"Intergradation" between Opuntia parryi and O. ganderi

Specimens of O. parryi (=Cylindropuntia californica var. parkeri) are usually easily separated from specimens of O. ganderi (=Cylindropuntia ganderi); see Opuntia echinocarpa, O. ganderi, O. parryi, and O. wolfii: Locations and Pictorial Identification Guide which also gives additional synonyms for the scientific names. However, many specimens in San Felipe Valley and Earthquake (Shelter) Valley are not easily determined at a glance, and appear as if they were hybrids between these two species.

In fact, plants in this area and northward for over 20 miles have been considered to be hybrids between O. parryi and O. ganderi due to this visual appearance. The Flora of North America says:

In the north part of its range, Cylindropuntia ganderi intergrades with C. californica var. parkeri.

Jon Rebman, in his 2005 article Gander's Cholla: Our Common But Barely Known Cactus, in the Anza-Borrego Desert Natural History Association's The Sand Paper, 34, Issue 2, p. 3, says:

It is also suspected that some populations in the USA and northern Baja California, especially those in San Felipe Valley north of Scissors Crossing, have been introgressed with C. californica var. parkeri because of the presence of a stouter central spine and smaller flowers and fruits. Individuals in these intermediate populations also usually exhibit a less strict, ascending habit.

The major characteristic that causes trouble in determining these plants is actually the presence of densely-spiny, bur-like fruit. This has been thought to be a characteristic only of O. ganderi, and not of O. parryi. For example, the Flora of North America separates these two species as follows:

3    Spines 0-few per fruit (deciduous glochids may also be present); fruits not burlike ... (leads to O. parryi)
+   Spines many per fruit; fruits sometimes burlike ... (leads to
O. ganderi)

However, many populations of O. parryi, and possibly the majority of all O. parryi plants in southern California, in fact have moderately- to densely-spiny fruit, and the densely-spiny fruit is almost always burlike. (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.)

In particular, all O. parryi specimens in the San Felipe and Earthquake Valleys have burlike, densely-spiny fruit, and some of these have been mistakenly called O. ganderi. For example, Benson, in his 1969 book The Native Cacti of California, plots two locations of O. parryi near Scissors Crossing (POM288520 (Benson 15773); 2 miles east of Banner; POM311542 (Benson 16385); 1 mile west of Scissors Crossing) and one location of "O. ganderi" from 1.5 miles north of Scissors Crossing (POM305021 (Benson 16146).

O. parryi populations near Aguanga, 23 miles north of San Felipe, have moderately-spiny to densely-spiny fruit, and some of those fruit are also burlike. Benson plots six locations from Aguanga to San Felipe for O. parryi, so he considered nearly all of these plants O. parryi. However, he also plotted one location four miles north of Aguanga for "O. ganderi" (POM305018 (Benson 16141)), over 20 miles away from the nearest actual O. ganderi.

My fieldwork and analysis has clearly shown that all "O. ganderi" specimens from Aguanga and San Felipe Valley are actually O. parryi. See the transition between Opuntia parryi and O. ganderi along Highway S2 in San Diego County for the strong evidence that all specimens in the San Felipe Valley are O. parryi. Many specimens from Aguanga are also in the plot of measurements of tubercle length and maximum stem length, and they are also consistent only with a determination of O. parryi. See also the plot of locations for each of these species in southern California determined from my fieldwork to date: all of southern California and detailed view of San Diego County. In the plots, Scissors Crossing and the San Felipe Valley are where the two species meet in roughly the middle of the San Diego County plot; Aguanga is at the bottom of SR371 in Riverside County, at the top of the San Diego County plot.

Densely-spiny stems and densely-spiny fruit are not evidence of intergradation with O. ganderi, even though that idea seems perfectly reasonable and "obvious" if you believe that "normal" O. parryi are sparsely spiny. Specimens along the Santa Clara River in the northwestern San Gabriel Mountains exhibit both of those characteristics, yet are almost exactly 100 miles away from the closest O. ganderi. Four specimens of O. parryi from that population have also been incorrectly determined as "O. ganderi" (the actual name on the vouchers is O. acanthocarpa, since O. ganderi used to be called O. acanthocarpa var. ganderi).

In fact, 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.

The idea that O. parryi usually has few-spined fruit apparently originated from over-emphasis on the relatively-small coastal population of O. parryi, which typically does have fruits that are markedly non-spiny. Pictures in Benson (1971), of O. parryi show only plants from coastal areas (Fig. 18, p. 87; and Plate 1), which have fruit that look nothing like the densely-spiny, bur-like fruit of O. echinocarpa (Fig 21, p. 92). That densely-spiny, bur-like fruit is identical to the fruit of specimens of O. parryi closer to both the Mojave and Sonoran Deserts.

This idea was probably perpetuated by a misinterpretation of brief key statements, such as this one from Jepson (1925):

Fruit-spines stoutish, in bundles of 8-12; deserts ... O. acanthocarpa
Fruit-spines acicular, solitary or few; interior ...
O. parryi

In the description, Jepson makes it clear that O. parryi has fruit ... with 3 or 4 rows of prominent tubercles, the upper ones each bearing a single acicular spine. Since there are typically six tubercles per row, this would make a total of 12-18 spines per fruit. However, a quick reading of the key alone might lead one to believe there are only a few spines per fruit, especially if one has seen only the coastal populations that do have at best a few spines per fruit.

The actual range I've found from extensive surveys of O. parryi is zero spines per fruit to well over 100 spines per fruit. There is no clear separation of populations with few spines per fruit from the populations with many spines per fruit; only the general trend mentioned above for the number of spines per fruit to increase in drier locations.

Of all the major floras, only the Jepson Manual treatment by Parfitt and Baker correctly described O. parryi as having spine number variable, and keyed it under both Fr densely spiny and Fr spineless.

Thus it appears that the evidence for intergradation near Aguanga and the northern San Felipe Valley rests primarily on the incorrectly-determined Benson vouchers of O. ganderi, along with the erroneous beliefs that the fruits of O. parryi cannot be densely-spiny and burlike, and that the stems of O. parryi cannot be spinier than typical coastal specimens.

As far as I can tell, there actually is no major intergradation between O. parryi and O. ganderi in the north part of the range of O. ganderi. There is an extensive population of O. ganderi in the northernmost part of its range in the Santa Rosa Mountains, and in the eastern slope of the San Ysidro and Hot Springs mountains west of Borrego Springs, and not one of these plants shows any similarity to O. parryi.

See also my detailed numeric study of the transition between these two species in Earthquake (Shelter) Valley along S2 in San Diego County.


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Copyright © 2007 by Tom Chester
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Last update: 28 February 2007