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Keystone Species and Trophic Cascades

Two of our primary goals at Wildlands Network are to protect ecologically important lands and to safeguard threatened species. But maintaining certain types of species interactions is also critical to maintaining natural processes, ecological resilience, and the diversity of life.

Brown bear standing on a rock in a waterfalls
Photo: Robert Long

To varying degrees, everything in the natural world is connected to everything else. Plants and animals are connected through the food web, and each species can affect the composition of the landscape. The presence or absence of one or several key species often determines the distribution and abundance of many other species.

Scientific research from a variety of ecosystems has revealed that some species are particularly vital to functional ecosystems.

Strongly Interactive Species

A species is considered strongly interactive when its absence leads to significant changes in the ecosystem(s) it inhabits. Such changes include structural or compositional modifications, alterations in the import or export of nutrients, loss of resilience to disturbance, and decreases in native species diversity.

Close-up of a beaver swimming with a stick in his or her mouth.
American beaver. Photo: William C. Gladish

Species whose activities affect and enhance physical or biological habitat structure have been referred to as ecosystem engineers. These engineers significantly modify their habitat in ways that often increase local species diversity. Beavers, for instance, create wetlands by building dams in streams. Elephants can convert woodlands into scrublands or grasslands. Other examples of ecological engineering include mound-building by termites, burrowing and grazing by prairie dogs, and habitat conversion by bison.

Keystone Species

Ecologist Robert Paine first used the term keystone species in his classic 1969 publication, “A Note on Trophic Complexity and Community Stability” (The American Naturalist). Keystones are species whose activities maintain other species and habitat diversity, and whose effects are disproportionate to their abundance.

When the density of a keystone species falls below some threshold, the species diversity in the area may decrease, triggering ecological chain reactions and ending with degraded or simplified ecosystems. Apex predators like cougars and wolves have been identified as keystone species.

Foundation Species

Many strongly interactive species are too abundant to be classified as keystone species, and are considered instead to be foundation species. Examples include a number of deciduous and mast-producing trees (trees that produce edible nuts and seeds), such as fig trees in the tropics and oaks and chestnuts in temperate regions. Bison, in their historical numbers, are another case in point.

Trophic Cascades

The elimination of apex predators destabilizes ecosystems, setting off chain reactions that eventually cascade down the trophic ladder (or food web) to the lowest rung, often reducing habitat complexity and species diversity. Paine coined the term trophic cascade to describe such multi-level trophic interactions. Once again, the altered state of the system is less biodiverse.

Paine famously removed a species of starfish from study plots on Washington’s coast and noted an unpredictably intense set of changes occurring in his plots, which were quickly overtaken by mussels that were usually kept in check by the starfish.

Another widely known example is provided by gray wolves in Yellowstone National Park, where the wolves’ local extinction led to an irruption in the numbers of elk—causing changes in vegetation structure, species composition, and diversity. Without wolves keeping elk in check, the elk achieved much higher population densities and shifted their behavior to a more concentrated feeding pattern, which led to the virtual disappearance of major vegetation types such as aspen and willow-beaver wetlands in some areas. Once wolves were restored to the park, over-browsed vegetation was given a chance to recover and the park’s natural vegetative pattern began to return—as did the beavers and other wetland species.

A coyote turns to look toward photographer, with lake in distance.
Photo: William C. Gladish

Coyote numbers also increased when wolves were missing from Yellowstone. Because coyotes are major predators of young pronghorns, the pronghorn population declined in the absence of wolves. When wolves returned, coyote numbers dropped by about 50%, followed by an increase in pronghorn numbers by about 50%.

Large carnivores are not the only type of keystone species (remember the starfish example). Other examples include prairie dogs, flying squirrels, and sapsuckers.

What You Can Do

The successful conservation of North America’s natural heritage requires us to preserve, and where necessary, reintroduce populations of keystone and other strongly interactive species. These species must be conserved at ecologically effective population numbers—in other words, in enough abundance that the species fulfill the ecological roles described above.

Protecting habitat is pivotal but insufficient on its own for saving biodiversity. The habitats we conserve will unravel if not regulated by the full complement of native keystone and other strongly interactive species.