Grand Canyon National Park - Layers of Life
Before the river makes its plunge into the earth's deep past, it crosses a stretch of tilted rock -- called, aptly enough, the Grand Canyon Super-group -- left over from the last of the vanished mountains. Early in this interlude is a place where vistas open upon a rolling desert valley banked by low cliffs on sandstone. Atop these cliffs, slopes of greenish shale, interleaved with purple, flatten into a broad desert platform, where the Anasazi coaxed crops to grow near spotty springs. Beyond, wide bands of color rise in clear-cut layers along the sculptured temples and distant canyon walls.
The canyon's life is layered, too, though not with the precision of the bands of stone. As the elevation increases, so does the degree of moisture; and temperatures drop. The changing climate creates distinct zones, each a mesh of interwoven life where many plants and animals are confined by their needs and natures. The dividing lines are not sharp. The zones blend into each other, and many mammals and birds move freely back and forth. Differences in soil quality and exposure to the sun, and such vagaries as rising currents of warm air and invisible waterfalls of cool air, mean that the dividing lines are not constant either. But they are there, separating communities of trees and smaller plants, of insects, birds, and beasts.
The North Rim, the high point of the humped Kaibab Plateau, gets much more wet weather than any place else in the canyon -- about 26 inches of precipitation per year, as against the South Rim's 16 inches. Summers are brief, but life is lush. A short distance back from the rim, mule deer browse in deep, still forests of spruce and fir. The spindly trunks of aspen trees grow arched and bent, in memory of the weight of 10-foot snows. Spotted coralroots and other wild orchids bloom amid dense stands of bracken fern on the mossy forest floor. Here and there, wild strawberries spread and white geraniums lift their delicately regal blossoms. Turkeys strut and porcupines waddle among blue columbines and larkspurs, and the hush is sometimes sweetened by the fluted song of a hermit thrush -- or shattered by the strident chatter of a red squirrel, vehemently protesting the presence of a trespasser on its domain.
In places where the forests open onto broad mountain meadows, the yellow blossoms of mountain dandelions, owl clovers, buttercups, goldenrods, and salsifies make a brazen backdrop for the bright reds of Indian paintbrushes, skyrocket gilias, and scarlet buglers. Hummingbirds whir, and migrating monarch butterflies pause over fields of tall, orange butterfly weed to sip nectar and deposit eggs.
Near the rim itself, this verdant world blends into a drier, more open forest of ponderosa pines, much like the forest that covers the South Rim. Shrubby clumps of Gambel oak, mountain mahogany, locust, snowberry, and elderberry flourish among the copper-colored trunks of the tall, orange butterfly weed to sip nectar and deposit eggs.
Near the rim itself, this verdant world blends into a drier, more open forest of ponderosa pines, much like the forest that covers the South Rim. Shrubby clumps of Gambel oak, mountain mahogany, locust, snowberry, and elderberry flourish among the copper-colored trunks of the tall, sedate ponderosas.
The ponderosa forests are home to two of the Grand Canyon's best-known animals: the Abert squirrel on the South Rim, and the Kaibab squirrel on the North. Both are striking tassel-eared squirrels -- large tree-dwellers, with tufted ears and long, flowing tails -- whose lives depend on ponderosa pines. They build their nests high in the great trees' crowns, and live largely on ponderosa seeds and twigs. Those on the South Rim look much like the tassel-eared squirrels found in ponderosa forests throughout the Southwest; though a few black ones appear, most are gray to dark reddish brown, with white bellies and salt-and-pepper tails. But the Kaibab squirrels on the North Rim are quite different: black with pure white tails, they are the only squirrels of their kind in the world. They owe their unique qualities to their long isolation from their kin.
During the most recent Ice Age, when the climate in northern Arizona was much cooler and wetter than it is now, ponderosas grew deep in the canyon, and all the tassel-eared squirrels could mingle together and breed. But as the climate warmed and the land became more arid, the ponderosas were confined to higher elevations. The forests on the North Rim were isolated by the canyon to the south and by deserts in all other directions. Over the intervening 35,000 years or so, the tassel-eared squirrels in these forests, cut off from others of their kind, became a new kind of squirrel.