January 16, 2025

A keystone species is an organism that helps define an entire ecosystem. Without its keystone species, the ecosystem would be dramatically different or cease to exist altogether.

Keystone species have low functional redundancy. This means that if the species were to disappear from the ecosystem, no other species would be able to fill its ecological niche. The ecosystem would be forced to radically change, allowing new and possibly invasive species to populate the habitat.

Any organism, from plants to fungi, may be a keystone species; they are not always the largest or most abundant species in an ecosystem. However, almost all examples of keystone species are animals that have a huge influence on food webs. The way these animals influence food webs varies from habitat to habitat.

An example of a predator acting as a keystone species is the presence of gray wolves in the Greater Yellowstone Ecosystem. The Greater Yellowstone Ecosystem (GYE) is an enormous and diverse temperate ecosystem stretching across the boundaries of the U.S. states of Montana, Wyoming, and Idaho. The GYE includes active geothermal basins, mountains, forests, meadows, and freshwater habitats.

The elk, bison, rabbit, and bird species in the Greater Yellowstone Ecosystem are at least partly controlled by the presence of wolves. The feeding behavior of these prey species, as well as where they choose to make their nests and burrows, are largely a reaction to wolf activity. Scavenger species, such as vultures, are also controlled by the wolf activity.

When the U.S. government designated land for Yellowstone National Park in the late 19th century, hundreds of wolves roamed the GYE, preying primarily on abundant herds of elk and bison. Fearing the wolves’ impact on those herds, as well as local livestock, governments at the local, state, and federal level worked to eradicate wolves from the GYE. The last remaining wolf pups in Yellowstone were killed in 1924.

This started a top-down trophic cascade in the Greater Yellowstone Ecosystem. A trophic cascade describes changes in an ecosystem due to the addition or removal of a predator. A top-down trophic cascade describes changes that result from the removal of an ecosystem’s top predator. (A bottom-up trophic cascade describes changes that result from the removal of a producer or primary consumer.)

Lacking an apex predator, elk populations in Yellowstone exploded. Elk herds competed for food resources, and plants such as grasses, sedges, and reeds did not have time or space to grow. Overgrazing influenced the populations of other species, such as fish, beaver, and songbirds. These animals rely on plants and their products—roots, flowers, wood, seeds—for survival.

The physical geography of the Greater Yellowstone Ecosystem was also impacted by the loss of wolves and subsequent elk overgrazing. Stream banks eroded as wetland plants failed to anchor valuable soil and sediments. Lake and river temperatures increased as trees and shrubs failed to provide shaded areas.

Starting in the 1990s, the U.S. government began reintroducing wolves to the Greater Yellowstone Ecosystem. The results have been noteworthy. Elk populations have shrunk, willow heights have increased, and beaver and songbird populations have recovered.

For the full article, you can visit this National Geographic article: https://education.nationalgeographic.org/resource/role-keystone-species-ecosystem/