How the Desert Got into California
CHUCKWALLA LAND: The Riddle of California’s Desert. David Rains Wallace. xxii + 255 pp. University of California Press, 2011. $27.50.
Chuckwalla Land, by David Rains Wallace, is an intriguing account of California’s deserts and the people who have explored, studied and theorized about them over the past two centuries. Those who want to learn more about these beautiful open spaces, their geologic and evolutionary history, and their plant and animal inhabitants will find this well-written book a useful guide.
Wallace eloquently describes stark landscapes, extreme environments and startlingly modified organisms. His prose conveys well the captivating beauty of the desert, the sense of isolation it can create, his initial impressions of its emptiness and his elation at soon discovering “the presence of so much interesting life in an apparently dead place.” In the course of a series of trips to the desert, he tells us, he realized that “although there have been many ideas on the subject—nobody really knows how the bushes and lizards got into the desert or how the desert itself got into California” and that nobody knows the exact age of the California desert. The book is about those enigmas, presenting a history of the various hypotheses scientists have come up with over the decades to explain when and how the desert originated and its inhabitants evolved.
Much of Chuckwalla Land consists of a vivid and detailed account of the ideas and arguments of paleobotanist Daniel Axelrod, botanist and geneticist G. Ledyard Stebbins, and other scientists, who spar verbally with one another in the course of the narrative. In the 1930s and 1940s Axelrod began to challenge the then-prevailing view that desert climates and floras were “earth old.” He thought that most desert species had evolved during the past three to five million years, and that these plants were not ancient invaders from the south, as some had theorized; rather, they were descended from the plants that had lived in the woodlands and savannas that had preceded the area that is now desert. In the early 1950s, Stebbins argued that aridity encouraged rapid selection, making the desert an evolutionary frontier that provided unusual opportunities for the expression of genetic variability. He believed that the rise of the California mountains, which formed desert by creating rain shadows, resulted in an environmental shift that caused evolution to accelerate. However, he thought that modern desert plants were descended from ancient xeric or semi-xeric ancestors that were present in the Middle Miocene and later times. By 1974 Stebbins had come to regard desert not as an evolutionary frontier, but as an evolutionary museum; he concluded that the constancy of the desert environment had slowed evolution there, allowing species to accumulate. This suggested that the desert was much older than Axelrod had proposed. Since the late 1990s, Axelrod’s basic premise has been called into question by papers estimating that California’s major mountains might have arisen more than 70 million years ago and may have been much higher than they are now.
That the origins of California’s desert have been controversial is understandable. Those origins are only vaguely visible through the depths of time. Recently derived lineages, evidence of rapid speciation, and unique morphological and physiological modifications have been interpreted by some as evidence of a recent origin. Current geologic evidence suggesting that California’s mountains, and the rain shadows they cast, may be much older than was previously thought has led others to conclude that the deserts are ancient. The relatively few fossils of distinct desert organisms, and the greater representation of shrubland and woodland species from which desert history must be inferred, have further confounded the issue. As more fossil evidence has been uncovered and geologic histories have been refined, it has become plausible to maintain that no desert—in the sense of the extensive, angiosperm-dominated ecosystems of the present—is ancient, and that deserts originated through the coalescence of the dry elements that were favored in the extremely arid areas that developed in the Middle Miocene and later times. Newly evolved species are now intermixed with the relatively few surviving plants and animals that were present in the Cretaceous and Paleogene. This mixture, in a geologic context still unsettled in its detail, is what has presented such a conundrum to those who have attempted to understand the mysteries of these arid regions.
Current thinking holds that the areas of California that are now desert were once occupied for the most part by shrublands or woodlands, and that the unique plants and animals that inhabit the desert today are derived from ancestors that were present in the woodlands and closed-canopy vegetation found in the area before the Middle Miocene Period some 15 million years ago. On a global scale, California’s deserts would be considered dry scrub: Annual precipitation in them is much higher than in, say, the Sahara, the Atacama or the Namib, and their cover of vegetation is much denser and more durable seasonally. The total biomass and species diversity is also much higher than in those much drier areas. California’s deserts are, as Wallace emphasizes, intermixed with woodland and scrub in border areas that receive higher levels of precipitation on certain slopes and exposures.
In a sense, the plants of the California deserts are an extreme expression of the vegetation that has survived increasing drought and seasonality over the past 15 million years. During that time climates became more diverse and drier, and a Mediterranean climate (dry in summer) eventually developed. Continental glaciations that spread from the north resulted in the appearance of cool currents that formed powerful gyres down the West Coast of North America. Westerly winds blowing over this cool water were depleted of moisture and arrived on land as desiccating air. Few species survived from earlier, moister periods, and those animals and plants that could do so underwent rapid evolution. The numerous closely related species found in the region today are the result of that evolution.
The plants and animals that survived were those with adaptations that particularly suited them to life in a dry and seasonal climate—and those that lived in places where there was in fact abundant water, even in a dry land. Water running down grooves in cliffs in desert regions all over the world provides locally wet habitats in dry regions, and unique plants and animals can often be found in them.
Reading the book, we were both reminded of the story of Senator Carl Hayden, the first senator from Arizona, who remarked in his maiden speech in the Senate, “Arizona would be paradise if only it had water”—only to hear a senator in the back of the room call out, “So would Hell, Senator, so would Hell.” The relentless “mining” of aquifers in the West is now limiting prospects that present levels of intensive human occupation in these regions will be possible in the future. Explorer John Wesley Powell’s predictions about the limited capacities of the western deserts are now coming back to haunt the inhabitants of the sprawling cities that have developed over the years in these regions.
Wallace has interwoven historical, biological, and environmental threads to produce a fascinating account of the interesting theories that have been propounded about the California desert. Readers will come away with a better understanding of the desert’s unique nature and of its geological and evolutionary past.
Alan Graham is Curator of Paleobotany and Palynology at the Missouri Botanical Garden in St. Louis. He is the author most recently of A Natural History of the New World: The Ecology and Evolution of Plants in the Americas (University of Chicago Press, 2010). Peter H. Raven, President Emeritus of the Missouri Botanical Garden, is George Engelmann Professor of Botany at Washington University in St. Louis and an adjunct professor of biology at the University of Missouri–St. Louis and at St. Louis University.