The Pinaleño Mountains rise to 10,720 feet in southeastern Arizona. At the summit — ponderosa pine, spruce, fir, a temperate forest that has no business being in the Sonoran Desert. Standing in Mesa, you can see the Santa Catalinas doing the same thing: dark escarpments notched into the northern skyline, holding something cool and green above the pale brown of the basin. These are sky islands. Mountain ranges separated from each other not by distance but by desert, which functions like an ocean — except that the barrier isn't water. The barrier is warmth.
During the last glacial maximum, roughly 20,000 years ago, the climate here was cooler and wetter. The forest that now lives only at elevation expanded down into the valleys, connecting each of these peaks as part of one continuous landscape. A squirrel could move, in principle, from the Pinaleños to the Chiricahuas to the Santa Catalinas along wooded corridors. They were not islands then. They were one place.
Then the climate warmed. The Pleistocene ended around 10,000 years ago. The forests retreated uphill. The desert advanced. Each mountain range became its own enclosed world, separated from every other by terrain that its inhabitants couldn't cross. The islands didn't form by eruption or tectonic drift — no new geography appeared. The sea came up.
The Mount Graham red squirrel lives only near the summit of the Pinaleños. Ten thousand years of the desert between it and other red squirrel populations in nearby ranges. Genetic studies show it's now a distinct subspecies — measurably different from populations in the White Mountains, less than a hundred miles away. The gap is small enough to walk. The barrier might as well be the Pacific.
What's strange is that sky islands are among the most species-rich places in North America. More than half of all North American bird species occur in the Madrean Archipelago, the cluster of ranges along the Arizona–New Mexico–Mexico border. More mammals per unit area than anywhere else in the United States. The isolation that should reduce diversity has instead produced something closer to a collision — species from the temperate north and the neotropical south both find habitat within a few miles of each other, stacked by elevation. A single day's walk from desert floor to summit crosses two distinct ecological zones. The isolation concentrates the overlap.
But part of the abundance may also be a lag. Ten thousand years isn't long enough to lose everything that was present when the forest was continuous. The species richness isn't fully in equilibrium yet. What we're watching may be a slow depletion, invisible at human timescales, that the MacArthur-Wilson theory of island biogeography predicts will eventually drive toward a lower equilibrium number. Not gone — just fewer. The richness as artifact of a connection that no longer exists.
The process that isolated the sky islands is still running. As temperatures rise, the desert advances higher on the mountain flanks. Species that once held range at lower elevations find themselves pushed upward. Fire, invasive species, and warming have all compressed the Mount Graham red squirrel's range. Higher has limits. The mountain ends.
This is the geometry that keeps catching me. An oceanic island can only grow if new land rises from below. A sky island can only grow if the climate cools. The barrier isn't a wall; it's a temperature threshold. The island is defined not by its own edges but by what isn't present in the surrounding lowlands — the absence of the right conditions. Change those conditions and the island doesn't sink. It just ceases to be an island. The sea retreats and takes the island's meaning with it.
The squirrel at the top of Mount Graham didn't choose its isolation. It was sitting in continuous forest ten thousand years ago and the forest changed around it. By the time the process was complete, going down had become impossible — not because anything blocked the way, but because the way down no longer led anywhere the squirrel could live. No wall went up. No flood cut off the valley. Just warmth, moving slowly uphill, taking away the route.
Ten thousand years later: a distinct subspecies, a shrinking range, a mountain that is the whole world.