“Look closely at nature. Every species is a masterpiece, exquisitely adapted to the particular environment in which it has survived. Who are we to destroy or even diminish biodiversity?” — E.O. Wilson

“We're losing biodiversity globally at an alarming rate, and we need a cornucopia of different plants and animals, for the planet's health and our own.” — Diane Ackerman

“Biodiversity” is shorthand for the sum of all species occupying a particular area. This definition is often narrowed to include just those species unique to a region. Under this convention, broad swaths of the northern parts of North America, despite being clothed in dense forests, are considered low in biodiversity since those forests are so young. Just 18,000 years ago, give or take a few thousand years, those regions were covered in by a massive ice sheet; only since that sheet retreated have the trees and shrubs and birds and frogs reinvaded those lands. Young by geologic and evolutionary standards. Too young for many species to have evolved into distinct varieties. South of the influence of that vast ice sheet is where northern species escaped and where southern species did not need to move. Species accumulated there and had time to evolve.

This “macroscopic” view explains why California is so rich in biodiversity, whereas northern tier states, from Idaho to Maine are relatively poor in species unique to their borders. Except in the Sierra Nevada, massive glaciers never formed in much of California, and the Pleistocene’s cold climate was moderated by the ocean and the deserts. The result is that California’s biodiversity exceeds that of all other continental states added together. That both gives us bragging rights and puts an onus on us to ensure those species remain protected; we have far more to lose than any other region north of Mexico.

For unique species to form they need to be isolated from other populations. Isolation restricts gene flow and allows mutations to flourish when they allow species to be better adapted to local conditions without being “watered down” by genes from beyond the borders of those local conditions. This explains why islands are typically populated by species found nowhere else on earth. Think the Galapagos or Hawaiian Islands. Oceanic islands certainly provide that necessary isolation, but California is also rich in ecological islands. Sand dunes require specialized adaptations for species to exist there and so sand dunes are very much “ecological islands”; every sand dune habitat in the California desert has beetles, plants, and/or lizard species unique to that dune system and found nowhere else on the planet. Their adaptations to living on and in loose sand are so complete that they cannot live anywhere else. For them, the dunes are like islands surrounded by an inhospitable “sea” of rocks and gravel. Isolated mountain ranges are also ecological islands. These mountains can have soils, and climates that are distinct from the surrounding lower elevation landscape. While birds can easily move between mountain ranges, and so rarely if ever form varieties unique to a montane island, plants, and reptiles, are much less capable of moving between mountains and so can adapt to the unique conditions those mountains provide. Our desert mountain ranges provided refuges for the northern waifs, pushed south by Pleistocene ice sheets, and create the isolation that fosteres the formation of new varieties and new species.

Mountains, especially desert mountains, have lots of exposed rocks. These rocks provide habitat for rock-loving (saxicolous) lizards, but because the lower elevation, intermountain landscapes are usually sandy, not rocky, the mountains act as isolated ecological islands for those saxicolous lizards. The male chuckwalla lizards in the Santa Rosa and San Jacinto Mountains have cream-colored backs whereas those across the valley in the Little San Bernardino Mountains have brick red backs. In Phoenix, there is an isolated mountain within the city limits (South Mountain) where the male chuckwallas are gray with carrot-orange tails. In northern Baja California they are gray with nearly white tails. These between population color differences may not have any adaptive value in the sense of making them better able to find food or escape predators; often such color differences are driven by female preference for mates, and the males that attract the most females will father the most sons and daughters. To be clear, none of these isolated chuckwalla populations are considered separate species, yet, but they illustrate how isolation and ecological islands can foster diverging genetic trajectories. However, in some cases the differences are sufficient to warrant calling saxicolous lizards in different mountain ranges different species; collared lizards in the Santa Rosa and San Jacinto Mountains are Baja California collared lizards, while those in the Little San Bernardino Mountains and beyond are Great Basin collared lizards – different species separated by just 20 miles of a sand “sea."

Areas that were ice-free during the during the Pleistocene, and with soils and topography that result in ecological isolation or islands, have accumulated species, and are now considered “biodiversity hotspots." They functioned during those glacial maxima as climate refugia, and while many of the Pleistocene megafauna (mammoths, horses, camels, ground sloths, and more) went extinct, every animal and plant we see today survived, many because of those climate refugia. That worked for climate fluctuations between frigid to mild, but today we hare headed in a different direction, from hot to hotter. The mountain species can shift to higher elevations, north-facing slopes, steep-sided canyons that limit the amount of solar radiation the rocks receive. If the mountains are high enough and provide other critical habitat needs (food, nesting sites, and habitats that allow them to escape predators) they should continue to provide climate refugia. In some cases, as in Joshua Tree National Park with its highest elevations just barely topping out at around 6,000’, Blainville’s horned lizards are already bumping up against that elevation ceiling. And of course those sand dune species have nowhere to go.

Nullius in verba

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