“Nature is not more complicated than you think, it is more complicated than you CAN think” ~Frank Edwin Egler

Nevertheless, a goal of conservation-minded naturalists is to document and understand nature, at least enough to be able to identify patterns and drivers of biodiversity and abundance, and ideally to then be able to identify problems or threats that might challenge species’ ability to sustain their populations. Once we know the threats, we then have an option to mitigate or control those threats through positive intervention. 

Despite perceptions of a vast wasteland, deserts can be deceptively complex and rich in biodiversity. While deserts lack much of the vertical layering that forests have, they make up for it in creating temporal niches, depending on species tolerances to temperature and aridity extremes, different species can occupy the same ground but at different seasons. Taking this idea further, desert sand dunes have even less vertical structure (fewer shrubs and trees) but far from being inert piles of sand, dunes often have higher levels of biodiversity (especially darkling beetles and other arthropods) than the surrounding desert, or even more than most forests and shrublands for that matter. In addition to diversity, desert dunes can support species of lizards, snakes, and arthropods at much higher abundances than the surrounding desert. To be clear, dune-dwelling reptiles and arthropods are often different species than those occurring on lands surrounding the dunes. The characteristics of dunes, low vegetation cover, uniform fine-textured uncompacted sands, and mobile sand surface, make sand dunes such a different habitat, requiring unique adaptations for plants and animals to occupy and exploit the dunes, that compared to the adjacent desert a different species mix lives there. One could imagine the dune to be much like an island habitat surrounded by a “desert sea”, and like many oceanic islands, the isolation and different habitat characteristics foster the evolution of new species. Every desert dune habitat in the California desert is occupied by at least one or more species of arthropod, lizard, and/or plant that is found nowhere else on earth. The bigger the dune system, the more unique species that are found there.

Many of California’s desert dunes owe their existence to sand eroded from sandstones by the Colorado River and its tributaries. During flood events, the river, heavily laden with suspended sand grains eroded out of what would become the Grand Canyon as well as other red rock formations in northern Arizona and southern Utah, would spread those sands across a floodplain well beyond the normal riverbanks. Once the floodwaters receded back into the river channel and the floodplain dried, prevailing winds would push the sands into dunes. The sand dunes of the Coachella Valley have a different origin. Those sands eroded out of granitic formations in the Sand Bernardino and Little San Bernardino Mountains, again during flood events, often spawned by hurricanes that wandered up the Gulf of California and then stalled in the basin bounded by the high mountains that surround the Coachella Valley. The resulting sands are a mix of salt and pepper colors derived from the quartz, feldspar, and mica that comprise granite. Most of the other California desert sand dunes are shades of tan and pink, comprised almost solely of quartz grains eroded out of the various red rock formations along the middle to upper reached of the course of the Colorado River. Those different colors are copied by the various species of fringe-toed lizards occupying those dunes, the color matching allowing them to blend into their sandy habitat and avoid detection by predators.

The Coachella Valley sand dunes once covered over 100 square miles, but today due to railroads, freeways, golf courses, and residential development, total just five square miles, fragmented into several small sand islands. Like sand dunes elsewhere, the Coachella Valley dunes provide habitat for a lizard, a plant, a cricket, a beetle, and other arthropods found nowhere else. A multi species, multi habitat conservation plan is in place to do its best to ensure the survival of these endemic species. However, the challenges are many. Sand source corridors are blocked, invasive plant species cover the less active portions of the dunes, off-road vehicle trespass, and climate change threaten these species. To apply appropriate management actions that might reduce these stressors, we need to how and where the threats are most acute. 

To that end, we have a rich data set that extends back two decades that includes metrics for the abundances of the endemic species, as well as for the stressors. In deserts, rainfall equates to life and at least for the endemic Coachella Valley fringe-toed lizard rainfall is a good barometer for predicting the shifts in abundance in the lizard population. Rain sustains and germinates plants, plants feed insects, and insects and plants feed fringe-toed lizards. We can then measure to what extent a reduced sand supply or an invasive species such as Sahara mustard disrupts that chain, and when appropriate send out volunteers to reduce the mustard population or perhaps truck in more sand. In a severe drought like the one we are now experiencing, no mustard germinated, but neither did the native plants. Nevertheless, the lizards on the wetter western dune fragments still look plump and healthy. However, those lizards on the drier eastern sites are thin and are not growing. This may be a harbinger of the future if climate change continues, with increased drought and aridity becoming the norm rather than just a periodic weather event. At least for now those western dune fragments appear to be wet enough, even in a drought, to sustain the fringe-toed lizard populations occurring there.

If rainfall provides such a good means of creating expectations for changing abundances of fringe-toed lizards, then it should work just the same for other lizards. Except it does not. Another lizard species we follow is the flat-tailed horned lizard. This species prefers somewhat more stabilized dunes than the fringe-toed lizard and as such is not quite as limited in their distribution. Flat-tails extend into Anza Borrego, into the few remaining natural habitats in Imperial County, and into the Gran Desierto in northwest Sonora, Mexico. They reach their northern most range in the Coachella Valley and are one of the species protected under the conservation plan that also covers the fringe-toed lizard. Flat-tails mostly eat ants, which eat seeds, which are the products of plants growing in wetter years. Rainfall levels should predict seed abundance and seed abundance should predict ant densities and ant densities should predict lizard abundance, but the data do not support that logic. For a few years rainfall and ants and the horned lizard population may respond in lockstep, but then it becomes asynchronous for several years and then unpredictably might become synchronous again. One problem may be that there might be so many seeds after a wet year that the ants are able to thrive for many years, even if a drought cycle returns. Still, we try to measure the ants directly and the ant-lizard pattern remains seemingly random. Part of the problem could be that our pitfall-ant sampling is inadequate. One clue may be that the while horned lizard and fringe-toed lizard abundances are not synchronous, the horned lizard and shovel-nosed snake populations are. Shovel-nosed snakes are not known to specialize on ants like the flat-tails do, but they do specialize on beetles and beetle larvae, and they and the ants may be tracking food abundance and availability similarly. Maybe, but after two decades it still is more complicated than we CAN think.

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