perognathus californicus bernardinus; and in Swarthout Valley D. a. perplexus, Reithrodontomys megalotis longicaudus, and Lepus californicus deserticola are seemingly restricted to the sagebrush flats.

Joshua Tree Woodland Association

MAJOR PLANTS

Yucca brevifolia Lycium Andersonii Eriogonum fasciculatum Tetradymia spinosa Ephedra sp. Larrea divaricata

This association is on the piedmont that dips toward the Mojave Desert from the interior base of the San Gabriels. The widely spaced Joshua trees with low bushes between, and the dry washes breaking the level terrain below the mouths of canyons are typical of this area. Field work was extended no farther down into the desert than about the 3500 foot level, where this association was still dominant.

Although the vegetation of this area is scattered and sparse, presenting a barren and sterile aspect, the area supports a rather high population of rodents. The soil at the bases of many large box-thorn- and creosote-bushes is perforated by burrow systems of Dipodomys panamintinus or Dipodomys merriami, and those burrows abandoned by kangaroo rats are used as retreats by Onychomys torridus and Peromyscus maniculatus. The mammals of this association are all characteristic of the fauna of the Mojave Desert, with the ranges of such species as the coyote and jack rabbit extending well up the desert slope of the mountains.

The mammals listed below were taken in 1948 in roughly 400 trap nights in the Joshua belt, at an elevation of 3500 feet, one mile below the mouth of Graham Canyon.

TABLE 9.--YIELD OF 400 TRAP-NIGHTS IN THE JOSHUA TREE BELT.

====================================================== | Number | Per cent | | of total -----------------------------------+--------+--------- Dipodomys panamintinus mohavensis | 36 | 59.0 Dipodomys merriami merriami | 15 | 24.6 Onychomys torridus pulcher | 4 | 6.6 Peromyscus maniculatus gambeli | 6 | 9.8 -----------------------------------+--------+---------

Populations of Dipodomys merriami and D. panamintinus fluctuate widely, possibly in response to weather cycles. In November of 1948 trapping in the Joshua belt showed that panamintinus outnumbered merriami approximately three to one, whereas in December of 1951, after a succession of unusually dry years, merriami was the more numerous. Further, merriami occurred in the lower parts of the juniper belt in 1951 where in 1948 it seemed to be absent.

Dipodomys merriami merriami and Onychomys torridus pulcher are diagnostic of the Joshua tree woodland association in the San Gabriel Mountains area, since few individuals of either species occur outside of this association.

ACCOUNTS OF SPECIES

Family DIDELPHIDAE

=Didelphis marsupialis virginiana= Kerr

Virginia Opossum

The opossum is common in and near small towns and cultivated areas at the Pacific base of the mountain range and does not thrive away from human habitation; extensive trapping in the coastal sage and chaparral belts produced no specimens except immediately adjacent to citrus groves. Pequegnat (1951:47) mentions that opossums in the Santa Ana Mountains of southern California are in the lower parts of the larger canyons, especially near human habitation.

Specimens examined.--Los Angeles County: Claremont, 1600 ft., 2 (PC).