"Lizards of every temper, style, and color dwell here, seemingly as happy and companionable as the birds and squirrels." — John Muir

By DR. CAMERON BARROWS

There are more species of lizards here than anywhere in the U.S., perhaps anywhere in North America. By here I mean the junction of northern Baja California, the southern Mojave Desert, and the edge of the Southern California Coastal plain, centered on the Coachella Valley, at the northwestern end of the Colorado Desert. At least 33 lizard species. Why so many? The answer is embedded in the happenstance and juxtaposition of so many different and variable climates, habitats, and geographic origins. Where most other more homogeneous regions of a similar size might have a single species of night lizard, collared lizard, fringe-toed lizard, or horned lizard, here we have four or five species of night lizards (possibly more will be found whenever a better genetic analysis is conducted), two collared lizards, four spiny or Sceloporus lizards, three fringe-toed lizards, and three horned lizard species. All within an hour or so road trip from the center of the Coachella Valley. That same geographic juxtaposition has created the canvas for a similar multitude of species for butterflies, native bees, darkling beetles, Jerusalem crickets, cacti, and more. For now, let’s focus on lizards.

There is an axiom in ecology taught in every beginning ecology course at every college and university around the world, that no two different species with similar lifestyles, diets, and temperature preferences, otherwise known as a niche, will occupy the same space at the same time. The reason given for this strict separation is competition. Over time one species will or should out-compete the other, pushing the less competitive species elsewhere or to extinction, so that the winner has exclusive access to limited resources, usually food. This axiom holds true much of the time for lizards, but not always. Up in our sky island mountains we can find three species of spiny or Sceloporus lizards, granite spiny, western fence, and southern sagebrush lizards sitting on the same boulder, sometimes at the same time. A fourth spiny lizard, the desert spiny lizard, is usually at lower elevations than the other three, although there is overlap at times between granite and desert spiny lizards (often in palm oases), and more often between western fence and desert spiny lizards. Perhaps they eat different insects, or perhaps they occupy different parts of the boulder, maybe at different times, based on temperature preferences? Or perhaps food and boulders are not limiting resources, and their respective populations are kept below levels that might use up those resources by something else, such as predators? Or perhaps those food resources fluctuate so greatly up and down within a season and from year to year, keeping the lizard populations far below maximums, below levels where competition might happen?

Our three species of horned (Phrynosoma – “toad-bodied”) lizards mostly adhered to the ecological axiom. Desert horned lizards and Blainville’s horned lizards both live in rocky and coarse sand habitats but separated by elevation, while flat-tailed horned lizards are restricted to fine aeolian sands and sometimes silty habitats. However, years ago, before much of the human transformation of our desert, there were areas where the rocky habitats mixed with those dominated by aeolian sands. At that time flat-tails and desert horned lizards could sometimes be found together. Over time, the desert horned lizards were the only ones still found in those areas where there was once niche overlap. Blainville’s horned lizards were typically found at desert elevations above 3,000 feet, and at lower elevations on the coastal plain west of our desert mountains. Those coastal plain habitats are largely gone, turned into the sprawling suburbia that is Southern California, so the mountains are increasingly their last refuge. As our climate has warmed over the past couple decades Blainville’s horned lizards have moved into higher elevations. They now are mostly found at or above 4,000 feet on the desert side of the mountains, up to about 4,500 feet. While there is still plenty of that elevation and higher available in the Santa Rosa and San Jacinto Mountains, the Joshua Tree National Park population is now pressing up against a habitat ceiling. The Joshua Tree desert horned lizards are also moving up, occupying habitats where decades ago only Blainville’s were previously found, and the Blainville’s appear to be getting squeezed out.

Flat-tailed horned lizards have the smallest range of any horned lizard species anywhere, and it is getting smaller. They once occupied most if not all the fine sand habitats of the Coachella Valley, Borrego Valley, Imperial Valley and over the international border toward Mexicali, Puerto Peñasco and Laguna Salada. Today, sprawling cities, suburban and agricultural development has left only a fraction of that habitat still suitable for flat-tails. In the Coachella Valley, their last refuge is on the Coachella Valley National Wildlife Refuge where they occupy at most 4-5 square miles, compared to the roughly 100 square miles or more that was available to them just a century before. Still, within the National Wildlife Refuge they seemed secure, at least until the last couple years. While fringe-toed lizards and desert iguanas and sand-treader crickets and Palm Springs pocket mice numbers within the Refuge continue to be strong, the flat-tails’ population appears to have crashed. What has changed, and why has the impact been only on flat-tails?

If it was a climate change-related impact, then I would expect the impact to be felt across more species, and it’s not. One thing that has changed is that ravens are now ever-present. Early desert naturalists rarely saw ravens, whereas today they are always here. They are exceedingly intelligent creatures and have learned to take advantage of the food waste and trash left by our species. I now see raven tracks in the sand walking back and forth, leaving little untrodden, day after day after day. If the ravens are preying on flat-tails, why aren’t the fringe-ted lizards and iguanas also being impacted? The answer could be in the species-specific predator responses. Fringe-toed lizards and desert iguanas run fast for long distances, and then either dive into the sand or into a burrow when they feel threatened. Flat-tails’ fear response, along with most if not all horned lizard species, is to first freeze and depend on their excellent cryptic, habitat-matching, coloration to avoid detection. Only when that does not work do they move, but they are not built for speed. Ravens may be smart enough to have figured out the flat-tails’ behavior. Compared to previous years, the very few remaining flat-tails appear exceedingly wary.  That new wariness along with very low numbers may, hopefully, eventually convince the ravens to move elsewhere and allow the flat-tails a chance to rebuild their population.

Where once you could identify each horned lizard species in the desert by their specific habitat, today they are moving or hiding. If you do see a horned lizard there are a few species-specific clues to check. Flat-tails are the only ones with a dark thin dorsal line. They also have long occipital horns that attach to the back of their skull, forming a V-shape. Desert horned lizards’ coloration is exceedingly variable, but they never have a dark dorsal line. Desert horned lizards’ shorter occipital horns also form a V-shape. Blainville’s horned lizards’ coloration is less variable but is still a good match to the color and texture of their habitat. Rather than their occipital horns forming a V, there is a space between the horns, making their attachment point more U-shaped. In the middle of the “U” there is a tiny rudimentary horn.

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