One Hundred Hikes in Yosemite

The Living Yosemite — Biotic Influences

Soils can affect the type and amount of vegetation, but vegetation can also create and change soils. A soil is more than just a combination of loose, moist sand and gravel—it also has an organic component. Each year the Merced River deposits sterile silt, sand, and gravel at the south end of Washburn Lake. Grasses and sedges take root in these new accumulations and stabilize them. Then, as these plants die over the years, they add an organic component to the sediments, converting them to soil. As the soil develops here, it becomes ripe for invasion by willows, which further modify the new soil. These trap more sediments, and they add detritus to the soil, enriching it and making it more suitable for other species. Additionally, transpiration by both herbs and shrubs helps to lower the water table: aspens and lodgepole pines can now invade. In this way, a sterile beach eventually becomes—through activities by a succession of plant species—a rich forest soil.

Animals also influence the development of soils and thus affect the development of vegetation. Most of the Yosemite landscape stands above the range of the lowly earthworm, but at mid-elevations, nature has other soil processors. Foremost among these is the industrious pocket gopher, who works year-round, even in the cold of winter when other rodents are either hibernating or living off their stored cache of food. Winter is actually a safer time for the gopher to work, for it can burrow along at the base of a snowpack without danger from its summer predators—hawks, owls, gopher snakes, weasels, badgers, foxes, and coyotes. After the snow melts, the gopher's winter tunneling appears as "gopher ropes." (After a gopher digs a tunnel through the snow, he later fills it with soil from his diggings in the ground beneath the snow. When the snow melts, this core of soil is then exposed, looking like a piece of thick rope.)

In a mountain meadow a gopher population will churn up tons of soil each year. This process has numerous benefits all leading to the development of a richer soil. To a much lesser extent, burrowing and digging by ground squirrels, moles, badgers, and coyotes also contribute to soil development. And we must not forget the decomposers—bacteria, fungi, lichens, invertebrates, and even a few plants, without whom dead plants and animals would continually accumulate until the entire forest lay smothered beneath their mass. Decomposers, which are usually minute and unseen, busily convert dead plants and animals into litter and humus. However, because too much litter and humus can prevent seedling germination, fire is an integral part of the ecosystem in all but Yosemite's subalpine and alpine areas, which are too sparse to support forest fires.

It has been said that the giant sequoia might not be around today were it not for the roles played by fire, the chickaree (Douglas squirrel), and a cone-boring beetle, all three aiding with the dispersal and germination of seeds. Momentarily we'll overlook the role of fire and concentrate on the roles of organisms. More than 150 species of insects depend in part on the giant sequoia, and it is very dependent on one beetle. This tree is not alone in this respect. In Yosemite, insect species outnumber plant species about ten to one. Without insects, most wildflowers would disappear for lack of pollination. Then too, wildflowers receive aid from birds and rodents in the form of seed planting and seed dispersal. Even preying on plants is beneficial; otherwise plants would undergo a population explosion, cover the earth, and die in their own debris.

Published: 29 Apr 2002 | Last Updated: 15 Sep 2010
Details mentioned in this article were accurate at the time of publication


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