“The best journeys are the ones that answer questions that at the outset you never even thought to ask” — Rick Ridgeway

By DR. CAMERON BARROWS

In North America’s arid west, we recognize four to six major subdivisions of deserts, depending on who’s talking.  The Great Basin Desert sits at the northwestern corner of North America's arid zone. Cold winters and warm summers. Great Basin sagebrush, scattered junipers, sometimes pinyon pines, sage grouse, sage thrashers, sage sparrows, and common sagebrush lizards. Head south and you are in the Mojave Desert. Cool winters and hot summers. Like the Great Basin Desert, the Mojave Desert has patches of junipers and pinyon pines (although different species). Unlike the Great Basin Desert, here you find iconic Joshua trees, other Yucca and Nolina species, blackbrush, and creosote bushes, LeConte’s thrashers, Mojave Desert tortoises, and Mojave Desert fringe-toed lizards. Both the Great Basin and Mojave Deserts get most of their rain during the winter from storms originating in the Bering Sea.

At the southeastern edge of North America’s arid west is the Chihuahuan Desert. Warm year-round, limited to just summer rain from monsoons originating over the Gulf of Mexico. Trees are scarce, but cacti are abundant and diverse, with 1,500 cactus species found there – but not the giant columnar cacti of the Sonoran and Baja California Peninsular Deserts. Lechuguilla (Agave lechuguilla) and Sotol (Dasylirion leiophyllum) are endemic, and ocotillo and creosote bush are common throughout this desert. The Bolson tortoises (Gopherus flavomarginatus), two species of fringe-toed lizards (Uma paraphygas and U. exsul), and more than 30 species of Sceloporus lizards can be found within the boundaries of the Chihuahuan Desert (only one Sceloporus occurs within the Great Basin Desert, and just two within the Mojave Desert).

North and west of the Chihuahuan Desert lies the Sonoran Desert, with its characteristic columnar saguaro cacti, Carnegiea gigantea. The Sonoran Desert gets some winter rain and some from summer monsoons. Like the Chihuahuan Desert, ocotillos and creosote bush are common throughout. Although pale in comparison to the Chihuahuan Desert, the Sonoran Desert is also relatively cactus and Sceloporus lizard-rich, with up to 300 cacti and six Sceloporus lizard species found there. The saguaros provide homes for many species of birds and bats (long-nosed bats are the main pollinator of saguaro cacti). Elf owls, ferruginous pygmy owls, Gila woodpeckers, gilded flickers, and Harris’ hawks found within these cactus “forests.”

The remaining two deserts are sometimes lumped in with the Sonoran Desert, the Colorado and Baja California Deserts. The Baja California Peninsular Desert does have fairly high cactus species richness, about 60 species, along with columnar cacti, but only a few are the same species found in the Sonoran Desert. The Baja cactus giant, the cardon, (Pachycereus pringlei) is larger than saguaro but not closely related to any Sonoran species. While the Baja California, Colorado, Sonoran and Chihuahuan Deserts are all numerically dominated by creosote bushes and ocotillo, the Baja desert has a giant ocotillo relative, the boojum tree, Fouquieria columnaris, which rivals and exceeds the saguaro in height. The common English name for the boojum comes from Lewis Carroll’s story, “The Hunting of the Snark,” a fictitious account of exploration to far-away places in which the explorers encountered a mythical plant at the end of the earth called a boojum. When the well-read explorer Godfrey Sykes first saw Fouquieria columnaris in the wild, he said “it must be a boojum,” and the name stuck. No species of Sceloporus lizards occur in both the Sonoran Desert and Baja California.

By comparison, the Colorado Desert is depauperate in cactus with just 39 species, similar to the Great Basin Desert in numbers, but sharing none of the same species. Along with creosote bushes and ocotillo, the Sonoran, Baja, and Colorado Deserts also share ironwood (Olneya tesota) and paloverde, (Parkinsonia florida). The species of plants and lizards that occupy the Colorado Desert are a mix of species aligned with each of the North American deserts. Pinyons and Junipers and cacti like those in the Mojave and Great Basin Deserts; lizards, cacti, elephant trees, and palm trees akin to Baja California; Ironwood, ocotillo, and palo verde like the Sonoran and Chihuahuan Deserts. A mixing of floras and faunas, likely because it was at a crossroads of species moving north and south during the repeated wet and cold then warm and dry periods of the Pleistocene Epoch. The Colorado Desert is also the hottest and driest desert region of North America. Similarities to all, but unique in the mix of species found there.

Heading east on I-10, climbing up out of the Coachella Valley, you are in the heart of the Colorado Desert – or is it the western edge of the Sonoran Desert? Crossing the Colorado River, the first saguaro cactus can be seen at 1.5 miles into Arizona. At 8.5 miles foothill/little leaf palo verde (Parkinsonia microphylla) dot the hillsides and buckhorn cholla (Cylindropuntia acanthocarpa) are abundant on the alluvial fans. At the 40-mile signpost velvet mesquite (Prosopis velutina) borders the highway. At the 50-mile signpost, devil cholla, (Grusonia emoryi) form thorny patches between the creosote bushes. Devil cholla are very similar in appearance to the matted cholla (Grusonia parishii) we see on the Pine City Trail in Joshua tree, but clearly, according to the botanists, a very different plant. Incrementally, mile by mile, species not found in the Colorado Desert appear and the landscape becomes different. By the time we are north of Phoenix, chain-fruit cholla, Cylindropuntia fulgida, appears and the saguaro density looks like the tines on a hairbrush. Except for the creosote bushes and ocotillo, every plant is a different species than those occurring in the Colorado Desert. The question then is are the Sonoran and Colorado Deserts separate deserts or is it all one desert along a gradient of temperature and rainfall? If your answer is that it’s all the Sonoran Desert, then one could argue that the Mojave, Colorado, Sonoran, and Chihuahuan Deserts are just a single desert, with gradual changes along climate gradients. If the answer is that they are different, the Colorado Desert being distinct from the Sonoran Desert, then where along the I-10 “transect” did they become separate?  Was it with that first saguaro cactus at 1.5 miles into Arizona? Or was it closer to Phoenix when the species differences exceeded the similarities?

One answer might be that desert boundaries are simply human constructs, meaningless to the past, current, and future of the plants and animals that have found ways of thriving in some of the harshest landscapes on our planet. Those plants and animals, and bacteria and fungi, interact with each other, and over long enough spans of time and changing climates, those interactions change. What we call those associations, those interactions at any point in time, is less vital than ensuring that those species are given a landscape free of boundaries so they can change when necessary. The opposing argument might be that by defining deserts as separate units, we can then better understand species interactions and changes, by giving them a structure, whether the associated species see it that way or not. Nobody should ever say that life, nature, and all that it includes, was simple.

Nullius in verba – Go outside, tip your hat to a chuckwalla (and a cactus), and think like a mountain