“In wilderness there is the salvation of the world.” — Henry David Thoreau 

“Perhaps this is the hidden meaning in the howl of the wolf, long known among mountains, but seldom perceived among men.” — Aldo Leopold

This quote comes from Leopold’s essay titled, “Thinking Like a Mountain,” where he relates his epiphany for considering the importance of longer time scales and in doing so incorporate different, not human-centric points of view when trying to understand the ebb and flow of natural systems.

The catalyst of his epiphany was when he was a young man working for the then fledgling federal agency, the US Forest Service during the early decades of the 1900s. He was charged with managing wildlife in forests along the Arizona-New Mexican border region and was expected to institute policies that increased the human utility of these forests, which in this case meant increasing the density of deer to provide better hunting opportunities for people. He and others reasoned that the fastest way to increase the deer herd was to eliminate the deer’s “other predator," Mexican wolves.

Leopold later realized that the wolves were key to keeping the deer herd size within that habitat’s carrying capacity and key to overall ecosystem sustainability. Without wolves the deer population does quickly increase, but does so by ultimately outstripping their food supply, damaging the forest’s ability to maintain itself, and eventually resulting in widespread starvation in the deer population. One day Aldo and a companion came across a pack of wolves, and they shot and killed one of them, watching the green fire dim and then go out in the wolf’s eyes as it expired. In remembering that experience he wrote: “I was young then, and full of trigger-itch; I thought that because fewer wolves meant more deer, that no wolves would mean hunters' paradise. But after seeing the green fire die, I sensed that neither the wolf nor the mountain agreed with such a view."

Thinking like a mountain was good advice then and still is today. The science of ecology, our ability to understand nature, is scale-dependent, both from a spatial and temporal perspective, and can suffer from a human-centric time scales. Larger spatial scales and longer time scales increase our ability to make sense of patterns in nature that at smaller and shorter scales seem rather chaotic. The exceptional biological richness of our deserts, an environment that at first glance seems so inhospitable and unforgiving for living creatures, can only be understood by “thinking like a mountain."

Baldwin Lake, perched at the southeastern edge of the San Bernardino Mountains, occurs on an ecotone, the border between the Mojave Desert and its Joshua tree woodlands, a pinyon pine forest, and a dense yellow pine forest, and as such is ideally located for monitoring change. The lake itself is rarely wet, at least when viewed through the lens of the past few decades. Dr. Katherine Glover earned her doctorate from UCLA in 2016, studying the changes in vegetation surrounding the lake across the past 125,000 years. She accomplished this task by examining cores into the lake bottom and used changes in species’ pollen abundance to infer when and what was growing along the lake’s shores. When viewed under a microscope, pollen grains are species-specific in their shape and texture and they are extremely resilient to erosion or decay. Even though the most dramatic changes were occurring far to the north, at that time scale Dr. Glover was able to detect vegetation changes before, during, and after the last glacial maximum (20,000 years ago). What she detected was an ecosystem that seemed to be in slow flux, seemingly stable at any given point but at the viewpoint of the mountain varying from wet and dry cycles, from shrublands to conifer forests and back again, and again. Fires (as evidenced by charcoal layers) were also variable in their intensity and periodicity. What seems to be today a stable forest system is only a moment in time for a dynamic landscape. We now understand that the lack of stability over time created space for more species to occupy this landscape. Our species is often constrained by the myopic perspective that stability is good, and change is to be avoided, when in fact, at larger time scales, change is the rule.  

There have been multiple glacial maxima over the past two million years. During each of those periods there were massive glacial ice sheets that covered most of what is now Canada and the northern tier states of the US. During glacial maxima species occupying those lands during the glacial minima either went extinct under advancing ice sheets or, were able to move south. Of course, that southward movement did not necessarily entail movement of individuals; rather the advancing ice was slow enough that shifts in distributions could be accomplished by higher reproductive success at the southern edge of a species’ range. When the glaciers eventually retreated, the process was repeated in reverse. As it got warmer and drier, species either went extinct or their reproductive success was then higher at the northern limits of a species range and so their ranges shifted northward. Or they found the same preferred conditions by incrementally shifting to higher elevations along mountain slopes. Over millennia tall southern mountains accumulated species that are today otherwise associated with more northern latitudes. Or sometimes, isolated from that northern population center, species who found refuge in mountains evolved into new species, adapting to the increasing temperatures and aridity in their southern homes. Add in southern semi-tropical species that shifted north into our deserts, and the seeming enigma of high biodiversity in a hot and dry landscape starts making sense.

Sagebrush lizards, Sceloporus graciosus, occur today in the higher latitude, cold winter Great Basin Desert. While the huge glacial icesheets never got so far south to cover their habitat, during glacial maxima it almost certainly was so cold that much, if not all their population shifted further south. Then in the current glacial minima they shifted back north again, except not all of them. One population got isolated in southeastern New Mexico and western Texas and adapted to a sand dune habitat there. They are now considered a separate species, S. arenicolus, the sand dune sagebrush lizard. Other populations shifted upslope into the desert border mountains of southern California and as far south as the Sierra San Pedro Martir in Baja California, Mexico. Some of these populations currently are found as high as 9,000-feet in elevation. So far, they have not received as much attention from scientists as they deserve, but are genetically distinct from the Great Basin populations, and are believed by some to also be a separate species, the southern sagebrush lizard, S. vandenburgianus. In these southern mountains they occur on isolated mountaintops with no apparent connectivity between populations. Are each of these mountaintop populations now genetically distinct? Can their genetics identify when they were isolated from each other and from the Great Basin lizards? How secure are these mountaintop populations from the impacts of modern climate change? Someday we will have those answers.

Nullius in verba

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