“This [biodiversity] is the assembly of life that took a billion years to evolve. It has eaten the storms-folded them into its genes-and created the world that created us. It holds the world steady.” — E.O. Wilson

Describing and understanding patterns of biodiversity is the avocation of naturalists. Animals and plants are not evenly distributed across landscapes, rather they are clumped at scales ranging from individuals, populations, species, and whole communities of organisms. Why clumped and why not evenly distributed is due to the same variables driving those patterns for you and me, the availability of food, water, thermal protection, and a safe haven from things and individuals that might cause you or your offspring harm, including predators, species that might envision you on their dinner menu. Where these variables are met, populations thrive. However, conditions can change over time, sometimes abruptly like when a hurricane like Hilary arrives at your doorstep. Other times these changes are incremental, such as when the climate gets warmer and drier.

This past week I helped survey fringe-toed lizards on the Coachella Valley National Wildlife Refuge. Of the remaining fragments of that original 100 square mile “sea of sand dunes” that once dominated the floor of the Coachella Valley, the Refuge protects by far the largest fragment left, but it also has the lowest mean rainfall and the highest average temperatures. Whereas other lizard species’ populations can and do incrementally shift toward higher elevations and north-facing slopes in response to a warmer-drier climate, fringe-toed lizards are so tied to living on sand dunes, with exquisite adaptations allowing them to thrive on what seems to be such a foreboding landscape, they cannot live elsewhere. Their existence is linked to the existence of healthy sand dunes. Because of its aridity and hotter temperatures, if a changing climate is going to negatively impact this lizard, the Refuge is where that impact will likely be felt first.

The frequency and severity of hurricanes in the Coachella Valley and elsewhere is likely a reflection of climate change; as I walked out onto the Refuge’s sand dunes the impacts of this summer’s tropical storm Hilary on these sand dunes were readily apparent. Where fringe-toed and flat-tailed horned lizards had occurred last spring, now there were basins of dried mud with the density of concrete, fractured into countless geometric patterns identifying where lakes, ponds, and temporary streams had existed just a month or two before. Nothing lived on these sunbaked expanses of dried mud, although the imprints of coyotes and horned larks indicated where some had explored the mud flats while the mud was still damp. Fortunately for the sand dune lizards and beetles and crickets and ants, sand dunes still exist where they were spared the water and mud flows, where the dunes were tall enough to stand above those flows.

Nevertheless, on those remaining sand dunes, the density of fringe-toed lizards was patchy, and generally much lower than it had been just last spring. There were, however, a few areas where the lizards were abundant. Pondering these patterns, I noticed that where the adult and hatchling lizards were abundant coincided with healthy stands of four-winged salt bushes, Atriplex canescens. Elsewhere, where the lizards were now sparse and there was little or no evidence of hatchling fringe-toed lizards, there were only skeletons of now dead saltbushes and creosote bushes. The on-going drought and weeks of 120˚F+ temperatures this past summer appeared to have taken their toll on some of these native shrubs, but not everywhere.

The link between high lizard densities and ample successful reproduction, with healthy saltbushes is easy to understand. The saltbushes’ branches and foliage are typically so dense as to repel most predators, certainly coyotes, shrikes and roadrunners, and at the same time buffering the lizards from incoming solar radiation, creating a cooler microclimate. Both lizards and insects (including lizard food) thus thrive below these shrubs. What is more challenging to figure out is why so many shrubs died this summer, while some appeared to do well. The obvious answer is that the surviving and thriving shrubs had better access to groundwater. Assuming that over such a relatively small areas as the Refuge that rainfall amounts are more or less homogeneous, having better groundwater access may reflect the below ground structure of the dunes, the layering of sand and finer clay particles.

Over the next months and years and decades those clay-lined basins left by Hilary will fill with blowing sand. The surface evidence of Hilary’s power will disappear. Since clay, compared to sand, is relatively impervious to water movement, I wonder if those clay-lined basins will then support perched water tables, providing easier access to abundant water for future saltbushes, and in doing so providing a thermal and food refugia for future generations of lizards. Maybe those patches of thriving saltbushes (and lizards) I saw on the dunes are the result of past flooding, past clay-lined basins from previous hurricanes. Maybe hurricanes and their lasting impacts on the sand dune landscape will be one way the amazing biodiversity we can find on desert sand dunes can survive.

Nullius in verba       

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