Rarest Species in the Borrego Desert Flora

Table of Contents

Rarest Species Seen In Our Surveys
Rarest Species From Vouchers without Taxonomic difficulties
Rarest Species From Vouchers with Taxonomic difficulties
Species Classified as Rare-In-California that are Not Rare In This List


The lists on this page were made for the first time on 9 February 2014, and it will take some time, and lots of review, before they can be considered mature, especially since it is very difficult to compile a list of the rarest species anyplace. If you know of other species that should be added to the lists, or know that we have missed other populations are of any of the rare species, or have any other comments on this page, please let me know. For now, I'm concentrating on trying to make the lists complete; I'll work on the abundance numbers, and species comments, later.

I consider only plants native to San Diego County in all of the following lists. This list concentrates on species of the Borrego Desert, defined roughly as the northern half of Anza-Borrego Desert State Park. It includes a few species we have seen in surveys in the southern part of the Park, or know about from other means.

For this page, I consider a species to be rare if it is rare in the Borrego Desert area, and is found in a small number of locations in all of San Diego County, typically six or seven, with fewer than 1,000 plants known from all the populations combined. This is very different from the criteria used by CNPS and others to define rare species in California as a whole. Many times, species rare overall in California can be very abundant in the few places where they live. Such rare-in-California species would not make this rare list, simply because there are many other species that are rarer when one considers just the Borrego Desert.

To be clear, none of the lists contain species that are rare in the Borrego Desert, but are common elsewhere in San Diego County. There are many species that are rare in the Borrego Desert simply because they are coastal / montane species that barely make it into the Borrego Desert, and it would be silly to include them in a list of truly-rare plants in the Borrego Desert.

The list of rare species on this page includes plants that may be common outside of San Diego County. Thus these species listed here are by no means all rare species statewide, and should not be confused with those species.

Compiling a list of rare plant species in a given area is a very difficult task for a number of reasons:

Since I and colleagues have done ~400 quantitative surveys of many different areas in the Borrego Desert, we have an excellent knowledge of the abundance of species we have seen, and have pretty good evidence that the species we have not seen are uncommon. In fact, five of the species on this rare plant list were ones discovered in the course of our surveys. Our surveys also allow us to ascertain the actual abundance here of species that are classified as rare or uncommon overall in the state, which often results in them not being present in the list of rare species given here.

Vouchers take advantage of over one hundred years of many people scouring the Borrego Desert for plants, and thus give names of species we haven't seen. But locations of older vouchers are often vague, and sometimes wrong, and determinations of vouchers are sometimes suspect.

As a result, I present the rarest species in four different tables. Table 1 gives species that are rare in our surveys, defined as occurring in roughly six or fewer locations, with fewer than 1,000 plants found in our surveys and from our estimate of the number of plants from additional vouchered locations. We have excellent abundance information for those species from the areas we have surveyed.

Table 2 gives vouchered species that are probably correctly determined, with good locations, that we haven't seen, and which appear to be rare in San Diego County as a whole. Rare here is defined as having fewer than eight locations, and which are likely to have fewer than 1000 plants (that is very much a judgment call, since vouchers rarely give abundances). We will target these species in future surveys to see if we can verify the determinations and locations, and get good estimates of their abundances. Not all the species in Table 2 have been studied in detail, so some of them might be removed to Table 3 if it becomes apparent that they suffer from taxonomic difficulties.

Table 3 gives vouchered species that are rare, but which have taxonomic problems that raise a question about the correct determination of the vouchers. We will look at these vouchers to try to verify the determinations, as well as target their locations for field study. Often the determination of a single voucher can be problematic, but when looks at the population in the field the determination becomes clear. I.e., the species in Table 3 might not be present at all in the Borrego Desert.

Table 4 gives species that are classified as rare or uncommon statewide, but which are not rare in the Borrego Desert using the criteria given here.

Rarest Species Seen In Our Surveys

The column header #Pls gives the number of plants of each species we have seen in our surveys, augmented by my estimate from voucher locations. Estimates from voucher locations are of course extremely uncertain except in the very few cases where the voucher label reported how many plants were seen.

Since a population of 20 or fewer plants is very likely to be extirpated by normal population fluctuations, it is highly likely that species with fewer numbers in Table 1 are more abundant than has been seen so far in our surveys and in vouchers. Most of those species have never had targeted surveys done for them, which might have discovered areas in which they are more abundant. These are all species that need targeted surveys done for them!

No abundance estimates are given for some species, and for many others the estimates are just wild guesses. I'll attempt to refine the numbers in the future. The first priority was just to get the names on this list. Some of the numbers for observed species were from my memory; I'll look up the actual numbers from our survey data in the future.

The column header #Areas gives the number of areas includes locations seen from our surveys, as well as other locations from vouchers in San Diego County. The number of locations is generally from the clustering algorithm of the Consortium, obtained by scaling the map so that San Diego County fills the screen.

Species discovered for the first time in San Diego County in our surveys are marked with ^ after the species name.

Some species have links to Borrego Desert pages for them.

Table 1. Rare Species Seen In Our Surveys

11Phacelia rotundifolia^
21Horsfordia alata
22Astragalus pachypus var. pachypus
3?1Boechera not-xylopoda^
3?2Astragalus pachypus var. jaegeri
81Abutilon abutiloides
10?2Stanleya pinnata var. pinnata
102Andropogon glomeratus var. scabriglumis
10?3Glycyrrhiza lepidota
104Petalonyx linearis
172Prenanthella exigua
20?3Chamaesyce pediculifera
20?4Penstemon thurberi
30?1Echinocactus polycephalus
301Eriogonum pusillum^
341Thymophylla pentachaeta var. belenidium
50?3Lycium parishii
50?6Crossosoma bigelovii
50?7Salvia eremostachya
943Nemacladus twisselmannii var. botanywomaniae^
1001Chamaesyce revoluta
100?3Cryptantha barbigera var. fergusoniae
100?1Atriplex elegans var. fasciculata
100?5Quercus palmeri
1102Trianthema portulacastrum^
14311Matelea parvifolia
200?3Tiquilia canescens var. canescens
200?4Chaenactis carphoclinia var. peirsonii
200?5Boerhavia coulteri var. palmeri
200?5Chamaesyce arizonica
2005Lessingia glandulifera var. tomentosa
200?7Chamaesyce abramsiana
3001Herissantia crispa
3006Scutellaria mexicana
8003Calliandra eriophylla
1000?6Cylindropuntia wolfii
 2Camissonia campestris ssp. campestris
 2Cryptantha holoptera
 3Hoffmannseggia microphylla
 3Plagiobothrys jonesii
 4Astragalus aridus
 4Sphaeralcea angustifolia
 5Allenrolfea occidentalis
 5Boerhavia wrightii
 5Helianthus niveus ssp. canescens
 5Linanthus jonesii
 5Malperia tenuis
 5Monolepis nuttalliana
 5Nemacladus tenuis
 5Opuntia chlorotica
 6Cryptantha ganderi
 6Linanthus bigelovii
 6Nemacladus sigmoideus
 6Stylocline micropoides
 7Cryptantha costata

^ Species discovered for the first time in San Diego County during our surveys.

Rarest Species From Vouchers without Taxonomic difficulties

Table 2. Rare Species From Vouchers without Taxonomic difficulties

1Astragalus magdalenae var. peirsonii
1Astragalus sabulonum
1Eriogonum deserticola
1Eucnide rupestris
1Gilia mexicana
1Gutierrezia microcephala
1Lepidium flavum var. felipense
1Opuntia polyacantha var. erinacea
1Phacelia ivesiana
1Spermolepis echinata
1Wislizenia refracta ssp. palmeri
2Helianthus niveus ssp. tephrodes
2Nama hispidum var. spathulatum
2Phacelia neglecta
2Salvia pachyphylla
2Spermolepis infernensis
3Abutilon palmeri
3Anemone tuberosa
3Aristida californica
3Eriophyllum multicaule
3Leptosiphon floribundus subsp. hallii
3Mimulus rubellus
4Astragalus insularis var. harwoodii
4Astragalus nuttallianus var. imperfectus
4Cuscuta denticulata
4Lupinus shockleyi
4Mentzelia desertorum
5Stylocline intertexta
6Eriogonum saxatile
6Mimulus parishii
7Gilia scopulorum
7Poa bigelovii

Rarest Species From Vouchers with Taxonomic difficulties

Table 3. Rare Species From Vouchers with Taxonomic difficulties

Baileya pleniradiata
Boechera pulchra
Eriogonum angulosum
Camissonia boothii ssp. decorticans
Chylismia brevipes
Chylismia claviformis ssp. aurantiaca
Chylismia claviformis ssp. claviformis
Ephedra trifurca
Eriogonum ordii
Phacelia longipes
Phacelia tanacetifolia
Phacelia umbrosa

Comments on some of these species:

Species Classified as Rare-In-California that are Not Rare In This List

My use of rare in this section is in the same sense as given above, which differs from the use of the same word in the Jepson Manual First Edition (JM1) and in the CNPS Lists.

CNPS considers a species to be rare in California, at rank 1 or 2, if there are fewer than 50 populations of it. Rank 4 is for species with 50 to 150 populations. Since there have to be fewer than eight populations to make my list here, many species correctly determined as being rare by CNPS might not make my rare list.

My placement of the species in this section therefore does not necessarily imply that any of these species should be removed from the rare category used by others. In fact, most of the species are on this list simply since there are more than six or seven locations in San Diego County, although some of these are very abundant species (Ferocactus cylindraceus and Selaginella eremophila). I need to collect abundances from our surveys for most of them to see whether some of them should be placed in Table 1 based on the number of plants.

I'll add our observed abundances of these species in the future.

Table 4. Species Classified as Rare-In-California that are Not Rare In This List

SpeciesRarity Designation in JM1
Acmispon haydoniiRARE
Astragalus crotalariaeUNCOMMON
Astragalus lentiginosus var. borreganusUNCOMMON
Ayenia compactaRARE in CA
Bursera microphyllaRARE
Carlowrightia arizonicaRARE in CA
Caulanthus simulansRARE
Cylindropuntia fosbergiiUncommon
Delphinium parishii ssp. subglobosumUNCOMMON
Ferocactus cylindraceusUNCOMMON; threatened by collecting
Galium angustifolium ssp. borregoenseRARE
Lupinus excubitus var. mediusRARE
Lyrocarpa coulteriUNCOMMON
Mirabilis tenuilobaUNCOMMON
Penstemon clevelandii var. connatusUNCOMMON
Pilostyles thurberiUNCOMMON
Selaginella eremophilaUNCOMMON
Senna covesiiRARE in CA
Streptanthus campestrisRARE
Xylorhiza orcuttiiRARE

I thank James Dillane for a number of additions to the initial list, and Adrienne Ballwey for noticing that Herissantia crispa was missing from the list.

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Updated 5 June 2020.