Yosemite National Park (California, USA)
Starting 10 million years ago, vertical movement along the Sierra fault started to uplift the Sierra Nevada. Subsequent tilting of the Sierra block and the resulting accelerated uplift of the Sierra Nevada increased the gradient of western-flowing streams. The streams consequently ran faster and thus cut their valleys more quickly. Additional uplift occurred when major faults developed to the east, especially the creation of Owens Valley from Basin and Range-associated extensional forces. Uplift of the Sierra accelerated again about two million years ago during the Pleistocene.
The uplifting and increased erosion exposed granitic rocks in the area to surface pressures, resulting in exfoliation (responsible for the rounded shape of the many domes in the park) and mass wasting following the numerous fracture joint planes (cracks; especially vertical ones) in the now solidified plutons. Pleistocene glaciers further accelerated this process and the larger ones transported the resulting talus and till from valley floors.
Numerous vertical joint planes controlled where and how fast erosion took place. Most of these long, linear and very deep cracks trend northeast or northwest and form parallel, often regularly spaced sets. They were created by uplift-associated pressure release and by the unloading of overlying rock via erosion.
A series of glaciations further modified the region starting about 2 to 3 million years ago and ending sometime around 10,000 BP. At least four major glaciations have occurred in the Sierra Nevada, locally called the Sherwin (also called the pre-Tahoe), Tahoe, Tenaya, and Tioga. The Sherwin glaciers were the largest, filling Yosemite and other valleys, while later stages produced much smaller glaciers. A Sherwin-age glacier was almost surely responsible for the major excavation and shaping of Yosemite Valley and other canyons in the area.
Glacial systems reached depths of up to 4000 feet (1200 m) and left their marks in the Yosemite area. The longest glacier in the Yosemite area ran down the Grand Canyon of the Tuolumne River for 60 miles (95 km), passing well beyond Hetch Hetchy Valley. Merced Glacier flowed out of Yosemite Valley and into the Merced River Gorge. Lee Vining Glacier carved Lee Vining Canyon and emptied into Lake Russel (the much-enlarged ice age version of Mono Lake). Only the highest peaks, such as Mount Dana and Mount Conness, were not covered by glaciers. Retreating glaciers often left recessional moraines that impounded lakes such as the 5.5 mile (8.9 km) long Lake Yosemite (a shallow lake that periodically covered much of the floor of Yosemite Valley).
With its scrubby sun-baked chaparral, stately groves of pine, fir, and sequoia, and expanses of alpine woodlands and meadows, Yosemite National Park preserves a Sierra Nevada landscape as it prevailed before Euro-American settlement. In contrast to surrounding lands, which have been significantly altered by logging, the park still contains some of old-growth forest. Taken together, the park’s varied habitats support over 250 species of vertebrates, which include fish, amphibians, reptiles, birds, and mammals.
Along much of Yosemite’s western boundary, habitats are dominated by mixed coniferous forests of Ponderosa Pine, Sugar Pine, Incense-cedar, White Fir, and Douglas Fir, and a few stands of Giant Sequoia, interspersed by areas of Black Oak and Canyon Live Oak. A relatively high diversity of wildlife species are supported by these habitats, due to relatively mild, lower-elevation climate and the mixture of habitat types and plant species. Wildlife species typically found in these habitats include American Black Bear, Bobcat, Gray Fox, Mule deer, Mountain Kingsnake, Gilbert’s Skink, White-headed Woodpecker, Brown Creeper, Spotted Owl, and a wide variety of bat species. In the case of bats, large snags are important as roost sites.
Going higher in elevation, the coniferous forests become purer stands of Red Fir, Western White Pine, Jeffrey Pine, Lodgepole Pine, and the occasional Foxtail pine. Fewer wildlife species tend to be found in these habitats, due to their higher elevation and lower complexity. Species likely to be found include Golden-mantled Ground Squirrel, Chickaree, Fisher, Steller’s Jay, Hermit Thrush, and Northern Goshawk.
Reptiles are not common, but include Rubber Boa, western fence lizard, and Northern Alligator Lizard.
As the landscape rises, trees become smaller and more sparse, with stands broken by areas of exposed granite. These include Lodgepole Pine, Whitebark Pine, and Mountain Hemlock that, at highest elevations, give way to vast expanses of granite as treeline is reached. The climate in these habitats is harsh and the growing season is short, but species such as Pika, Yellow-bellied Marmot, White-tailed Jackrabbit, Clark’s Nutcracker, and Rosy Finch are adapted to these conditions. Also, the treeless alpine habitats are the areas favored by Sierra Nevada Bighorn Sheep. This species, however, is now found in the Yosemite area only around Tioga Pass, where a small, reintroduced population exists.
At a variety of elevations, meadows provide important, productive habitat for wildlife. Animals come to feed on the green grasses and use the flowing and standing water found in many meadows. Predators, in turn, are attracted to these areas. The interface between meadow and forest is also favored by many animal species because of the proximity of open areas for foraging and cover for protection. Species that are highly dependent upon meadow habitat include Great Grey Owl, Willow Flycatcher, Yosemite Toad, and Mountain Beaver.
Despite the richness of high-quality habitats in Yosemite, the California golden bear, California Condor, and Least Bell’s Vireo have become extinct in the park within historical time, and another 37 species currently have special status under either California or federal endangered species legislation. The most serious current threats to Yosemite’s wildlife and the ecosystems they occupy include loss of a natural fire regime, exotic species, air pollution, habitat fragmentation, and climate change. On a more local basis, factors such as road kills and the availability of human food have affected some wildlife species.
The black bears of Yosemite were once famous for breaking into parked cars to steal food. They were also an encouraged tourist sight for many years at the park’s garbage dumps, where bears congregated to eat park visitors’ garbage and tourists gathered to photograph the bears. Increasing encounters between bears and humans and increasing damage to property led to an aggressive campaign to discourage bears from relying on human food or interacting with people and their property. The open-air dumps were closed; all trash receptacles were replaced with bear-proof receptacles; all campgrounds were equipped with bear-proof food lockers so that people would not leave food in their vehicles, which were easy targets for the powerful and resourceful bears. Because bears who show aggression towards people usually are eventually destroyed, park personnel have continued to come up with innovative ways to have bears associate humans and their property with unpleasant experiences, such as being hit with rubber bullets. Today, about 30 bears a year are captured and ear-tagged and their DNA is sampled so that, when bear damage occurs, rangers can ascertain which bear is causing the problem.