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The species in an ecosystem can be thought of loosely as a sports team, where each species has a different role to play. While the loss of any player might hurt the team’s prospects, the loss of certain ones, such as the quarterback in football, would be absolutely devastating. In nature, it’s also the case that some species appear to be more vital to the integrity of communities and ecosystems than others.

Species that exert significant effects on biodiversity despite low abundance within the community are called keystone species. Losing one of these species is like the collapse of a stone archway following the removal of its keystone. Keystone species are usually top-level predators like sea stars, sea otters, and jaguars that influence the structure of communities through their feeding activities (Figure 4.9). When restored to Yellowstone National Park, the gray wolf revealed its role as a keystone species. Before the restoration of wolves, woody vegetation along the Lamar River was limited to a scattering of mature cottonwood and willow trees, with very few young trees in the populations. Elk had been feeding on the young cottonwood and willow trees so that few grew to maturity. However, soon after the reintroduction of wolves, elk avoided riverside areas, where they were vulnerable to wolf attacks. With the elk gone, young trees survived at a higher rate (Figure 4.10).

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These effects trickled down through the ecosystem. As willows increased in size and abundance, there was an increase in the feeding and breeding habitat available to birds. Beaver populations grew, too, and physically altered the landscape as they became more abundant (Figure 4.11). Species, like the beaver, that manipulate the physical environment and influence ecosystem structure and processes are called ecosystem engineers. By preying on elk throughout the year, wolves also left behind carrion that other species, including grizzly bears, coyotes, foxes, ravens, and eagles, could feast on.

Foundation species create a physical framework for the community with their own bodies. Think, for example, of a forest, where the trees form a canopy and where shrubs and herbaceous vegetation on the forest floor create a habitat for other species. In the ocean, corals or kelp create a complex structural framework for fish and other animals to live in and feed in (Figure 4.12).

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Willows and cottonwoods, since they appear to have been released from control by elk grazing, may resume the status of foundation species in the Yellowstone National Park ecosystem. In addition to providing food for beavers, willows are a nesting habitat for a diversity of birds, including the endangered Western willow flycatcher. They provide shelter and food to a great diversity of insects, as well as cover for small mammals. By shading the ground, willows change the temperature and moisture conditions experienced by soil organisms, including worms, insects, and fungi. Again, wolves, by chasing elk out of streamside and riverside areas, seem to be releasing willows from herbivore control, further underscoring the keystone role of wolves (Figure 4.13). These far-reaching effects of wolf reintroduction show how the presence, or absence, of a single species in an ecosystem can make a profound difference to biodiversity.

There are many other species that can play important practical roles as we work to conserve ecosystems and landscapes. Viable populations of some species can demonstrate the health of the ecosystems on which they depend. Consequently, monitoring these indicator species, such as the California gnatcatcher, which is associated with coastal sage scrub (see Chapter 3, page 77), can provide information about the state of the ecosystem in which they live. Management designed to sustain populations of indicator species, as well as others such as the grizzly bears of Yellowstone or the spirit bear of the temperate rain forests of western Canada (see Figure 3.21, page 78), is another entry into ecosystem conservation. Indicator species can also act as umbrella species, since protecting them provides protection for the entire ecosystem on which they depend. Yet other so-called flagship species can attract and sustain human interest in protecting ecosystems. Examples of such charismatic species, which have become symbols for ecosystem conservation, include redwood trees, giant pandas, and the great whales.

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