A keystone species is an organism that has a disproportionately large effect on its natural environment relative to its abundance. These species are important for maintaining the overall structure and health of an ecosystem. They influence which other types of plants and animals constitute that ecosystem, making their presence fundamental for ecological stability. This article explores the consequences that unfold when a keystone species is removed from its natural habitat.
Immediate Ecological Disruption
The removal of a keystone species triggers immediate impacts on dependent species. For instance, if a keystone predator is eliminated, its prey might experience a rapid population increase. This unchecked growth can lead to overconsumption of vegetation or other resources, affecting species that rely on them.
Conversely, if a keystone herbivore is removed, the plant species it consumes may overgrow, altering the physical structure of the habitat. These shifts quickly change the availability of food or shelter for other organisms. This initial disruption sets off a chain reaction, affecting species linked to the keystone species in the food web or through habitat modification.
Cascading Impacts Across the Food Web
Beyond immediate effects, the removal of a keystone species often initiates widespread “trophic cascades,” which are indirect interactions that reverberate throughout the food web. These cascades occur when changes at one trophic level, such as the disappearance of a top predator, affect non-adjacent trophic levels. For example, a decline in predators can lead to an increase in herbivores, resulting in a decrease in primary producers like plants.
This domino effect impacts species not directly interacting with the keystone species but part of the interconnected ecological network. The food web’s stability and complexity can decrease as these links are broken. Such disruptions can lead to a simplification of the ecosystem, making it less resilient to further disturbances.
Long-Term Ecosystem Transformation
Over time, immediate disruptions and cascading effects lead to broader transformations of the ecosystem’s structure and function. A consequence is the widespread loss of biodiversity, with species extinctions or population declines across various groups. The physical environment can also degrade, leading to habitat loss for many species.
The ecosystem becomes less resilient to disturbances like disease or climate change. This instability can result in a shift in the ecosystem’s character, sometimes leading to new species becoming dominant. Such changes can alter the landscape and the types of life it can support, sometimes irreversibly.
Illustrative Examples
The ecological importance of keystone species is demonstrated through examples. Sea otters, for instance, are keystone predators in kelp forest ecosystems along the Pacific Coast. They feed on sea urchins, preventing overgrazing of kelp forests. When sea otter populations declined due to hunting, sea urchin numbers exploded, leading to the destruction of kelp forests and reduced marine biodiversity.
Another example is the reintroduction of wolves to Yellowstone National Park in the mid-1990s. Before their return, elk populations proliferated, overgrazing riparian vegetation. The reintroduction of wolves helped control elk numbers and altered their grazing patterns, allowing riparian vegetation to recover, benefiting beavers, fish, and birds.
Similarly, the ochre sea star (Pisaster ochraceus) in intertidal zones is a keystone species by preying on mussels. Without the sea star, mussels can outcompete other species, leading to a monoculture and a reduction in species diversity within the tide pool community. Ecologist Robert Paine’s experiments demonstrated that removing this predator altered the ecosystem, reducing species by nearly half.