“Though we sometimes think of the 'last ice age' as one long, cold period, it wasn't. Ice advanced and retreated as the Earth cycled between glacial (colder) and interglacial (warmer) periods.” —  Anonymous (thehorseshoecrab.org)

Oscillations of the earth’s orbit around the sun, involving both the tilt of the earth and how near the sun the earth gets during its orbital passage — called Milanković Cycles — precipitated the fluctuating ice ages of the Pleistocene Epoch. As a result of those cycles, over the past million years or so, the earth’s northern hemisphere experienced glacial maxima about every 100,000 years separated by warmer and drier interglacial periods when the glaciers retreated northward. During those glacial maxima, glaciers thousands of feet thick covered nearly all of what is now Canada, and extended well into the northern tier states of the U.S. Beyond the ice sheets, frigid air poured off the glaciers pushing plants and animals still farther south. Plants and animals see-sawed north and south depending upon whether warmer or colder Milanković Cycles prevailed. During the warm interglacial periods most cold-adapted species shifted back north and desert species became established in their present configuration. However, rather than shifting back north, some cold-adapted species became stranded, isolated on southern mountain slopes. These mountains, isolated from other high elevation habitats by lower elevation deserts or near-desert habitats are called “sky islands.”

It is through this historical lens that we can begin to understand species occurrences on sky islands. The cold glacial maxima caused vast conifer forests found today from Alaska to the Rocky Mountains and Cascades to shift south and then return to their present range when warm and drier glacial minima conditions returned. Except rather than shift back north, some tracked their preferred climate envelopes up slopes into our sky islands. Nine species of conifers can be found on the San Jacinto and San Gorgonio Mountains, with one, the limber pine reaching its current southernmost location on Mount San Jacinto. Otherwise, limber pines extend into the Canadian Rockies. Another, the lodgepole pine, does have a population on another sky island in Baja California in the San Pedro Martir Mountains as well as on Mount San Jacinto. Otherwise lodgepole pines extend into the Rocky Mountains and through the Cascades, all the way into central Alaska.

As interesting as these now disjunct southern populations are, species absences from sky islands both north and south are just as interesting. Why aren’t limber pines and lodgepole pines present at the higher elevations of the Santa Rosa Mountains? Although there is overlap in available elevations in the Santa Rosa Mountains with where these conifers occur on the adjacent San Jacinto Peak, the Santa Rosa Range is drier, and maybe that is explanation enough. However, there are other species occurrences on San Jacinto that are absent on Santa Rosa, such as otherwise widespread black oaks. Black oaks were and are extensively used for food by indigenous people throughout California. They are abundant at middle elevations from 4,000-7,000 feet on south-facing San Jacinto slopes, and are found even lower and further south on Mount Palomar, the Julian-Cuyamuca region, and the Laguna Mountains. Current aridity alone does not seem to explain this species’ absence from the Santa Rosa Mountains. Perhaps it is that the Santa Rosa Mountains jut out into and are surrounded by the hottest, driest, lowest elevation deserts of southern California? Maybe those low elevations created a filter, a barrier to some species’ access even during the cooler wetter conditions of the Pleistocene’s glacial maxima?

Sky islands, like oceanic islands, are isolated from other similar habitats. Isolation, either by oceans, or by deserts, can then foster incremental changes in species over time as they lose genetic connections with parent populations, adapt to local conditions, or simply by genetic drift. If genetic differences are sufficiently large, they may be deemed to be distinct species or subspecies. If restricted to a single or several sky islands, such species are then endemic to that specific region. One wide-spread North American conifer is absent from the Southern California Sky islands is Douglas-fir, Pseudotsuga menziesii. However, it left its genetic “off-spring” here in what is now considered a separate species, bigcone Douglas-fir, Pseudotsuga macrocarpa. Isolation and then speciation. bigcone Douglas-firs occur on the seaward (cooler-wetter) mid-elevation (3000-5000 feet) slopes of each of the Southern California sky islands, except again they are not in the Santa Rosa Mountains.

There is a lizard with a similar distribution. Common sage brush lizards, Sceloporus graciosus, are distributed throughout the colder Great Basin Desert from Washington and Oregon to Colorado and Wyoming to the eastern slopes of the Sierra Nevada and Cascade Mountain ranges. During the repeated glacial maxima, they were pushed south and then moved back north during the warm interglacial periods. Except some moved up into the southern California sky islands. They now occur mostly above 6,000 feet across the Transverse and Peninsular Mountain Ranges as far south as the Sierra San Pedro Martir Range in Baja California. Due to their isolation and genetic differences they are now considered a separate species, the southern sagebrush lizard, Sceloporus vandenburgianus. Because elevations above 6,000 feet are not continuous across that range, there is considerable separation and isolation between this lizard’s populations; some aspiring graduate student will someday analyze their genetics across these isolated populations and perhaps find many that now differ at a sub-species level, or perhaps even a new species will emerge.

Of the Southern California sky islands, the San Jacinto Mountains seem to have fostered an inordinate number of endemic plants and a cricket found nowhere else. The cricket, the San Jacinto shieldback, Phymonotus jacintotops, has only been seen from a few acres along the Round Valley Trail. That’s it, nowhere else. Tom Chester, among the leading botanists exploring these sky islands, has documented seven endemic species, the San Jacinto bedstraw, California bedstraw, Tahquitz ivesia, San Jacinto keckiella, San Jacinto prickly phlox, and Hidden Lake bluecurls occurring in the San Jacinto Range and nowhere else. Another three species also occur in the Santa Rosa Range, Southern California rock draba, shaggy-haired alum root, and white-margined oxytheca. Just two are restricted to just the Santa Rosa Range, Ziegler’s aster and the Santa Rosa Mountains linanthus. One additional species, California Marina, Marina orcuttii, mostly occurs in the Santa Rosa Mountains, but a couple populations also occur further south in the Sierra Juarez and San Pedro Martir Ranges in Baja California. Similar to the San Jacinto Mountains, t San Gorgonio Mountain and theSan Bernardino Mountains are chock full of species found nowhere else on earth; twelve such species have been identified. The greater number appears to be due to specific soil types, calcareous limestone soils and pebble plains soils that further isolate populations and foster endemism.

Viewing our sky island mountains from below, it may seem hard to imagine that such biological diversity exists on what appears from afar to be brown, treeless slopes. In part due to their isolation, and in part due to the periodic inputs of northern species pushed south by glacial winds and frigid temperatures, these mountains are exceptionally rich in species found nowhere else. Part of this richness is explainable, but part is due to the low probabilities of species finding just the right climate envelope, a climate refugia, as warmer and more arid conditions returned. How many species failed to find that “just right” climate niche, and so perished will likely never be known. Those that found the dark, cool, canyon, that rock crevice with perennial water, or reached high enough elevations, won the survival lottery.

Nullius in verba

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