“The study of nature is a limitless field, the most fascinating adventure in the world” – Margaret Morse Nice

Understanding why a creature lives here but not there is among the basic pursuits of a naturalist. Sometimes the answers are obvious: frogs need water; no water, no frogs. Sometimes it takes a bit more thought and a deep-time knowledge of changing climates.

Alfred Russel Wallace, the lesser known co-discoverer of natural selection with Charles Darwin, is more celebrated as the “father” of biogeography (the study of the distributions of organisms), writer of “The Malay Archipelago," and the discoverer of Wallace’s Line. Wallace was in the Malay Archipelago (now the 1000s of islands that make up the countries of Indonesia, Borneo, Timor, and New Guinea) when he sketched out his ideas on natural selection and sent them to Darwin to see if they might be worth publishing. Over a 20-year span Darwin had prepared detailed drafts of his own ideas on the subject, but fully aware of the controversy that would be unleashed, rather than publish, he kept experimenting and analyzing hoping to ensure no other explanation was possible. Upon reading Wallace’s sketch, it was if he had written those words himself. Darwin’s own code of honor dictated that he must see that Wallace’s sketch get published, and so then cede priority to Wallace on what had been Darwin’s life work.  However, Darwin’s colleagues interceded and convinced Darwin to publish his and Wallace’s natural selection sketches simultaneously. The following year Darwin wrote and published his book, “The Origin of the Species." Years later, when Wallace returned to England, he never once expressed misgivings of how question of priority had been handled and the subsequent acclaim Darwin received with the “Origin." Wallace received his own acclaim upon publishing the “Malay Archipelago," still a must-read for anyone interested in adventure and biogeography. 

One of the great puzzles for Wallace while collecting specimens in the Malay Archipelago was that there was a clear association of birds, mammals, and reptiles with origins in Asia on part of the Archipelago, and with Australia on the other. Monkeys on one side and tree kangaroos on the other. The dividing line was so narrow that Wallace could stand on one side with the monkeys and see the islands with tree kangaroos across a narrow channel. He then measured the water depths and found that the channel that separated the two sides to be much deeper than the waters separating the islands within the Asia-origin fauna or within the Australia-origin fauna. He speculated that changing sea levels in the past would have provided land connections to the islands within each region and to their respective continents of origins but the two regions, separated by that deep channel would have always been separate. That channel became known as Wallace’s line, and those sea-level changes were due to the multiple glacial and inter-glacial periods of the Pleistocene. 

Those multiple glacial and interglacial periods have had profound impacts on the distribution of plants and animals throughout the northern hemisphere. The massive glaciers shaved all life off of the earth’s crust as they moved south. Animals and plants (via their seeds) that could escape further south did so or perished. Today the lodgepole and limber pines at the top of Mt. San Jacinto owe that occurrence to the legacy of those glaciers pushing them south. During the various glacial maxima, our deserts would have been covered with junipers, oaks, Joshua trees and pinyon pines. When the glaciers retreated (glacial minima) those species moved up slope and were replaced by creosote and cacti at the lower elevations. 

Of course, it was not only plants that shifted, so did the animals. Today, within Joshua Tree National Park, as well as within the Santa Rosa and San Jacinto Mountains National Monument, western fence lizards and Blainville’s horned lizards are restricted to those higher elevations, living among the junipers, oaks, and pinyon pines. Moving toward the coast and cooler-wetter climates, both lizards are found at lower elevations; if not for the houses with ocean views, they would be found down to the bluffs overlooking the ocean. During those glacial maxima they almost certainly occurred at lower elevations in what we now call deserts, but today both lizards are absent from the hyper arid deserts and are now restricted to those higher elevations. 

Within the center of the National Park we can find fence lizards at elevations approaching 4400’ and above; below 4360’ no fence lizards. At the somewhat wetter western edge of the Park, fence lizards occur at 4200’, but not below 4,000’. Last week we surveyed lizards on the Maze trail right at that western Park elevation boundary. Five years ago, I found fence lizards there at 4035’, but not since. It is getting hotter and drier. These elevation boundaries seem to delimit where we can find fence lizards and where we can’t. 

This week we surveyed along the lower elevation 49 Palms trail. The trail ands at the palm oasis and one of the very few permanent water courses in the Park. The oasis is at 2700’ and is a lush island of habitat amid scorching hot and dry slopes of creosote bush. There are fence lizards in and among the palm trees. But how? Otherwise they are found more than 1600’ higher up the mountain. I suspect that as plants and lizards moved back and forth up and down the mountain between  glacial maxima and minima, this tiny, isolated population of fence lizards found a climate refugia where the shade and water met all of their needs. These lizards have been isolated here for perhaps 1000s of years. 

It would be fascinating to test the genetics of this population with those at higher elevations to see just how long they have been isolated. Small isolated populations can develop different genetic traits. Notice the attached image of one of the fence lizards we saw. Take a close look at her feet; her toes are extraordinarily long – longer that I have ever seen on a fence lizard. Perhaps the better to climb palm trees, or perhaps just a product of 1000s of years of inbreeding. Nevertheless, the existence of fence lizards here demonstrates the huge value of climate refugia for buffering species from changing climates. It perhaps provides a glimmer of hope as temperatures continue to rise, hope that plants and animals will find such places that buffer them from aridity – until we can reverse this trajectory.

Go outside, tip your hat to a lizard, and be safe.