“The diversity of lifeforms, so numerous that we have yet to identify most of them, is the greatest wonder of this planet.” — E.O. Wilson

By CAMERON BARROWS

The origin and accumulation of the species that make up the warm North American deserts are processes that have been occurring for tens of millions of years. However, details in terms of where and when and how are still not well understood. Deserts are a poor landscape for creating a fossil record that could allow us to then “read and interpret” the past. Rather fossilization more readily occurs in regions with rivers and lakes that can then entomb leaves and bones in mud and silt. Not deserts. What fossil records we have, such as the “mother lode” of fossils excavated from the Anza Borrego Desert badlands, record a period in time when that region was not a desert at all. Those fossils reveal a landscape from 4 to2.6 million years ago (the Pliocene) and extending into perhaps tens of thousands of years ago (through the Pleistocene glacial periods), filled with large creatures, mammoths and camels, big predators, and a forest comprised of Washingtonia palms, oaks, ash, bay laurels, avocados, and walnut trees. Not a desert. No fossils of creosote bushes, cacti, ocotillo, or agaves. Today only the palms remain as a component of our current desert flora and as testaments to what once was. The descendants of many of the other trees still occur well above the desert floor in our local mountains.

Yet, when those bones, leaves, and tree trunks were being entombed in what are now the Borrego badlands, the current mountains and the rain shadow they create today were then rapidly rising if not already in place. The Colorado River was untethered by today’s concrete dams, and so would periodically send floodwaters across this landscape, providing ideal conditions for fossilization. Then, in a geologic “blink of an eye," today’s deserts formed. Not only an arid climate, but a spectacularly rich, biologically diverse landscape filled with thousands of plant species, dozens of reptiles, and an uncalculated multitude of insects, all uniquely adapted to thrive in our desert. Therein lies the mystery.

There are no fossils to guide explanations as to how our deserts came into being, apparently so quickly. All we have are hypotheses. One hypothesis is that concurrent with those oak and walnut and palm forests and savannas, there were isolated arid “islands” of proto-desert habitats, well away from lakes and floodplains and so where fossilization might never occur. This archipelago of arid islands might have occurred on south-facing, low elevation, leeward mountain slopes, providing conditions much like today’s deserts. On each arid “island” over tens of thousands and even millions of years, plants, insects, and reptiles would have had time to “experiment” with desert living, leaving only those descendants that had the genetic fortitude to survive and thrive within such an arid world. What is particularly attractive about this hypothesis is that each arid island would have had its own set of “experiments” resulting in new and different species. So when the palm-oak-walnut forest-savanna reign ended, each island’s species could have spread out and would account for the rich desert biodiversity we enjoy today, with many species with relatively narrow distributions.

Another hypothesis, not exclusive of the “arid island” hypothesis, was that during the Miocene, Pliocene, and Pleistocene there was a large desert developing in what is now central Mexico. Today we call it the Chihuahuan Desert. There, over those millennia, ocotillo, agave, yuccas, and cacti developed. Then, with the increasingly arid and warmer conditions developing to the north and west, those species were able to invade that landscape abandoned by the palm-oak-walnut forest-savanna. That expansion is well documented by use of pack rat midden analyses. Some pack rat middens have been in continual use by generation after generation of rodents, sometimes for tens of thousands of years. Each generation adds a new layer of collected debris, providing a legacy of the surrounding changing habitats. From carbon-14 dating analyses of each layer within a midden, ecologists have been able to document when creosote bushes (and other species) arrived. Under this hypothesis there would have been more of a wide-ranging wave of invading desert species, accounting for those broadly distributed desert plants such as creosote, palo verde, and ocotillo.

By combining both the arid island and Chihuahuan Desert expansion hypotheses we can account for both the more narrowly endemic desert species as well as the wide-ranging species. These combined hypotheses provide a plausible explanation for the rapid development of California’s and Arizona’s warm deserts and the overall patterns and rich biodiversity we see today. It also makes me ponder the labels we have attached to our deserts, the Chihuahuan, Sonoran, Colorado, Mojave, and Baja Peninsular Deserts. On the one hand they each are comprised of many species found only within their borders. On the other hand, they are all part of an integrated landscape with many species shared and providing that legacy of a common history.

Nullius in verba

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