Species That Skip the Borrego Desert

Table of Contents

Analysis of Geographic Ranges of Species Approaching the Borrego Desert
List of Species That Skip the Borrego Desert
Coastal / Peninsular Range Species Not Considered Here
Species With Possible Taxonomic Difficulties Not Considered Here


The list on this page were made for the first time on 27 February 2014, and it will take some time, and lots of review, before it can be considered mature, especially since it is very difficult to compile a list of the species missing from a flora that might otherwise be expected to be there. If you know of other species that should be added to this list, or have any other comments on this page, please let us know. The list appears incomplete beyond a distance of 20 miles, since the number of species declines with distance beyond about 20 miles, instead of increasing with distance as would be expected.

Species distribution is a fascinating topic. Most analyses concentrate on where a species is found. But it is just as interesting to note where species are not found.

This page presents a list of native desert species that might conceivably live in the Borrego Desert as defined below, but have not been recorded there. It also contains an analysis of the geographic ranges of the species whose closest location is in different directions, and at different distances, from the Borrego Desert boundaries. That analysis reveals considerable clues as to the make-up of species in the Borrego Desert Flora: the areas with which they share, or do not share, some species.

The lists do not include species found in the Peninsular Range to the west, some of whom venture into the Desert Transition Zone while others do not. Nor do the lists include riparian species that are not widespread in the desert.

We originally had separate lists for two categories of such species. First, there are species that are found just outside the borders of the Borrego Desert, but which stop short of entering it (see Fig. 1 below). Second, there are species with disjunct distributions that skip over the Borrego Desert (see Fig. 2 below).

However, the separation between these categories is often ambiguous, and it is more convenient to have all the species found outside the borders of the Borrego Desert in one table. But since it seems of interest to identify those species that for some reason completely skip over the Borrego Desert and nearby areas, from those that approach the Borrego Desert closely, we've added the notation Gap to the locality of those species that seem to do that.

For this page, we define the Borrego Desert to mean the portion of the Borrego Valley / Coyote Creek / San Felipe Creek drainage areas that are considered to be in the Sonoran Desert floristic province. Most of this area is below 3000 feet elevation except for the Vallecito Mountains, with elevations up to Whale Peak at 5349 feet. This area is entirely in San Diego County except for a few fingers reaching into Riverside County. We do not include the Fish Creek drainage area that is mostly on the south side of the Vallecito Mountains, but do include the Elephant Tree area that drains into the lower Borrego Valley. We also include Blair Valley, Little Blair Valley, and upper Smuggler Canyon since that is a natural part of the Whale Peak / Vallecito Mountain area.

Fig. 1 shows the definition of the Borrego Desert used for this page, along with the best example of a species that approaches the Borrego Desert, but is not found within it, Hoffmannseggia microphylla = Caesalpinia virgata.

It is possible that the elevation preference of this species in the immediate area accounts for its close approach to, but absence from, the Borrego Desert. In its closest approach from the north and the south, it lives at elevations of 200 feet or so, lower than the elevations of the nearby portions of the Borrego Desert. Although it is found up to elevations of 2000 feet on the southern border of Joshua Tree National Park, the conditions in that area are significantly different from those in the Borrego Desert.

Fig. 1. Geographic distribution map for Hoffmannseggia microphylla, obtained from the Consortium of California Herbaria on 27 February 2014. The black dots give locations of georeferenced vouchers. The color-shaded regions correspond to the Floristic Geographic Subdivisions given in the Second Edition of the Jepson Manual: the Sonoran Desert (DSon); Mojave Desert (DMoj); Desert Mountains (DMtns); and Peninsular Ranges (PR). The subdivisions are labeled on the map.
The region defined as the Borrego Desert in this page is a subset of the DSon region outlined on the east by the thick black lines that follow the northeast border of San Diego County; on the south by a straight line along the southern boundary of the higher elevations of the Vallecito Mountains, whose highest point is Whale Peak; and on the west and north by the limits of DSon as given by the Consortium mapping.
Note that the black dots come just about as close as possible to the Borrego Desert without entering it.
See also a map without any extra lines or shading.

Fig. 2 shows a geographic distribution map for a species that has a larger gap in its distribution consisting of the Borrego Desert and surrounding area, Astragalus coccineus. This species is found on the desert side of every southern California mountain range, as well as on the desert side of the Sierra Nevada, except for the mountain ranges to the west of the Borrego Desert area. The Borrego Desert area is clearly a gap in the distribution of this species.

Fig. 2. Geographic distribution map for Astragalus coccineus, obtained from the Consortium of California Herbaria on 28 February 2014. The red dots give locations of georeferenced vouchers, which are found on the desert side of all southern California mountain ranges except for the mountain ranges west of the Borrego Desert area. Note that this map covers a much larger area than was shown in Fig. 1.

Analysis of Geographic Ranges of Species Approaching the Borrego Desert

Somewhat surprisingly, some insights into the Borrego Desert Flora can be gleaned from the species which are found only in the areas just outside the borders of the Borrego Desert! The nearest locations of such species turn out not to be uniformly distributed, but instead are found in clumps. Furthermore, the clumps at different distances and in different directions come from species with very different geographic distributions, which reveal areas with different connections to the species in the Borrego Desert flora.

Fig. 3 shows the nearest locations of the species given in Tables 1 and 2 below as of 1 March 2014, when there were 59 species in the table (more have been added since). The Consortium clustering algorithm was used, so that one can see hotspots where many additional species are found closest to the Borrego Desert. The northwest part of Joshua Tree National Park (nw JTNP) area and the Mecca Hills / Box Canyon area contribute the clusters with the highest number of these species, 11 each, since those areas contain the southernmost limit of many Mojave Desert Species. The third and fourth richest hotspots are the Mason / Vallecito Valley / Canyon 41 area immediately southwest of the Borrego Desert, and the north side of the Santa Rosa Mountains (n side SnRsMtns), each with seven species. Each of these hotspots is labeled in Fig. 3.

Fig. 3. Geographic map of the closest locations for each of the species not found in the Borrego Desert. The number inside the circle indicates how many species have approximately the same location. The color of the circle changes with the number of species. The four richest hotspots are labeled.

For the four richest hotspots, we have analyzed the geographic distribution of each species found within it. Within each hotspot, it turns out that there is a considerable similarity in the distributions of many of the species. There is also an interesting variation in the geographic distribution of the species from one hotspot to another.

Fig. 4 presents the geographic distributions of the species for each of the four richest hotspots, with each species given a different symbol, after removal of two species each from two of the hotspots that do not fit the dominant pattern within those hotspots (the removed species are identified below). Fig. 5 presents the same geographic information on an actual map, but without separate symbols for each species.

northwest Joshua Tree National Park hotspot

Box Canyon, Mecca, Shavers Well Area hotspot

north side Santa Rosa Mountains hotspot

Mason, Vallecito Valleys; Canyon 41; Inner Pasture area hotspot
Fig. 4. The full geographic distribution of the species whose closest location to the Borrego Desert is in each labeled hotspot, after removal of two species from each of the bottom two plots whose geographic distribution is very different from the rest of the species in each of those hotspots. The black rectangle gives the approximate location of the Borrego Desert. See the text for details. Click on the pictures for larger versions that include the legend and axis labels.

northwest Joshua Tree National Park hotspot

Box Canyon, Mecca, Shavers Well Area hotspot

north side Santa Rosa Mountains hotspot

Mason, Vallecito Valleys; Canyon 41; Inner Pasture area hotspot
Fig. 5. Same as Fig. 4, but with the locations plotted on a geographic map, and using the same red dot for each species. Click on the pictures for larger versions.

Figs. 4 and 5 show the following patterns:

This information from the "closest neighboring species" gives clues about where some of the species in the Borrego Desert came from, as well as about areas which have little affinity with the Borrego Desert. Note in particular how remote the Borrego Desert area is from many of the species found in the northwest Joshua Tree National Park hotspot, which are concentrated on the desert sides of the Sierra Nevada and the Transverse Range.

Note that the above analysis does not mean that the Borrego Desert shares no species at all with, for example, the desert side of the Sierra Nevada or Transverse Range! There certainly are such species, such as creosote, that are widely distributed in the entire desert. The percent of the Borrego Desert flora that contains such widespread species as creosote, and the percent that shares no species with a given region, awaits a detailed analysis of the species actually found in the Borrego Desert flora. Remember, all of the above analysis is on species found just outside the boundaries of the Borrego Desert.

That said, the analysis here is consistent with the conventional wisdom that the Borrego Desert and San Jacinto Mountain area have a strong affinity with the Sonoran Desert and the Arizona upland, and the Mojave Desert and Transverse Ranges have a strong affinity with the Sierra Nevada and the Great Basin. Those affinities are probably driven by the glacial cycles, of which there have been roughly 25 in the last 2.5 million years. During the cold phase of each glacial cycle, the higher-elevation species retreat into the closest lower-elevation areas. The species in the TR and SN retreat into the DMoj and Great Basin; the species in SnJt, the PR and the Arizona uplands retreat into the DSon.

When we do the similar geographic analysis for species found in the Borrego Desert, it will be interesting to see how many species fit these same patterns. It should be possible to quantitatively estimate what percent of the Borrego Desert flora came from different areas.

Species That Skip the Borrego Desert

The nearest location in this table comes only from georeferenced vouchers; there might be a closer location from a non-georeferenced voucher. The georeferenced position has been checked for reasonableness, in terms of nearby georeferenced vouchers, and has often been checked to see if it is consistent with the locality, but not always.

This table will eventually be pruned to only include species within 20 or 30 miles, but includes some species found at farther distances for now so that we know we've checked how close they come. Since the Mexican border is 31 miles away, and we've only been searching California plant species, the list would be incomplete beyond that distance.

Table 1. Species That Approach the Borrego Desert

SpeciesClosest Location
Distance (miles)Location
Stylocline intertexta       0.1Freeman Properties, Truckhaven, north fork of Big Wash, 0.1 miles east of San Diego / Imperial County line.
Brandegea bigelovii       0.5One mile west of Travertine Rock at Riverside-Imperial County Line, 0.5 miles east of San Diego County border.
Cylindropuntia fosbergii   1Mason Valley; Hornblende Canyon; Vallecito Valley
Hoffmannseggia microphylla       1.01/2 mile southwest of the intersection of Split Mountain Road and Trestle Road near Gypsum Plant.
Nemacladus orientalis       1.2Along S22 1.2 miles east of San Diego / Imperial County Line
Sporobolus contractus   2Two miles east of San Diego County / Imperial County line in Riverside County just north of its county line.
Chamaesyce pediculifera   3Bisnaga Alta Wash south of Whale Peak
Horsfordia alata   3Bisnaga Alta Wash south of Whale Peak
Eriophyllum lanosum      3.5Smuggler Wash, Vallecito Valley
Acalypha californica   6Canyon 41, just south of Agua Caliente County Park, at mile marker 41 on S2
Eriophyllum ambiguum var. paleaceum   6Inner Pasture west of Tierra Blanca Mountains
Lycium parishii   6Canyon 41, just south of Agua Caliente County Park, at mile marker 41 on S2
Linanthus maculatus var. emaculatus   7Carla Hoegen and Fred Melgert found a population in the Inner Pasture on 30 March 2016, which is now the closest population to the Borrego Desert.
Astragalus coccineus   8Gap; found on the desert side of the Sierra Nevada, San Gabriel Mountains, San Bernardino Mountains, Santa Rosa Mountains and the In-Ko-Pah Mountains, but not on the desert side of the Laguna Mountains. Closest location is at the Dolomite Mine, Cactus Spring Trail, Santa Rosa Mountains on north.
Phacelia lemmonii   9Pinyon Flat, Santa Rosa Mountains
Purshia tridentata var. glandulosa   9Gap; widespread on desert side of Sierra Nevada and southern California mountains; abundant along SR74 on north side of Santa Rosa Mountains to the north; in Laguna Mountains and Jacumba area to the south, but skips the Borrego Desert Area
Tragia ramosa   9Santa Rosa Mtns Cactus Springs Trail near Horsethief Creek
Cylindropuntia wolfii10Mouth of Carrizo Canyon (there is a closer voucher from near Harpers Well, but that is way outside the previously-known range of C. wolfii.
Atrichoseris platyphylla11near Mecca, due north of the San Diego / Imperial County line.
Ditaxis claryana11E side of Jefferson Rd across from entrance to Cahuilla Lake County Park, S of Indio
Sporobolus flexuosus11Santa Rosa Mtns. Junction of Hwy 74 and Carrizo Road
Chylismia brevipes ssp. brevipes12near Mecca. There may be closer vouchers if those vouchers are correctly determined.
Eucnide rupestris12Near mouth of Indian Gorge, southern ABDSP
Pseudorontium cyathiferum120.25 mi SW of Boyd Deep Canyon Desert Research Center, south of Palm Desert
Astragalus lentiginosus var. coachellae13Painted Canyon, Mecca Hills
Sphaeralcea emoryi var. emoryi14Gap; north base of Santa Rosa mountains on north; San Sebastian Marsh area on east; Coyote Wells on south.
Xylorhiza cognata14Box Canyon 6 miles east of Mecca
Eschscholzia glyptosperma15Aqueduct Rd between Indio and Mecca, due north of San Diego / Imperial County / Riverside County junction
Salvia greatae15Endemic to Chocolate-Chuckwalla Mountains area
Ephedra trifurca15San Sabastian Marsh
Psorothamnus polydenius16Coyote Mountains, Imperial County
Brickellia incana17Shavers Well, top of Box Canyon in Mecca Hills
Phacelia campanularia ssp. vasiformis17Shavers Well, top of Box Canyon in Mecca Hills
Psorothamnus arborescens var. simplicifolius19Box Canyon, Mecca Hills / Chocolate-Chuckwalla Mountains.
Linanthus filiformis20Sweeney Pass
Monoptilon bellidiform20Cottonwood Springs, Joshua Tree National Park
Tetracoccus hallii20eastern edge of Coachella Valley, north of I-10 NW of Cactus City
Yucca brevifolia20Along road to Juniper Flats, Joshua Tree National Monument
Panicum urvilleanum21Indio
Xylorhiza tortifolia var. tortifolia225 miles east of Shavers Well near I-10
Chylismia arenaria26Canyon Springs area N of the Bradshaw Trail, drainage between Orocopia and Chocolate Mtns
Coryphantha alversonii26Joshua Tree National Park, 2.5 miles from Cottonwood springs on road out to Hwy 60-70
Galium angustifolium ssp. gracillimum27Tahquitz Canyon, Palm Springs
Tiquilia canescens var. pulchella27western base of the Chocolate Mountains, small canyon c. 1.25 mi due north of Imperial Co. line and c. 3.75 mi due west of aqueduct
Eriogonum nidularium28Cottonwood Mountains, Pinto Basin Road, Joshua Tree National Park
Ipomopsis tenuifolia282 km. north of Dubber along railroad track in Carrizo Gorge
Polygala acanthoclada29Joshua Tree National Park, 1 mile N of Cottonwood Spring
Salvia mohavensis29Lost Palms Canyon in the Eagle Mtns, southern Joshua Tree National Park
Ziziphus obtusifolia var. canescens30Chocolate-Chuckwalla Mountains region West end of the Orocopia Mtns
Chylismia claviformis ssp. claviformis31Palm Springs area. There may be closer vouchers, and perhaps even one from the Borrego Desert, but our ssp. peirsonii is often misdetermined as another subspecies. The JM places ssp. claviformis only in DMoj and edges.
Castela emoryi33Hayfields, Chuckwalla Valley, just east of Chiriaco Summit
Colubrina californica34Joshua Tree National Park, Ruby Lee Mill Site
Camissoniopsis pallida ssp. hallii34Snow Creek Rd 1.4 mi. S of CA Hwy 111, north base of San Jacinto Mountains
Cylindropuntia munzii34W. slope Chocolate Mountains, 11 miles from Niland on road to Beal Well
Teucrium cubense ssp. depressum34Hayfields; Colorado Desert, Chuckwalla Valley
Nicolletia occidentalis35nw Joshua Tree National Park
Tetradymia axillaris var. longispina36Near Salton View, Joshua Tree National Park
Calochortus kennedyi var. kennedyi37Northwest of Keys View, Joshua Tree National Park
Gutierrezia microcephala37nw Joshua Tree National Park. There is a voucher of what is probably a waif in the Buckman Springs rest area of I-8 that is 19 miles away.
Hymenoxys cooperi37nw Joshua Tree National Park
Condalia globosa var. pubescens39Red Canyon Road, Chocolate Mountains
Ambrosia ilicifolia40North base of Signal Mountain, Yuha Desert
Tetradymia stenolepis40Ryan Mountain, Joshua Tree National Park
Croton wigginsii43East Mesa, east end of Montgomery Road at the Coachella Canal, ca. 1 miles northwest of the Mammoth Wash crossing of the canal
Ericameria cooperi var. cooperi43nw Joshua Tree National Park
Eriogonum heermannii var. argense43nw Joshua Tree National Park
Penstemon clevelandii var. mohavensis43nw Joshua Tree National Park
Adenophyllum cooperi44nw Joshua Tree National Park. There may be closer vouchers, but their determinations seem suspicious.
Krascheninnikovia lanata62New Dixie Mine Road Area, desert on northeast side of SnBr.

Coastal or Peninsular Range or Riparian Species Not Considered Here

Andropogon glomeratus var. scabriglumis
Aristida ternipes var. gentilis
Eustoma exaltatum ssp. exaltatum
Juncus acutus ssp. leopoldii
Eragrostis mexicana ssp. mexicana
Eriastrum densifolium
Eriodictyon trichocalyx var. trichocalyx
Eriogonum apiculatum
Laennecia coulteri
Marina orcuttii var. orcuttii
Pediomelum californicum

Species With Possible Taxonomic Difficulties Not Considered Here

Dieteria asteroides var. asteroides
Ditaxis serrata var. californica
Ephedra nevadensis
Nemacladus tenuis var. aliformis

We thank Jane Strong for comments that improved Fig. 1.

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Copyright © 2014-2017 by Tom Chester and James Dillane
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Updated 10 February 2017.