Plants of Southern California: Opuntia echinocarpa, O. ganderi, O. parryi, and O. wolfii: Locations

See Opuntia echinocarpa, O. ganderi, O. parryi, and O. wolfii: Pictorial Identification Guide for an introduction to this page, and how to discriminate these species.

Geographic Maps
Discussion of Location by Species
Tour Guide To All Four Species Along S2 in San Diego County

Geographic Maps

Locations for each of these species, from vouchers and from personal observations, are shown in the next two plots. Annual rainfall contours from Pryde (1992) and Climatic Atlas of the United States (1968) are also shown, but should be interpreted with considerable caution. Although there is little doubt that they correctly show the sharp decrease in rainfall toward the desert regions, the contours are almost entirely extrapolations due to the lack of weather stations in the desert regions. Pryde (2004) removed such maps for precisely that reason.

The first plot shows a large area of southern California; the second plot is a blow-up of San Diego County and a portion of Riverside County that contains the entire population of O. ganderi and O. wolfii that I know about. Note the "10 mile" scale given toward the upper right in both plots.

The third map is a blowup of the northern part of Anza-Borrego Desert State Park and just contains O. ganderi and O. echinocarpa. It is being updated regularly in the winter, and so contains much more data than is shown in the other two maps.

Elevations for these species are given in the plots at the bottom of this page.

In the above maps, I have separately plotted specimens of O. parryi with few- to moderately-spined fruit under the label of few-spined fr, and specimens with moderately- to densely-spined fruit under spiny fr. Note that specimens with moderately-spined fruit are in both categories. (I define few-spined fruit as having roughly 0-10 spines per fruit; moderately-spined fruit as having roughly 30-50 spines per fruit; and densely-spined fruit as having roughly over 100 spines per fruit.)

This separation shows the areas where specimens of O. parryi might be incorrectly determined in vouchers due to the presence of densely-spiny fruit. See Opuntia echinocarpa, O. ganderi, O. parryi, and O. wolfii: Pictorial Identification Guide for a discussion of the problems in determining specimens of O. parryi using floras.

There is no evidence that these are separate taxa. Many identical moderately-spined fruit are found under both labels, and there seems to be no consistent difference in other characteristics between plants with few-spined or densely-spiny fruit other than the following tendencies. O. parryi populations in wetter locations, which are usually coastal and montane, typically have fewer spines on their stems and fruit, as well as longer tubercles. O. parryi populations in drier locations have more spines probably simply because better-defended plants survive better in those locations where there are fewer food sources for animals.

Since many of the plotted locations are from drive-by botany, some cholla specimens in areas of transition between species remain to be determined. Also, some specimens of O. parryi did not have fruit present, and so could not be categorized on how spiny the fruit are. These incompletely-determined species are plotted separately as unknown determinations just to show where chollas of one of these species are present.

In order to avoid hopelessly cluttering the plot even worse than it already is with yet more symbols, I have not distinguished voucher locations from unvouchered personal observations. But it is easy to tell the difference in general. Voucher locations are few, and scattered. My observations are along roads and trails, and are generally dense in the areas surveyed. Locations without points are nearly all unsurveyed; the absence of symbols does not necessarily imply absence of any of these taxa! I will add a separate map showing surveyed areas in the future.

In particular, I have not yet done a GPS survey of S2 below Agua Caliente County Park, so it contains only scattered locations from my observations.

I have not plotted voucher locations of O. parryi, since I don't know whether those specimens have few-spined or densely-spined fruit and I am in the process of surveying most of the voucher locations.

Discussion of Location by Species

O. ganderi is by far the most common cholla species of these four in its range from southwest Riverside County to Baja California, concentrated on the desert slopes of the mountains at elevations of ~1000 to ~4000 feet. It is the most common cholla species west of Borrego Springs, and is often found in dense stands.

Examples of dense stands:

The map above significantly underestimates the abundance of O. ganderi. Within its limits determined by climate, primarily total rainfall, O. ganderi doesn't much care where it lives in terms of distance from drainages, slope aspect, presence of boulders or not, or soil type. Hence to a high approximation one can mentally shade in all the areas between populations with O. ganderi symbols. I.e., it has an almost-continuous distribution in latitude, inhabiting a distinct range of longitude.

O. parryi requires more moisture and a kinder, gentler climate than O. ganderi. In areas with both O. parryi and O. ganderi, O. parryi is in general found just inland from O. ganderi. Unlike O. ganderi, O. parryi cares very much exactly where it lives. If you don't see a nearby drainage, you won't find O. parryi.

O. parryi performs a delicate duel with chaparral in competing for locations. O. parryi cannot survive competition from climax chaparral which shades it out. It cannot survive frequent fire either, another weapon wielded by dense stands of chaparral.

Hence most of the population of O. parryi is found on the edge of the desert, in locations too dry to support dense stands of chaparral, but too moist to support most true desert species.

In the desert portion of San Diego County, O. parryi is found only in the San Felipe Valley and Earthquake (Shelter) Valley. It is the only cholla species in its range here except for contact with O. ganderi at the southern end of Earthquake Valley.

The "dry but not quite desert" portion of San Diego and Riverside Counties contains two major populations. In the north, the San Felipe Valley population extends along the upper San Luis Rey River and Temecula River drainages inland to Aguanga in the rain shadow of the Palomar Mountains. In the dry southern part of San Diego County a population extends along the Campo Creek drainage and uppermost Carizzo Creek drainage from near Campo to about Boulevard, where it then makes contact with O. ganderi and O. wolfii to the south. This population is in the rain shadow of the Otay - Tecate - Baja California Mountains.

A similar extensive population in a "dry but not quite desert" in Los Angeles County is found along the Santa Clara River in the rain shadow of the San Gabriel / Santa Susana Mountains.

Since O. parryi can thrive in moister locations, it is also found on the coastal side of the mountains, but those populations are not extensive since it has to find small niches that don't burn frequently and aren't densely covered with chaparral. Those niches are somewhat-disturbed areas that don't burn often, like benches in floodplains, or steep dry south-facing slopes above floodplains protected from burning by local topography and sometimes being too hot to produce large chaparral plants.

As a result, O. parryi is only sparsely found in coastal San Diego County. (I don't include var. serpentina in this article, but it is found in a similar habitat only near the coast in the dry extreme southwest San Diego County.) I have now extensively surveyed San Diego County along the major roads, and there are no voucher locations along roads that have not been surveyed. Hence this is a pretty complete mapping of O. parryi in San Diego County.

Populations on the coastal side of the San Gabriel Mountains are also sparse, and are probably declining due to habitat destruction, river damming, and increased fire frequency. For example, Dawson (1966) said that O. parryi occurs more or less commonly .. in Cajon Pass where it may be observed ... along Highway 395. O. parryi is no longer common there.

Jane Strong, Dick Swinney and I surveyed the remaining portion of SR395 in Cajon Pass on 9 February 2007. Although much of SR395 has been turned into I-15, the Highway still traverses the most likely habitat for this species along Cajon Creek. We observed only three total locations there with plants on an undisturbed alluvial bench. The O. parryi habitat on another section of alluvial bench has been destroyed by "development".

Furthermore, no plants were present at or near the location of a 1993 Swinney voucher near SR138. Fires are very frequent in the Cajon Pass now, often beginning from cars, and O. parryi has probably been extirpated from that heavily-trafficked location by fire.

Other coastal areas of the San Gabriel Mountains and San Bernardino Mountains are no longer creating new habitat for O. parryi to colonize, due to channelization of rivers, "development", and upstream dams preventing new alluvial benches from appearing.

In contrast to O. ganderi, the map is pretty complete for populations of O. parryi in San Diego County. Since O. parryi is strongly associated with drainages, probably the only missing locations are those in drainages connecting to those shown on the map.

The densest stands of O. parryi I have surveyed so far are:

O. wolfii has the smallest range of any of these species, and is concentrated in the extreme southeast portion of San Diego County and adjacent Imperial County and Baja California, and O. ganderi is also largely excluded from its range. There are vouchers of O. wolfii from Sentenac Canyon and Box Canyon, but I have been unable to verify the presence of this species at those locations in the field and have thus not plotted the voucher locations until I can verify their determinations.

The densest stand of O. wolfii is at Mountain Springs in extreme southwestern Imperial County along I-8.

O. echinocarpa is found over large areas, befitting a taxon that ranges throughout the entire DSon, and is found only in the driest, most-desert-y locations in flattish areas such as alluvial slopes and the desert floor.

The coastal-side boundary of O. echinocarpa seems firmly correlated with the 6-12 inch rainfall contour, and not with elevation. Thus in the western Mojave Desert in Los Angeles County, O. echinocarpa is found at elevations below ~4000 feet, but in San Diego County it is found only at elevations below ~1000 feet.

Interestingly, O. echinocarpa also seems to avoid the driest portions of the California desert as well. I found essentially no specimens in the Salton Sea area of Imperial County below ~300 feet in a survey of the eastern sections of S22 and SR78, along with the connecting portion of Highway 86 shown on the map. Furthermore, the locations plotted in Benson (1969) all seem to be in moister areas of the desert, at the base of mountains or along drainages, avoiding large areas away from those features.

The densest concentrations I know about are:

The map significantly underestimates the population of O. echinocarpa since I have only surveyed a few areas in its range.

Just for fun, here are the number of voucher specimens and locations I've determined as of 3 March 2007, in declining order of my locations:

Species# Voucher and Benson Locations# Chester Locations
O. ganderi32421
O. parryi(not compiled)316
O. echinocarpa75218
O. wolfii27119

This table is only indicative of the relative abundance in areas I've sampled and should not be used to deduce relative abundance in different areas. Since I've been concentrating on O. parryi and O. ganderi areas, it is not surprising that they are the most abundant in my records so far.

Tour Guide To All Four Species Along S2 in San Diego County

The easiest way to see all four species is to drive the entire length of S2 beginning at its northern end at mile marker 0. Beginning in the north is highly recommended since it allows one to get very familiar first with the two easiest species to recognize: O. parryi, and then O. ganderi. Beginning in the north also has the advantage of beginning with typical specimens of O. parryi. O. parryi becomes spinier to the south, which makes it harder to distinguish from O. ganderi at a glance.

After gaining extensive experience with these two species, it is then easier to pick up the differences of O. wolfii and O. echinocarpa than if one tries to start with those species first.

Except for isolated specimens, the first chollas appear near mile marker 7, and near mile marker 8, just south of the town of San Felipe, you quickly encounter dense stands of O. parryi near its type locality. This is to a very high approximation the only species you see until you reach the boundary of Anza-Borrego Desert State Park on the south side of Earthquake (Shelter) Valley, when there is an abrupt transition to O. ganderi.

O. ganderi is then the only cholla species until the very-different-looking O. bigelovii and O. fosbergii make sudden successive appearances in Box Canyon southwest of Blair Valley.

O. ganderi essentially disappears, and O. wolfii makes its first appearance, at the Carrizo Badlands Overlook, where many specimens are found. The Overlook has a side road that parallels S2 where you can park and walk about without having nearby traffic.

Just to the east, at the Imperial County line, O. echinocarpa makes its first appearance, and both O. wolfii and O. echinocarpa line S2 the rest of the way to Ocotillo.

The densest concentration of O. wolfii, in a nearly pure stand, is just a short distance away on I-8 west of Ocotillo, between the freeway lanes at Mountain Springs. This is also the type locality.


Elevations for all species except O. wolfii are plotted in the next series of maps. (I don't have enough of my own data to make interesting plots for O. wolfii.) I plot both elevation vs. longitude, keeping longitude along the horizontal axis, and latitude vs. elevation, keeping latitude along the vertical axis.

I show first plots with all species in them, followed by plots by individual species. Note that the scale for plots of individual species is usually expanded over that of the plots with all species in them.

As mentioned above, note that although the elevation of O. echinocarpa is only below 4000 feet in the San Gabriel Mountains, and only below 1000 feet in San Diego County, all of these plants are found in locations drier than the rainfall less than ~6 inches per year contour.

O. parryi and O. ganderi apparently have no strong elevation preference. Each of these species only drops out when the rainfall falls below the tolerance of each species.

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Copyright © 2007-2008 by Tom Chester
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Last update: 4 February 2008 (plot of northern Anza-Borrego Desert State Park may be more recently updated)