Mammals of the San Gabriel Mountains of California Part 1

Mammals of the San Gabriel Mountains of California.

by Terry A. Vaughan.

INTRODUCTION

This paper presents the results of a study of the mammals of the San Gabriel Mountains of southern California, and supplements the more extensive reports on the biota of the San Bernardino Mountains by Grinnell (1908), on the fauna of the San Jacinto Range by Grinnell and Swarth (1913), and on the biota of the Santa Ana Mountains by Pequegnat (1951).

The primary objectives of my study were to determine the present mammalian fauna of the San Gabriel Mountains, to ascertain the geographic and ecologic range of each species, and to determine the systematic status of the mammals. In addition, certain life history observations have been recorded.

Field work was done in the north-south cross section of the mountains from San Gabriel Canyon on the west, to Cajon Wash on the east; and from the gently sloping alluvium at the Pacific base of the mountains at roughly 1000 feet elevation on the south, over the crest of the range to the border of the Mojave Desert at an elevation of 3500 feet on the north. Camps were established at many points in the area with the object of collecting the mammals of each a.s.sociation and each habitat. Field work was begun in the San Gabriels in November 1948, and was carried on intermittently until March 1952. I was unable to carry on field work in any summer.

For advice and a.s.sistance in various ways I am grateful to Drs.

Willis E. Pequegnat, Walter P. Taylor, Henry S. Fitch, E.

Raymond Hall, Mr. Steven M. Jacobs and my wife, Hazel A.

Vaughan.

More than 350 mammals were prepared as study specimens; most of these are in the University of Kansas Museum of Natural History.

Approximately a fifth of them are in the collection of the Department of Zoology at Pomona College, and a few are in the University of Illinois Museum of Natural History. No symbol is used to designate specimens in the University of Kansas Museum of Natural History. Specimens from the Department of Zoology of Pomona College and the University of Illinois Museum of Natural History are designated by PC and IM, respectively.

[Ill.u.s.tration: FIG. 1. Map of the San Gabriel Mountain area showing the positions of places mentioned in the text.]

DESCRIPTION OF THE AREA

The San Gabriel Mountains are approximately sixty-six miles long, and average twenty miles wide. The main axis of the range trends nearly east and west, and extends from longitude 11725' to longitude 11830'. The widest part of the range is bounded by lat.i.tude 347' and lat.i.tude 3430'.

The San Gabriel Mountains connect the Sierra Nevada with the Peninsular Ranges of southern California and Baja California. On the west the San Gabriels are bordered by the Tehachapi Mountains, which stretch northeastward to meet the southern Sierra Nevada; to the east, beyond Cajon Pa.s.s, the San Bernardino Mountains extend eastward and then curve southward to the broad San Gorgonio Pa.s.s, from which the San Jacinto Range stretches southeastward to merge with the Peninsular Ranges.

The rocks comprising the major part of the San Gabriel Mountains probably were intruded in Late Jura.s.sic times, with severe metamorphic activity taking place concurrently. A long period of erosion followed after which deposition took place during much of the Tertiary.

Deformation and uplift beginning in Middle Miocene times resulted in the formation of east-west-trending faults along both sides of the range. By repeated movements along these faults the Late Jura.s.sic crystalline rocks were lifted above late Tertiary and Quaternary sediments and elevated above the surrounding terrain. Continued uplifts in post-Pleistocene time together with erosion in Recent times have shaped the San Gabriel Mountains (Oakeshott, 1937).

The alluvial slopes at the coastal base of the range give way to the foothills at roughly 1800 feet elevation; whereas the Mojave Desert merges with the interior foothills at elevations near 4000 feet. The crest or drainage-divide of the range varies from 6000 to 8000 feet in elevation, and many peaks are more than 8000 feet high. San Antonio Peak, the highest peak of the range, rises to an alt.i.tude of 10,080 feet. The mountains are characteristically steep and the slopes are deeply carved by canyons, the larger of which have permanent streams.

The abruptness of the Pacific slope is in many places impressive. The horizontal distance from the top of Cucamonga Peak, at an elevation of 8911 feet, to the base of the coastal foothills directly to the south, at 2250 feet, an elevational difference of 6661 feet, is only 3.8 miles.

From the base of Evey Canyon, at 2250 feet, to an unnamed peak to the northwest with an elevation of 5420 feet, the horizontal distance is 2.1 miles. Because of the steep, rocky nature of many of the slopes and the lack of soil on them, vegetation may be spa.r.s.e even at high elevations.

There are few meadows in the mountains.

Because the San Gabriels stand approximately thirty miles from the Pacific Ocean and are a partial barrier to Pacific air ma.s.ses sweeping inland, the desert side and the coastal side of the range differ climatically. The coastal slope receives much heavier precipitation than the desert slope. The precipitation, for 1951, of 25.36 inches recorded at the mouth of San Antonio Canyon on the Pacific slope contrasts with 7.17 inches recorded at Valyermo at the desert base. Nearly all of the precipitation comes in winter. The higher parts of the range, above approximately 5000 feet, receive much of their mid-winter precipitation in the form of snow. Snow often extends down the desert slope well into the Joshua Tree belt. When there are heavy winter rains the channels of the usually dry washes are filled with rus.h.i.+ng, turbid water. There are striking differences in temperature between the two sides of the range and between the lower elevations of the mountains and the higher parts.

For example, in December 1951, the mean temperature at the base of San Antonio Canyon (2225 feet) at the coastal foot of the range was 55.4F, while at Llano (3764 feet) at the desert base it was 43.7F. In this same year the December mean for Table Mountain (7500 feet), on the desert slope, was 33.4F. The temperature means for July, 1951, at San Antonio Canyon, Llano, and Table Mountain, were 77.3F, 82.1F, and 69.2F respectively. The weather records for 1951 were used for ill.u.s.tration because average temperature and average precipitation for many other years are lacking for most of the weather stations in the area. There is an important difference in the humidity on the two sides of the range, but actual data are not available. At certain times, especially in spring, fog banks moving in from the Pacific Ocean frequently blanket the coastal base of the mountains and the foothills.

On such days the fog generally "burns off" in the morning, but may persist into the afternoon or throughout the day. Never in my experience has fog spilled over the main part of the range far onto the desert slope, although the fog may push through the lower pa.s.ses to be dissipated quickly in the dry desert atmosphere. The obvious differences in the biota on the two sides of the range are probably due to the contrasting climates.

BIOTIC PROVINCES AND ECOLOGIC a.s.sOCIATIONS

Because of the elevational extremes and attendant climatic contrasts in the San Gabriel Mountains, there is a rather wide range of environmental conditions. Four life-zones are represented: Lower Sonoran, Upper Sonoran, Transition, and Canadian. Within these zones certain ecologic communities can be recognized; these represent several biotic provinces. Table 1 shows the relations.h.i.+ps between the environmental categories recognized by the writer in the San Gabriel Mountains. The biotic province and ecologic community system is that developed by Munz and Keck (1949), and the life-zone system is that of Merriam (1898).

TABLE 1.--RELATIONS OF THE MAJOR ENVIRONMENTAL CATEGORIES OF THE SAN GABRIEL MOUNTAINS.

======================================================================= Biotic province | Plant community | Life-zone | Slope ----------------+--------------------------+-----------------+--------- |1. Coastal sage scrub | Lower Sonoran | Pacific Californian |2. Southern oak woodland | Upper Sonoran | Pacific |3. Chaparral | Upper Sonoran | Pacific ----------------+--------------------------+-----------------+--------- Sierran |4. Yellow pine forest and | Transition | Pacific | limited areas of | Canadian | and | boreal flora | | Desert ----------------+--------------------------+-----------------+--------- Nevadan |5. Sagebrush scrub | Transition | Desert | | Upper Sonoran | ----------------+--------------------------+-----------------+--------- Southern Desert |6. Pinyon-juniper woodland| Upper Sonoran | Desert |7. Joshua tree woodland | Lower Sonoran | Desert ----------------+--------------------------+-----------------+---------

The Californian Biotic Province dominates the biotic aspect of the coastal slope of the range. Thirty-nine out of the seventy-two mammals recorded from the San Gabriels are typical of this Province. The coastal sage-flats at the Pacific base of the mountains and the vast tracts of chaparral of the coastal slope are included in this Province.

Forming a hiatus between the Pacific and the desert slope is the Sierran Biotic Province consisting of coniferous forests on the crest of the range. The chipmunk (_Eutamias speciosus speciosus_) and the introduced black bear (_Ursus america.n.u.s californiensis_) are the only two mammals which can be considered typical of this area. On the higher peaks of the range, such as Mount San Antonio and Mount Baden Powell, the Canadian Life-zone is represented by certain boreal plants.

At scattered points along the crest of the range and on the desert slope, the Nevadan Biotic Province is represented by the sagebrush scrub a.s.sociation. No mammals can be considered typical of this region.

The Southern Desert Biotic Province occurs below 6000 feet elevation on the interior slope of the range, and markedly influences the mammal fauna of this slope. Twenty-one species of mammals are typical of this Province.

SCIENTIFIC AND COMMON NAMES OF PLANTS MENTIONED IN THIS REPORT

_Pinus lambertiana_ Sugar Pine _P. monophylla_ One-leaf Pinyon _P. ponderosa_ Yellow Pine _P. contorta_ Lodge-pole Pine _Pseudotsuga macrocarpa_ Big-cone Spruce _Abies concolor_ White Fir _Libocedrus decurrens_ Incense-Cedar _Juniperus californica_ Juniper _Ephedra sp._ Desert-Tea _Bromus sp._ Brome Gra.s.s _Yucca Whipplei_ Spanish Bayonet _Y. brevifolia_ Joshua Tree _Salix sp._ Willow _Alnus rhombifolia_ Alder _Castanopsis sempervirens_ Chinquapin _Quercus Kelloggii_ California Black Oak _Q. agrifolia_ California Live Oak _Q. dumosa_ Scrub Oak _Eriogonum fasciculatum_ California Buckwheat _Umbellularia californica_ Bay, California-laurel _Ribes nevadense_ Gooseberry _R. indecorum_ Currant _R. Roezlii_ Currant _Planta.n.u.s racemosa_ Sycamore _Rubus vitifolius_ Western Blackberry _Cercocarpus ledifolius_ Mountain Mahogany _C. betuloides_ Mountain Mahogany _Adenostoma fasciculatum_ Greasewood _Purs.h.i.+a glandulosa_ Antelope-brush _Prunus virginiana_ Choke Cherry _P. ilicifolia_ Holly-leaved Cherry _Larrea divaricata_ Creosote Bush _Rhus diversiloba_ Poisonoak _R. trilobata_ Squaw Bush _R. laurina_ Laurel Sumac _R. integrifolia_ Lemonadeberry _R. ovata_ Sugarbush _Rhamnus crocea_ Buckthorn _Ceanothus sp._ Lilac _C. cordulatus_ Snow-brush _Fremontia californica_ California Slippery-elm _Opuntia occidentalis_ p.r.i.c.kly-pear _Arctostaphylos sp._ Manzanita _Salvia mellifera_ Black Sage _S. apiana_ White Sage _Lycium Andersonii_ Box-thorn _Haplopappus squarosus_ _Chrysothamnus nauseosus_ Rabbitbrush _Baccharis sp._ Mule Fat _Franseria dumosa_ Burroweed _Artemisia tridentata_ Basin Sagebrush _A. californica_ Coastal Sagebrush _Lepidospartum squamatum_ Scale-broom _L. latisquamatum_ Scale-broom _Tetradymia spinosa_ Cotton-thorn

Coastal Sage Scrub a.s.sociation

MAJOR PLANTS

_Artemisia californica_ _Salvia apiana_ _Salvia mellifera_ _Eriogonum fasciculatum_ _Rhus integrifolia_ _Opuntia occidentalis_ _Haploppapus squarrosus_

This a.s.sociation is restricted to the Pacific base of the range, is typical on the alluvium at the bases of the coastal foothills, and usually grades into the chaparral at about 1800 feet elevation. When seen from above, the rather level terrain of the a.s.sociation is broken sharply at the mouths of canyons by dry washes, and is limited below, to the south, by cultivated land. The coastal sagebrush is the most characteristic plant of this a.s.sociation, occurring in all undisturbed parts of the area.

There are several habitats within the coastal sage scrub a.s.sociation.

These differ from one another chiefly on the basis of soil type. The soil of the rather level sageland in most places is rocky or gravelly, or, as adjacent to washes, it is finely sandy in texture, and supports the major plants of the a.s.sociation. Most of the eroded adobe banks at the bases of the foothills support these same plants, with white sage being the dominant species. Locally, as in damp hollows or cleared areas, there is gra.s.sland. Jumbles of boulders, sand, gravel, and steep cutbanks, are characteristic of the channels of dry washes, these areas supporting spa.r.s.e vegetation. The fauna and flora of the washes are distinct from those of surrounding sage flats. Because they are included within the geographic limits of the coastal sage belt, however, the washes are discussed along with this a.s.sociation.

The abruptness with which one habitat gives way to another in this a.s.sociation causes sharp dividing lines between the local ranges of certain mammals. For example, in trap lines transecting dry washes and level sageland two a.s.semblages of rodents were found. That part of the line amid the boulders and cutbanks of the wash took mostly _Peromyscus eremicus fraterculus_ and _Neotoma lepida intermedia_, while _Perognathus fallax fallax_, _Dipodomys agilis agilis_, and _Peromyscus maniculatus gambeli_ were taken in the adjacent sage flats. The steep adobe slopes of the foothills, which const.i.tute the upper part of the coastal sage scrub a.s.sociation, are commonly inhabited by _Peromyscus californicus insignis_, which rarely occurs in the level tracts of sage a few yards away. Thus, this a.s.sociation is not h.o.m.ogeneous with regard to its rodent population; many of these species have local and discontinuous distributions.

The following list gives the results of about 500 trap nights (a trap night equals one trap set out for one night) in typical coastal sage-scrub a.s.sociation one-half mile southwest of the mouth of San Antonio Canyon, at 1700 feet elevation.

TABLE 2.--YIELD OF 500 TRAP-NIGHTS IN THE COASTAL SAGE SCRUB a.s.sOCIATION.

====================================================================== | Number | Per cent | | of total --------------------------------------------------+--------+---------- Perognathus fallax fallax | 31 | 30.7 Dipodomys agilis agilis | 20 | 19.8 Reithrodontomys megalotis longicaudus | 4 | 4.0 Peromyscus californicus insignis | 4 | 4.0 P. eremicus fraterculus | 7 | 6.9 P. maniculatus gambeli | 20 | 19.8 Neotoma lepida intermedia | 9 | 8.8 N. fuscipes macrotis | 2 | 2.0 Microtus californicus sanctidiegi | 4 | 4.0 --------------------------------------------------+--------+----------

The list below indicates the catch in 200 trap nights in San Antonio Wash, at 1700 feet elevation and within the realm of the coastal sage; all of the traps were set in rocky and sandy main channels of the wash.

TABLE 3.--YIELD OF 200 TRAP-NIGHTS IN SAN ANTONIO WASH.

====================================================================== | Number | Per cent | | of total --------------------------------------------------+--------+---------- Perognathus fallax fallax | 2 | 5.1 Peromyscus californicus insignis | 2 | 5.1 P. eremicus fraterculus | 26 | 66.7 Neotoma lepida intermedia | 9 | 23.1 --------------------------------------------------+--------+----------

The p.r.i.c.kly-pear cactus is of obvious importance to certain mammals of the coastal sage belt. This cactus is most common in disturbed areas such as sandy flats bordering washes, eroded adobe banks, and land once cleared by man. In these areas it is often the dominant plant with respect to area covered, usually growing in dense patches each covering approximately 150 square feet. It provides subst.i.tute nesting sites for _Neotoma lepida_ in areas devoid of rock piles, and is probably the major factor governing the distribution of this wood rat in the sageland. Cottontails and brush rabbits use p.r.i.c.kly-pear cactus extensively as refuge. Their forms and short burrows can be seen beneath many of the clumps of cactus.

This cactus serves as food for many mammals at least in the fruiting period in the fall. Usually only the fruit is eaten, but some pads are chewed by rabbits. The fruit or seeds of this plant are eaten by striped skunks, gray foxes, coyotes, pocket mice, kangaroo rats, wood rats, and probably white-footed mice.