The scientist who developed the concept of life zones, C. H. Merriam, based his theory on research he did in the Grand Canyon  region in 1889 as head of the U.S. Biological Survey. He proposed that plant and animal communities change not only with latitude but also with elevation.
The life zones, or biomes, identified by Merriam are the Lower Sonoran, Upper Sonoran, Transition, Canadian, Hudsonian, and Arctic-Alpine. Grand Canyon encompasses five of the six life zones Merriam identified, often likened to traveling from Mexico to Canada.
Merriam’s system is still used in the western United States, although most scientists today think in terms of biologic communities rather than zones. Even so, plants and animals aren’t aware of the neat scientific boundaries assigned to them. The canyon acts as a barrier to some species and a corridor to others.
Orientation to the sun, rainfall amounts, differences in terrain and soils—these factors create a range of microclimates within biologic communities. Desert species extend higher on sunny ridges and slopes, while deep, shady draws allow higher elevation species to move downward.
The intermixture between neighboring biologic communities is referred to as an ecotone, where flora and fauna mingle in amazing variety. Approximately 1,800 species of plants grow in Grand Canyon, nearly half of the flora found in Arizona, which is considered one of the most botanically diverse states in the country. A dozen plants are endemic, found only in the Grand Canyon area. One is the endangered sentry milk vetch (Astragalus cremnophylax var. cremnophylax). Its Latin species name means “gorge watchman.”
The Lower Sonoran life zone, found below 3,500 feet, primarily within the Inner Gorge , includes plants from the Sonoran and Mojave Deserts. Species vary east to west, with some Great Basin desert plants growing in Marble Canyon , and Mojave Desert species appearing as the canyon approaches Lake Mead.
Temperatures rise above 100°F on summer afternoons, and little rain falls. Saltbush and creosote are common. On talus slopes, Mormon tea, brittlebush, ocotillo, and crucifixion thorn join prickly pear, hedgehog, fishhook, and barrel cacti.
In sharp contrast, along the river and its tributary creeks or nearby springs, riparian communities nourish moisture-loving species. Tributaries may be cooler and more sheltered than the main canyon, nurturing redbuds, cottonwoods, willows, ferns, and monkey-flower.
Along the river, Apache plume, mesquite, catclaw, and saltbush grow above the old predam high-water line. Above the new high-water line, mesquite, coyote willow, arrowweed, and the common reed compete with thickets of tamarisk, an introduced species that has aggressively overtaken Colorado River beaches.
The Upper Sonoran life zone, at 3,500–7,000 feet, includes the yucca, agave, and cacti of the Tonto Platform and the piñon-juniper woodland of the South Rim. Within this zone are desert grassland, chaparral, sagebrush, and other plant communities, often intermixed. Blackbrush predominates the Tonto Platform, giving it a gray-green appearance.
In the canyon’s western reaches, Joshua trees make an appearance. Cliffrose, New Mexico locust, barberry, and other spring-blooming shrubs grow on and below the South Rim , appearing in sunny pockets below the North Rim . Along trails and roadsides, fleabane, asters, phlox, globe mallow, Indian paintbrush, snakeweed, goldeneye, and other wildflowers put on changing displays from spring to fall.
At 7,000–8,000 feet, the ponderosa forests of the Transition zone yield to thickets of Gambel oak near the rims. On the North Rim, lupine and butterweed make purple and yellow carpets below the ponderosas during the summer. Above 8,000 feet the North Rim Canadian zone is home to white fir, while the moister and cooler Hudsonian zone is typified by spruce-fir forests.
These two zones do not have distinct boundaries but interweave with each other according to topography, so they are often referred to in combination as the Boreal life zone. The Boreal life zone receives 25–30 inches of precipitation annually, supporting forests of spruce and fir, with open meadows edged by aspen, shimmering yellow and gold in fall. The first major snowfall often arrives in November.
Temperatures sometimes plummet below 0°F, and snows can last into May or even June. High-meadow lakes form from snowmelt in the spring, attracting coyotes, wild turkeys, deer, elk, and other wildlife.
Grand Canyon’s elevations vary from 1,200 feet at river level to 9,200 feet on the North Rim , creating myriad wildlife habitats that are mostly unbroken within the park’s 1,217,403 acres.
The varied environments support thousands of invertebrates, 17 fish species, 9 amphibian species, 17 reptile species, 355 bird species, and 89 mammal species.
Along the river, you may encounter caddis flies, numerous beetles, moths, and butterflies, such as the lovely yellow-and-black swallowtails. The showy sphinx moth is an important pollinator, often mistaken for a hummingbird.
There are blessedly few mosquitoes, but other biters include midges, fire ants, centipedes, millipedes, and the dreaded cedar gnats that plague piñon-juniper woodlands in late spring. Several scorpion species inhabit the desert areas along the river, including the tiny bark scorpion.
Higher up the canyon walls and along the rims are secretive black widow spiders and tarantulas, often venturing across roads and trails around the time of the summer monsoon. Preying on the harmless giant spiders are tarantula hawks, low-flying blue-black wasps.
The completion of Glen Canyon Dam changed the Colorado River from a sediment-laden river with widely fluctuating flows to a cold, clear river. Since then, several native fish species have become endangered, extirpated, or threatened, including the humpback chub, razorback sucker, and flannelmouth sucker.
Native species are more often spotted near the confluence with the warmer waters of the Little Colorado River . Introduced species include rainbow trout, present in such large numbers that in certain areas they attract bald eagles during spawning season.
Along the river and its tributaries, tree frogs and red-spotted toads serenade campers. Numerous lizard species live in the inner canyon and on the rims, including the chuckwalla, gecko, skink, yellow-backed spiny lizard, tree lizard, jewel-colored collared lizard, and short-horned lizard, colloquially but incorrectly referred to as a “horny toad.” On a hot summer afternoon, when many animals retreat, lizards continue to scramble around rocks and rims.
Snakes do not tolerate extreme heat or cold, spending winter months hibernating underground and reappearing in the spring to sun themselves on rocks. In midsummer, snakes are most active in early morning and at dusk.
Six rattlesnake species inhabit the inner canyon and its rims, including a couple not usually seen outside the area, the Grand Canyon pink rattlesnake and the speckled rattlesnake. Two rattlesnake predators are also present, with gopher snakes (also known as bull snakes) more common on the rim and king snakes seen more often in the river corridor.
From tiny black-chinned hummingbirds to enormous condors, birds are numerous throughout the canyon and its surrounding forests and woodlands. The diverse environments are a birder’s paradise, with more than 200 species on the North Rim  alone.
Willow flycatchers, phoebes, and kingfishers catch insects along the river while mallards, mergansers, teals, goldeneyes, and other waterfowl swim its waters. Great blue herons and spotted sandpipers ply the river’s edges. Canyon wrens sing from the canyon’s rocky walls, a descending, flutelike trill that delights hikers and river runners.
Peregrine falcons nest in cliffs, and red-tailed hawks and kestrels are fairly common. In the fall, the canyon becomes a flyway for other hawk species. Through the month of October, members of HawkWatch International station themselves at East Rim overlooks and count migrating raptors, usually 10,000–12,000 each season. (Public participation is welcome.)
Ponderosa forests are lively with scrub jays, Steller’s jays, woodpeckers, chickadees, towhees, wild turkeys, and other species. The most ubiquitous canyon avian is the common raven, exceptionally clever and equally at home raiding a river campsite, soaring across the canyon, or entertaining visitors at rim overlooks.
Mule deer often wander around the North Rim’s Bright Angel Point  and the South Rim’s Grand Canyon Village . Elk graze the high-country meadows between Jacob Lake and the North Rim in the mornings and evenings. Though they do occasionally wander the rim, desert bighorn sheep are seen more often scrambling inner canyon cliffs.
These large prey animals attract the canyon’s largest predator, the mountain lion. Coyotes are more likely heard than seen, and black bears, bobcats, and gray foxes are reclusive.
Many mammals are nocturnal or crepuscular, active around dawn and dusk. Hike the North Rim’s Transept Trail  early in the morning to see Kaibab squirrels in their native ponderosa pine forest. Found only on the Kaibab Plateau , the squirrels have been designated a National Natural Landmark. Their more common South Rim cousins, Abert squirrels, are often spotted near Grandview Point .
Trails along the rim and into the canyon are busy with chipmunks, golden-mantled ground squirrels, and rock squirrels. Unfortunately, accustomed to handouts, rodents and ringtails raid backpackers’ food stores. Deer mice, kangaroo rats, packrats, and, in riparian areas, raccoons and skunks can make unwelcome nighttime campsite visits.