“A million years is a short time — the shortest worth messing with for most problems. You begin tuning your mind to a time scale that is the planet's time scale. For me, it is almost unconscious now and is a kind of companionship with the earth.” — John McPhee

For many or us, perhaps most of us, it is hard to imagine landscapes where the mountains, sand dunes, and rivers, sheathed in the plants and animals adapted to those landscapes, have not always been where they are today. Yet the only thing constant about these landscapes is change.

Think a mere 200 years ago, when there were grizzly bears across California — the largest grizzlies south of Alaska. Such a large omnivore must have had an impact on all plants and animals within its domain. Yet people were here and coexisted with these enormous bears, sharing the landscape. Once and a while I hear of someone proposing that we re-wild California by bringing back the grizzlies. The quick counter argument is that there would be too much risk for people today to consider such a reintroduction. Yet California’s indigenous tribes coexisted with grizzlies for thousands of years. People today, indigenous and more recent arrivals, coexist with grizzlies today in Alaska and Canada. 

Or thinking a bit deeper in time, just 20,000-50,000 years ago when there were relatives of elephants, camels, horses, cheetahs, ground sloths, and saber-toothed lions occupying what are now the southern California deserts. Of course, back then much of what we now call the Colorado Desert was covered in what was more akin to what today we might find at the higher elevation fringes of the Mojave Desert. Joshua trees, junipers, and live oaks.

North America during that time has been described as having been populated by so many different large mammals that it must have rivaled what the Serengeti in Africa is today. The first people arriving on this continent would have witnessed and been in awe of that multitude of life. That was a time of plenty and was just one pulse in a series of pulses that would have occurred for a million years or more, the time we now call the Pleistocene, the time of glaciers. It was a time of an ebb and flow of hotter-drier conditions with wetter-cooler ones. Hot dry deserts with cooler and wetter savannas and woodlands. 50,000 years ago was at or near one of many glacial maxima that spanned the Pleistocene.

About every 100,000 years or so there has been a glacial maximum, interspersed with glacial minima. The cause of such dramatic climate swings is a repeating oscillation or wobble in the earth’s orbit of the sun. Variations in the Earth’s orbital movements affect how much solar radiation the Earth receives. These cyclical orbital movements, which are known as the Milankovitch cycles, cause variations of up to 25 percent in the amount of incoming insolation at Earth’s mid-latitudes.

Milankovitch cycles include the shape of Earth’s orbit, known as eccentricity; the angle Earth’s axis is tilted with respect to Earth’s orbital plane, known as obliquity; and the direction Earth’s axis of rotation is pointed, known as precession. When the Milankovitch cycle brought the northern hemisphere more toward the sun and got us just a wee bit closer to our star, it was a glacial minima, a period of warmer, drier conditions. When that wobble tilted us away from the sun, we had glacial maxima, as it was some 40,000-50,000 years ago, cooler and wetter. Then a massive ice sheet would cover what is now Canada and the northern tier and the United States. Animals and plants that could stay in well front of the ice sheet survived, those that could not perished. Areas south of the ice sheets became the new homes of species otherwise more adapted to cooler-wetter conditions. When the ice sheets retreated, as the earth headed into a glacial minima, those species then needed to move back north or perish as the south again returned to a warmer-drier climate. Unless there was a mountain to climb.

Sometimes, rather than retreating north with the onset of warmer-drier conditions, species climbed to higher elevations in mountains to find their preferred climates, otherwise known as a climate refugium. Over the many glacial minima-maxima cycles of the Pleistocene, desert mountains, what we sometimes call “sky islands," accumulated these northern waifs, giving the sky islands much higher levels of biodiversity than the surrounding landscapes. The sky islands need to be tall enough to provide the cool climates the more northern species require. When the surrounding desert reaches well above 110° F in the summer, desert mountains that reach above 6,000-10,000’ can still be in the more comfortable 70-80° F range. Northern facing slopes are still cooler and moister, and so are even better at providing climate refugia.

Locally, the Santa Rosa and San Jacinto Mountains are sky islands, and the taller, more westward San Jacinto Mountains are cooler and wetter. Just a few of the northern waifs that today call the San Jacinto Mountains home include southern sagebrush lizards, western burning bush, pacific dogwood, little prince’s pine, woodland pinedrops, limber pines and lodgepole pines, all species that otherwise have their closest relatives living in the Great Basin Desert, Sierra Nevada or Coast Ranges of California and further north. While we are currently in the midst of a glacial minima, modern anthropogenic climate change is creating warmer and drier conditions than these waifs have ever experienced. The climate-buffering effects of elevation and north-facing slopes are still providing habitats for these species, but for how long?

Going back still deeper in time, 5-20 million years ago, during the Pliocene and Miocene eras, there was a migration into our region from the opposite direction, from the tropical south. Then the Sierra Nevada and Peninsular Mountain ranges reached their present elevations, and (about 5 million years ago) the Colorado Plateau uplifted sufficiently to send the Colorado River westward and southward. As the river cut through the sandstones of the plateau, the Grand Canyon was formed, and the resulting eroded sands formed many of the dune systems of the southwest. The uplifted mountain ranges created rain shadows on their leeward sides beginning the deserts as we know them today. Largely tropical plants, adapted to warm conditions as well as surprisingly droughts (northern tropics have a distinctly wet and dry season, providing an opportunity for plants and animals to develop adaptations for surviving dry conditions) were able to then exploit these new rain shadow deserts. Legume (bean and pea) family trees and shrubs, ironwood, palo verde, mesquite, and acacias shifted north, as did cacti. 

Deep time thinking does not come easy to our species of nearly hairless apes, yet when we do it adds much to our ability to understand and interpret our world. 

Nullius in verba

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