“Perhaps I am just a hopeless rationalist, but isn’t fascination as comforting as solace? Isn’t nature immeasurably more interesting for its complexities and its lack of conformity to our hopes? Isn’t curiosity as wondrously and fundamentally human as compassion?” — Stephen Jay Gould

One of the scientific disciplines primarily populated by naturalists today is “biogeography," a field of study that asks the question, “What creatures live here, and why or why not?”

The “father” of biogeography was Alfred Russel Wallace, arguably the top English naturalist of the 19^th century. He explored the Amazon for years, supporting himself by selling the specimens he collected to museums and private collectors back in England. Then he explored the Indonesia (then referred to as Malay) archipelago for even more years, still funding his travels by selling plants, birds, beetles, and butterfly specimens. To a large extent these creatures, both from the Amazon and Indonesia were new to western science; except for the local indigenous people, he was the first “outsider” to see them.

Wallace noted the similar floras and faunas from adjacent islands and noted the exceptions to those similarities. He found a zone where on one side the islands were populated with plants and animals with affinities to the Asian mainland, but on the other side the affinities were to Australia. It was a sharp line between island clusters, that at first did not make sense. Wallace came up with a hypothesis that the ocean levels must have at one time been much shallower, connecting most of these islands into a single land mass, except between the zone that separates the biota with Asian versus Australian affinities. Here he postulated the sea must be deeper, and so islands on either side of this zone were always separate. To test his hypothesis, he took a long rope with an attached anchor and measured the ocean depths between islands with similar and dissimilar affinities. He was right. We now know that during the multiple ice ages of the Pleistocene, when much more of Earth’s water was trapped in glaciers, the world’s oceans were much shallower. Only islands separated by deep trenches stayed separate. However, during the warm inter-glacial periods the glaciers melted and the sea level rose, isolating what were mountain peaks of the larger land masses into island archipelagos. This was the “birth” of biogeography as a science, providing testable explanations as to why species are “here” and not “there." The line separating the species with Asian versus Australian affinities was dubbed “Wallace’s Line.”

Along with biogeography, Wallace independently developed a theory of evolution by natural selection. As such, he is given credit with Charles Darwin (another top naturalist) as the co-founder of that theory. The reason Darwin is so well-known, and Wallace largely fell into obscurity is complicated. Books have been written about this topic, some trying to argue that Darwin “stole” Wallace’s Theory while Wallace was still exploring the Malay Archipelago. That idea was been explored ad-nauseum and found to lack merit. When Darwin did hear of Wallace’s ideas and realized how similar they were to his own, Darwin was urged to publish his ideas more quickly, but he always acknowledged Wallace’s role. When Wallace returned to England and had read Darwin’s “Origin of the Species," he graciously accepted Darwin’s priority.  Noting that while their theories were essentially identical, Wallace knew his was just an idea, whereas Darwin backed his with many examples that tested its validity. Still, being the founder of Biogeography and a co-founder of natural selection as the driver of evolution, Wallace should be more well-known. Part of the problem was that while Wallace showed his genius with these two theories, he also believed in phrenology (a hypothesis that a person’s character could be predicted based on bumps on one’s head) and believed that “mediums” could, during seances, talk to his dead relatives. The scientific establishment was trying hard to dispel people’s belief is such charlatan practices, and so Wallace’s ardent and unrelenting support put him permanently on the outside of that establishment. Nevertheless, because Wallace was never able to hold on to enough wealth to live comfortably, Darwin used his considerable influence to convince the British government to award Wallace an annual pension that allowed him to be financially secure for the rest of his life.

Much of the patterns of where species occur today can be explained by the influence of those same multiple ice ages that created the patterns Wallace noted in the Malay Archipelago. The glaciers forced northern species to shift southward or perish. The conifers that top our desert mountains are a legacy of that southward push. When the glaciers retreated many of the conifers shifted back north, but some just climbed the desert mountains, finding sanctuary there then and today. Southern sagebrush lizards, and their island-like distribution across mountain peaks that rise above 6,000’ in elevation across the Transverse and Peninsular Mountain ranges, is yet another mark of the impact of those ice ages. Common sagebrush lizards were pushed south by frigid climates, and then like the conifers either moved up mountain slopes or returned to the shrub steppe habitats of the Great Basin Desert. Those that climbed the mountains have been isolated long enough from their northern cousins to now be genetically distinct.

Chuckwallas, relatively large, plump-bodied vegetarian lizards, currently can be found from near the southern tip of Baja California to the northern edges of the Mojave Desert. They probably evolved into the lizard we see today in those southern deserts of Baja (probably) or the Sonoran Desert, and then expanded northward after the retreat of those Pleistocene glaciers. As they shifted north, some isolated populations formed distinct color patterns indicating genetic separation, although not enough to warrant naming each color pattern as separate species. Still, there are some aspects of their current “biogeography” that seem to challenge easy explanations. Using iNaturalist records, chuckwallas in Joshua Tree National Park (“Joshua Tree”) can be found at all the lower elevations of the park as well as at upper elevations up to 4,500’-4,800’. However, in the Santa Rosa and San Jacinto National Monument (the “Monument”), just south of Joshua Tree across the Coachella Valley, the highest recorded chuckwallas have only been found up to 2,900’-3,300’, a 1,500’ difference. Is that a result of more than a color difference, but a physiological difference in their ability to handle cooler climates? If it is, does that mean a differential ability to handle a still warmer and more arid desert as a result of climate change?

One approach to answering these questions is to compare high elevations for other lizard species. Desert iguanas, another large vegetarian lizard, can occur at elevations of up to 4,000’-4,300’ in Joshua Tree, but the highest records for the Monument are just a little above and below 2,000’ - a +2,000’ difference. Desert Horned lizards in Joshua Tree can be found up to and just above 5,000’, whereas in the Monument the highest recorded elevation is 3,600’ – a difference of 1,400’. I found similar differences for every lizard species I looked at. Is it possible that each species evolved a tolerance for cooler temperatures in Joshua Tree? Another hypothesis is that the same elevations in Joshua Tee and the Monument do not correspond to the same climates. Rather, the steep largely north-facing mountain slopes of the Monument could be cooler than at the same elevations within the less steep and flatter locations within Joshua Tree. An elevation of 3,000’ in the Monument could have the same temperature profile as 4,500’ in Joshua Tree due to cold air drainage off the higher slopes and less solar radiation hitting the north facing slopes of the Monument. If that is true, then there is much more area within the Monument for lizards (and all living things) to move into if the climate keeps on its current trajectory of our deserts becoming warmer and more arid. More area would indicate the possibility of the Monument becoming a better climate refugia than Joshua Tree.

It is the complexities of nature ensure that we will never lack new things to learn, new wonders to explore.

Nullius in verba

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