“Of these three essential factors, space might be said to be one with which biogeography is primarily concerned. However, space necessarily interplays with time and form, therefore the three factors are as one of biogeographic concern.” — Leon Croizat

By DR. CAMERON BARROWS

Biogeography is a science aimed at understanding how life is distributed across our planet. It is the science of the patterns of life, giving us a framework for understanding similarities and differences between any two locations.

In 1967, Robert MacArthur and E.O. Wilson penned their “Theory of Island Biogeography,” where they explained the patterns of life found on oceanic islands. In a nutshell, their theory predicted that, assuming other variables being equal, bigger islands had more species and islands in closer proximity to each other shared more species, while those more distant shared incrementally fewer species. MacArthur and Wilson chose oceanic islands as a simple model because within a region the ocean tends to moderate climates and within that region the islands often had a similar origin (Croizat’s time and form). As we look beyond oceanic islands to continental landscapes, those “other variables” such as climate, topography, elevation, and age play an increasingly important role and so rapidly add complexity. Still, the “island model” can still be a useful starting point for understanding those patterns of life.

“Sky Islands” refer to isolated, high elevation mountains surrounded by an “ocean” of lower elevation desert habitats. The term first gained utility in referring to the isolated mountains of Southern Arizona, reaching above the surrounding Sonoran Desert, they are biologically an extension of the flora and fauna of the more consolidated mountains of north-central Mexico, the Sierra Madre, and are sometimes referred to as the Madrean Sky Islands. Here you can find species such as jaguars, trogons, thick-billed parrots (although no longer occurring wild north of Mexico), rock, twin-spotted and ridge-nosed rattlesnakes, and Yarrow’s spiny lizards, all species otherwise found south of the U.S. border.

The high mountains of the central and northern Peninsular Range and the Transverse Range, of California and northern Baja California, also fit the definition as sky islands. However rather than being refugia for Sonoran, otherwise southern sub-tropical species, these sky islands provide an archipelago of refugia for northern, cold-adapted species, species pushed south by the repeated glacial maxima of the Pleistocene, resulting in a deep time climate connectivity. The lower elevation “oceans” partially surrounding these massifs and so confining these species are the Mojave and Colorado Deserts. The species occupying these Southern California sky islands are aligned with those species otherwise occurring within the Sierra Nevada and Cascade Mountain ranges. These sky islands, San Gorgonio, San Jacinto, Santa Rosa, and Toro Peaks, are biogeographically similar, harboring mostly the same set of species. Yet there are differences. Black oaks, a prized food source (acorns) of the Cahuilla people, are abundant on the slopes below San Jacinto and San Gorgonio peaks, but are absent further south below the drier Santa Rosa and Toro Peaks. Similar patterns occur for maples, alders, and lemon lilies (although there is one small lemon lily population off the road heading up to Santa Rosa Peak).

In Northern Arizona there is another sky island, the San Franciso Peaks, just north of Flagstaff. Unlike southern Arizona’s Madrean sky islands, the San Francisco Peaks are biogeographically aligned to the Rocky Mountains. Like the Southern California sky islands, the San Francisco Peaks provide refugia for northern species, pushed south during the glacial maxima of the Pleistocene. Due to that Pleistocene connection, hiking in and around the San Francisco Peaks feels similar to hiking in the Southern California sky islands. Ponderosa pine, white fir, and Douglas fir are the dominant conifers. Mountain chickadees, white-breasted, spotted towhees (although with a different song), red-breasted and pygmy nuthatches are abundant, as are Steller’s jays, acorn and hairy woodpeckers, yellow-rumped warblers, black-headed grosbeaks, western tanagers, and dark-eyed juncos. These similarities seem to go against MacArthur and Wilson’s theory that the greatest similarity will be among “islands” that are in closest proximity to each other. The biological connectivity between the San Jacinto Mountains and the San Francisco Peaks extends up into Colorado, Utah, Idaho Washington and then down through Oregon, and through the Sierra Nevada Range to Southern California. Whereas by distance alone, the San Francisco Peaks are much closer to the Madrean sky islands. Rather than distance, biological similarities between the Southern California and San Francisco sky islands are more tied to deep time climate connectivity – aka glacial maxima.

However, while that deep time climate connectivity explains the similarities, it does not explain the differences. Species that are missing from the San Francisco Peaks are just as informative to our understanding as what species occur there. There are no incense cedars, no white-headed woodpeckers, and no canyon live oaks, each a direct connection between the Sierra Nevada and Cascades to Southern California, but with no connections to the Rocky Mountains. There are also no high elevation specialist lizards, like southern sagebrush lizards that thrive between 6,000 – 11,600’ in the Southern California sky islands, a species with links to the cold Great Basin Desert and Sierra Nevada, but not the Rocky Mountains. But there are Virginia’s warblers, red crossbills, white pines and Engleman spruce present in the San Francisco Peaks and the Rocky Mountains, but absent from the Sierra Nevada and Southern California. Deep time climate connections but lacking the broader Cascades to Rocky Mountains link.

Alfred Russel Wallace, dubbed by many as the father of biogeography, also identified the importance of deep time climate connections. He found an invisible line separating one half of the Malay (Indonesian) Archipelago flora and fauna from the other. During the Pleistocene glacial maxima, much of the Earth’s water was tied up in glaciers and so the sea levels were much lower. During that time many of those Indonesian islands were connected into two larger land masses, separated by a deep-water channel – the same line that Wallace found separating the floral and faunal affinities of today’s archipelago.

The distance and size relationships MacArthur and Wilson identified are another piece of the biogeography puzzle. As much as the San Francisco Peaks feel familiar to a naturalist accustomed to the Southern California mountains, there are also differences. Red-faced warblers, hepatic tanagers, and alligator junipers are among the species with direct affinities to the Madrean sky islands that also thrive in the San Francisco Peaks. MacArthur and Wilson’s description of close connections as well as Croizat’s time and form (deep time climates) are both part of understanding how life is distributed across the Earth.

Nullius in verba                                                                                                                                               

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