“The desert tells a different story every time one ventures on it.” — Robert Edison Fulton, Jr

Areas with high biodiversity, many unique species within a given geographic area, must have at least two ingredients, high primary productivity and isolation.

High primary productivity requires warm temperatures and sufficient moisture to foster plant growth. Both the Colorado and Mojave Deserts have plenty of warm temperatures, but by definition, moisture is in short supply. Except when it isn’t. Sometimes the ephemeral cool season rains fall in abundance creating an explosion of wildflowers along with their attendant insect pollinators. More dependable water occurs where groundwater wells up to the surface along earthquake faults and on high elevation desert mountains or sky islands, which create their own wetter weather. Sand dunes are also wetter than the surrounding desert landscapes.

Sand dunes might seem the most surprising and out of place on a list of “wetter” desert habitats. After all, sand dunes are the iconic epitome of aridity. However, the grains of sand that comprise sand dunes capture rainwater and can hold on to that water below the dune surface for months or even years. Water flows through coarser desert sands, flowing via gravity to deep aquifers, often too deep for the roots of many plants to reach. Finer desert soils are composed of more clay and are too fine, the between-grain air spaces are too small for the water to move through, and/or the clay particles absorb the water and expand, plugging any air spaces that exist. Sand dune sands are like the Goldilocks fable, not too coarse and not too fine. The clay particles have all blown away in the wind, leaving the sands just the right size for moving along the surface of a dune during strong wind events. And for allowing the rainwater to move into the inter-grain air spaces and remain there within reach of lizards, insects, and plant roots.

The challenge for life on a sand dune is not that it is too dry, the challenge is that the sand is constantly moving, like a river in slow motion. That surface dynamic means that only those plants and animals who have evolved coping mechanisms for summer heat and a flowing surface can live there. Those adaptations are of little use away from a sand dune habitat, and so can restrict species to the dunes and only there. Evolving for life on a sand dune therefore often results in species being completely restricted to a single dune. It is as if the dune is an isolated island of habitat surrounded by an ocean of unsuitable habitat. The result is that individual dunes support species found nowhere else on earth. Because of few competitors, the abundance of those dune-restricted species can be much higher than similar species living and competing with many more species off the dune. Animals that have become tied to sand dune habitats include little pocket mice, desert kangaroo rats, dune cockroaches, many species of darkling beetles, sand-treader crickets, and two lizards, flat-tailed horned lizards and fringe-toed lizards.

Fringe-toed lizards are related to zebra-tailed lizards, but to live on a sand dune fringe-toed lizards evolved a suit of morphological features that make them superably adapted to what otherwise seems to be such an inhospitable habitat. Elongated scales on their feet act like “snowshoes” providing increased surface area and traction, making them the fastest animal on the dune. Their nose is just the right size and shape to allow them to bury in the sand and to breath the air trapped between the sand grains without inhaling any grains of sand into their lungs. Their lower jaw fits up inside the upper jaw, allowing them to dive headfirst into the sand without the sand forcing their mouth to open. Their skin matches the color of the sand they live on. California desert sand dunes are mostly comprised of sands eroded by the Colorado River as it cut through what is now the Grand Canyon. Like the Grand Canyon, the sands are shades of brown, tan and sometimes a rusty red. The lizards’ colors match those colors so closely that until they move, they are almost invisible to possible predators. One dune system in the Coachella Valley has a different sand source. There the sands are eroded from granitic rock below San Gorgonio Peak. The result is a pale gray, salt and pepper-hued sand dune, and fringe-toed lizards that perfectly match that background. There are at least six species of fringe-toed lizards, restricted to the sand dunes of the California deserts (three species) and the adjacent dunes of Arizona and Sonora Mexico (three other species). This dune-specific evolution is repeated in little pocket mice, plants, crickets, and beetles.

Most desert mammals are nocturnal and rarely seen, avoiding the heat of the day and by doing so conserving precious water. One of the attributes of sand dunes is that whatever walks on them, from ants to coyotes, leaves species-specific tracks in the fine sand. So even if we never see a desert kangaroo rat or little pocket mouse, we can see how they spent the night by following their tracks in the sand. During the day these mammals sleep in their burrows, within the “wet zone” of the dune sands and where the humidity is always 100%. Their diet is comprised of seeds, often those having blown onto the dunes. When these species digest dry seeds, essentially carbohydrates and protein, water is one of the metabolic products. Between living during the day in a 100% humidity burrow and creating metabolic water, these supremely adapted mammals thrive, never needing to drink liquid water.

This island-like isolation and evolution also occurs in the desert mountains (sky islands). There, during the Pleistocene ice ages, northern species moved south to escape frigid climates and walls of ice. Then during the interglacial warm periods those northern species moved back to the areas from whence they came, or they moved up into the higher elevations of the sky islands. With thousands and perhaps millions of years of isolation new species evolved. One is the southern sagebrush lizard, pushed south from the Great Basin Desert during the glacial periods, sagebrush lizards were then stranded on the multiple sky islands, isolated from their parent species, the common sagebrush lizard, which still is found in the Great Basin Desert. Other, more mobile sky island bird species didn’t evolve new species, but still reside in theses desert sky islands. These include Clark’s nutcrackers, Williamson’s sapsuckers, red-breasted nuthatches, fox sparrows, mountain chickadees, and spotted owls.

Lower on the slopes of those desert sky islands, from the alluvial fans up to about 3,000 feet, is the habitat of an iconic desert mammal, the bighorn sheep. Bighorn sheep have a social system, much like that of African elephants, where the core groups of females and lambs are led by a matriarch ewe. The matriarch knows where and when food can be found, where dependable water holes exist, and where there are safe refuges from danger, like mountain lions. The mature rams exist alone or in in bachelor groups, periodically coexisting with ewe groups to assess the breeding condition of those ewes. The males move from ewe group to ewe group, and occasionally move from sky island to sky island. So, despite the insular character of the sky islands and the tight association with particular ewe groups with portions of those mountain slopes, there has been no genetic isolation between sky islands because those males, at least occasionally, roam far and wide spreading their genes. The fragmenting of the California deserts with highways and human development has constrained those roaming males, meaning genetic isolation could be the rule in the future.

Bighorn sheep in the Santa Rosa and San Jacinto Mountains have recovered from near extinction resulting from an upper respiratory disease transmitted from domestic livestock. Now, as their population is again healthy new concerns have emerged due to climate change and the associated drying of critical waterholes and the lack of forage in the easternmost portions of those mountains. Unlike the kangaroo rat, these sheep are exposed to the heat of the day, and so depend on both succulent forage plants and waterholes to quench their thirst. As a result of our changing climate, many of those water holes have already dried up.

Recruitment by successful reproduction within that population favors those individuals that are better adapted to the unique conditions occurring where they live. Fringe-toed lizards that better match their dune’s color are better at avoiding predation and so have a higher rate of survival. If that population is isolated, if there is no genetic connectivity with other populations, eventually an isolated population evolves into a new, unique species. Without isolating mechanisms, species tend to be generalists, adapting to the broad set of conditions that occur within the entire range of that species. The extreme aridity of deserts is one isolating mechanism; non-desert species can rarely survive under such dry conditions, while at the same time the adaptations that allow desert species to thrive have no value on cooler-wetter landscapes. The within a desert, additional isolating mechanisms, such as sand dunes and sky islands, foster the evolution of new species. The result is much higher biodiversity than would be expected in what seems at first glimpse to be such a harsh, unforgiving climate. However, if equipped with the right set of adaptations, our deserts are not so unforgiving. Those desert lizards, beetles, and kangaroo rats thrive here, often at densities far above those of their cousins living in more moderate climates.

Nullius in verba

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