“Biogeography typically trumps taxonomy and anticipates molecular phylogeny” — Dennis McCarthy

McCarthy’s quote aims to get us still closer to what is a species. How organisms are distributed in time and space provides insights as to their degree of isolation and perhaps how long that isolation has occurred. The more complete the isolation, how much or how little two populations interbreed, and how long that isolation has occurred, are strong hints as to whether those populations might in fact be different species. The “father” of the study of biogeography is Alfred Russel Wallace, the same Wallace who, along with Charles Darwin, independently came up with the theory of natural selection as the mechanism for the origin of species. 

The reasons that Darwin became widely known and Wallace was largely lost in obscurity are many. Wallace was a “working class” naturalist whereas Darwin was of a member of the upper class with a host of upper crust friends who kept his legacy alive. That is the typical explanation, but I think there is a better one. Wallace was much younger than Darwin, and while Darwin took over 20 years of careful study and experiments to finally have the confidence to shake the pillars of established science and publish his theory, Wallace’s insight apparently came to him while fighting malaria in a hut on an Indonesian island. Wallace was less concerned about shaking the pillars of science as he was anxious to make a name for himself and then be able to enter that elite society of English scientists. Darwin and Wallace’s independent but identical theories were presented together at the Linnean Society in London in 1858 with little or no fanfare. Only a year later when Darwin published his book, The Origin of the Species, did the world take notice. Wallace went on to publish his ideas on biogeography, ideas developed from years of study in both the Amazon rainforests and the Indonesian (Malay) Archipelago, to wide acclaim. Wallace had entered that scientific society, but perhaps it came too easily. He was self-taught, with barely a high school level of education, and yet he was now among the “Dons” of the scientific elite. 

His next efforts put him at odds with that elite. He was attracted to the then-popular world of seances, and ghosts communicating to the living through those seances. He conducted what he thought were careful studies to prove that the ghosts were real and published his findings. Those scientific “Dons," aware of the charlatans conducting the seances, from then on kept Wallace at arms-length. Wallace never admitted he was perhaps wrong about ghosts and seances. Being considered an outsider may have given him the license and freedom to then go further outside the boundaries of science and he wrote extensively about vaccinations being bad, and about social reform, challenging the “rights” of society’s upper class, including the leading scientists of the day. There is no question that within the boundaries of natural history that Wallace was a genius, but his explorations beyond natural history were often antithetical to good science and so put him in opposition to that same society that he so wanted to be part of. For Wallace money was always in short supply. Before he passed away, Darwin used his influence the secure a government pension for Wallace that allowed him to live the rest of his life in relative financial security.

Wallace created the science of biogeography. Whatever other ideas he espoused, biogeography stands as his crowning achievement. His field studies allowed him to identify the physical boundaries that separated species, be they oceanic islands, rivers, mountains, and other habitat barriers that limited connections between populations, and ultimately species. While in the Indonesian archipelago he puzzled over patterns of species occurrences between islands. One group of islands clearly had affinities to southeast Asia: elephants, tigers, monkeys, reptiles and birds and insects all of Asian origins. But there was an adjacent group of islands with affinities to Australia: marsupial tree kangaroos, birds, snakes and lizards and insects that were more closely related to that island continent. By meticulously measuring ocean depths between this island groups, he found that the channel separating these groups was much deeper than the within island group channels. Later, it was determined that during the series of ice ages during the Pleistocene era, when so much of the earth’s water was tied up in massive glaciers, the sea level was much lower. The Asian-affinity islands would have then been connected to each other and connected to southeast Asia, and the Australian-affinity islands would have been connected to each other and to Australia. However, the deep channel separating these island groups would have been still inundated throughout that period, creating a biological barrier to species dispersals. That channel was and continues to be referred to as Wallace’s line.

The biological upheavals wrought by the ebb and flow of walls of ice during the Pleistocene continue to be expressed in the patterns of biodiversity we see today. Our local mountains, the San Jacinto, Santa Rosa and San Bernardino Ranges owe a portion of their rich biodiversity because they became climate refugia for more northern species that were being pushed south by ice and frigid climates. Northern conifers such as white fir, lodgepole, limber, Jeffery, ponderosa, sugar, and knobcone pines, single needle pinyon pines, incense cedar, Utah and western juniper, and big cone spruce are all species with disjunct and isolated populations in our local mountains. Other northern tree species such as California bay trees, big leaf maples, dogwoods, alders, and walnuts follow the same pattern. Northern animals following this pattern include spotted owls, saw-whet owls, pygmy owls, and flying squirrels. During that ebb and flow of ice, topographic and climate gradients sorted out where each of these species could maintain sustainable populations. All those mentioned here are currently found in the San Bernardino Range. White fir, lodgepole, limber, Jeffery, ponderosa, sugar pines, single needle pinyon pines, incense cedar, big cone spruce, California bay trees, big leaf maples, dogwoods, alders, and saw-whet owls can also be found in the San Jacinto Range. White fir, limber, Jeffery, sugar pines, single needle pinyon pines, incense cedar, and alders make into the Santa Rosa Range. For some the gap between the San Bernardino and San Jacinto ranges may have been a distance too far to navigate, but I suspect that the historical and increasingly stark climate differences are part of the explanation. Although there is a single record of a Spotted Owl in the Santa Rosa Mountains (a pile of feathers), as recently as a decade or two ago there was a small population in the San Jacinto Mountains, but none have been seen in recent years. A healthy Spotted Owl population still exists in the San Bernardino Mountains, along with their favorite food, northern flying squirrels. 

Although imperfect comparisons due to differences in elevation, the mean annual rainfall and snow fall at the Big Bear Lake dam is 35.6” and 128.3”; at Idyllwild in the San Jacinto Mountains it is 25.2” and 38.3”, and for Pinyon Crest in the Santa Rosa Mountains it is just 9.4” (snowfall was not reported but snow there now is an increasingly rare event). The San Bernardino Mountain chain is higher in elevation, wetter and gets more snow. The Santa Rosa Range is much drier and significant snowfall is a rare event (although those living in the Santa Rosa Mountains just 50 years ago or more remember regular deep snow and creeks running through the spring months — they did not even need to bring canteens on their hikes). 

While there is a strong north to south gradient of otherwise more northerly distributed species from the San Bernardino to the Santa Rosa Mountains, there is an opposite south to north gradient of more southerly, Baja California species. More Baja species of plants and reptiles in the Santa Rosa Mountains and none in the San Bernardino mountains. Climate gradients, historical climate shifts, physical barriers and connectivity are all fodder for understanding why species are where they are. That is the science of biogeography.

Nullius in verba

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