“Earth’s geology, like its life forms, constantly changes as time passes. Seas rise and fall, mountain ranges erode, and new mountains ascend. What was once was a verdant forest can be replaced by a desert ... Earth’s geology is magnificently restless, and that is the key to understanding the generation and regeneration of Earth’s vast biodiversity.” — John Kricher

And with that restless geology comes a similarly restless climate. Climates vary with elevation and latitude, and as plate tectonics push continents into different positions, change proximity oceans, and push up mountains creating different windward or leeward climates. There is also an extraterrestrial factor; the Earth’s orbit and tilt toward or away from the sun also varies on time scales that span tens of thousands of years, changing the intensities of where our sun heats our planet’s surface. Though at timescales that often challenge our full understanding, change, not stability, is the norm for Earth, and that change is a key to understanding patterns of biodiversity.

Over the past roughly 1-2 million years, the Earth has been in an era named the Pleistocene, and some might argue we are still in that era. The Pleistocene has been characterized by oscillating climate patterns resulting in ice sheets covering much of the northern hemisphere for thousands of years followed by relatively warm periods. Each complete climatic cycle (ice to warm) lasted some one hundred thousand years. There have been at least ten ice ages, followed by warm periods throughout the Pleistocene – so far. Those climate shifts catalyzed the evolution of a glorious assortment of mammals that were keenly adapted to those cold ice age conditions. A mere 20-30,000 years ago wooly mammoths, wooly rhinoceroses, giant sloths, giant bears, giant armadillos, giant lions, horses, camels, and of course many, many more species roamed those ice age steppes, including what is now the Colorado Desert. Direct evidence of their presence here is being regularly excavated from the badlands of the Anza Borrego Desert. Of course, the cold climate changed, became warmer again, and those species finely tuned to the cold are now gone. Many went extinct, but some, like horses, camels, and cheetahs, found refuge in Asia. The cheetahs eventually found their current refuge in Africa. Wooly mammoths are closely related to Indian elephants, so some of those mammoth genes are still alive and well in southeast Asia.

For those that went extinct, their finely tuned adaptations that allowed them to thrive during the ice ages did not serve them well in the newly re-warmed Earth. What is arguably more interesting is that every single plant and animal alive today in the northern hemisphere both survived the ice ages and thrived under the post-ice age warmer conditions. What characteristics dictated who survived and who perished? One is size. Big animals perished. Smaller creatures typically have shorter lives and breed at earlier ages. Frequent reproduction has a greater chance of yielding hopeful mutations that result in better survival in a changing world. Another characteristic, although more difficult to quantify, is adaptability. Animals and plants that exist across a variety of habitats probably have greater survivorship than those confined by narrow requirements as habitats change. Think side-blotched lizards. They occur from the lowest elevation deserts to the lower margins of conifer forests, from below sea level to over 6,000 feet in elevation. If the deserts get too hot, side-blotched lizards have plenty of cooler habitats where, as a species, they will survive. If the high elevations get too cold, again they have warm options that should ensure their survival.

Does having survived the recent ice ages guarantee species’ survival under the still warmer conditions predicted as a result of modern climate change? Being not too big (frequent early age reproduction) and adaptability will help. The problem in making such predictions is that during the Pleistocene the climate oscillated between cold and warm climates. Our current trajectory has us headed from a warm climate to one that is much warmer, and for currently arid landscapes, a much drier climate as well. So, the predicted future includes a climate none of the Earth’s inhabitants have seen before. As deserts and mountain slopes become increasingly and incrementally hotter and drier, species that have found suitable habitat in arid deserts as well as cooler-moister higher elevations, like side-blotched lizards, should have genes already adapted to hot-dry conditions, and so should be good bets for being survivors at those somewhat higher elevations. Our Community Scientists have already measured higher side-blotched lizard densities and higher reproduction rates at the middle to higher elevation ends of their current distribution, and very low densities at the lower elevations. On a recent survey on the Pioneertown Mountains Preserve, at elevations between 1,200-1,500 meters (4,000-5,000 feet) we found 259 side-blotched lizards, most of them hatched in 2022, in just a six-mile hike. Having small size, frequent early reproduction, a broad range of habitats they can occupy (adaptability) and an elevation gradient (a sky island) to exploit should be a recipe for survival for this species.

However, nature includes complex inter-species connections. Side-blotched lizards will only survive if they can find suitable food at those higher elevations, plants will only survive if their pollinators are also present. One climate scenario for our deserts and sky islands is that there will be less winter rain (almost a certainty), but there could be more summer rain. Clearly the side-blotched lizards living at 4,000-5,000 feet, based on their reproductive success, are finding plenty of food. Their abundance also favors the survival of lizard-eating species such as collared and leopard lizards.

Will plants that are finely tuned to flower after sufficient winter rain be able to shift their phenology to bloom after summer rains? And if they can, will there be pollinators available to ensure that they produce seeds for future generations? The summer of 2022 has included more than one drenching rainfall event, patchily distributed across our desert. Our Community Science excursions are finding that in those areas that have received enough rain, some plants that are blooming as if it is spring, and importantly those flowers are being “serviced” by lots of hummingbirds, native bees, flies, and butterflies. Many of these blooming plants are well-known for being summer bloomers. They include many species of buckwheat (Eriogonum spp.) and species of rabbitbrush (Ericameria spp.). However, there are also those that are better known as spring bloomers: desert sunflower (Bahiopsis parishii), Acton’s brittlebush (Encelia actonii), and desert mallow (Sphaeralcea ambigua), together they are turning hillsides yellow and orange when in most other years those same hillsides in October are a crispy brown. Clouds of butterflies and bees are attending these flowers, offering an optimistic harbinger of what a future might look like.

What were mostly missing are the annual wildflowers. All those mentioned above are perennial plants with deep roots and a long-term strategy for both surviving drought, and apparently an opportunistic strategy for responding quickly, taking advantage of rain whenever it arrives. Conversely, annual wildflowers exist as a seed bank in the soil for most of their existence, waiting for just the right conditions – enough rain and warm temperatures - to quickly sprout, grow, flower, and go to seed. Waiting for just the right conditions might entail a very long wait for annual wildflowers, whereas the more opportunistic strategies of many perennial flowering plants may be the more fruitful approach.

No one has a perfect crystal ball for predicting how desert biodiversity will respond to a warmer and drier desert. Clinging to some good news, as evidenced by some lizards and some plants, is the best we have for now.

Nullius in verba

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