In ecology, a keystone species is a species that has a disproportionately large impact on its environment relative to its abundance. A keystone species plays a key role in maintaining the health and diversity of an ecosystem and is significant in conservation efforts.
- A keystone species has a major impact on other species in an ecosystem, even if it is not abundant.
- The gray wolf in Yellowstone is a good example of a keystone species. Re-introducing the wolf controls the herbivores in the park, massively impacting the diversity of the ecosystem. Even though wolves constitute a tiny fraction of the organisms, they have an immense effect.
- While familiar examples of keystone species are often mammals, they can be other animals, including insects, as well as plants, fungi, or other forms of life.
American zoologist Robert T. Paine introduced the term “keystone species” in 1969. During his studies of the intertidal ecosystem of the Pacific Northwest, Paine observed that certain species, such as the sea star Pisaster ochraceus, played a critical role in maintaining the structure and function of the ecosystem. When Paine artificially removed the sea star from a region, species diversity quickly plummeted, ultimately wiping out the benthic algae in the area. Removing the sea star reduced the diversity in the study area from fifteen species to eight. Paine likened the role of significant species to the keystone in an arch, which is a small part of the arch but holds the entire structure together.
Examples of Keystone Species
There are numerous examples of keystone species in ecosystems:
- Gray wolves control herbivores, which otherwise overgraze an area. Overgrazing reduces the number and diversity of plants and trees, reducing the available habitat for other species. Indications of the benefits of re-introduction of wolves into their native habitats include increased willow, beaver, and songbird populations.
- Sea otters regulate the sea urchin population, keeping their consumption of kelp forest in check. The kelp forests are host for a number of other species.
- The North American beaver builds dams which modify the habitat of an ecosystem and determine which species live there.
- The Saguaro cactus is a keystone species because it is the habitat for numerous other species.
- Bees are keystone species that act as pollinators, ensuring plant diversity in an ecosystem. In turn, the plants support insects and other animals and influence the nutrient profile and weather of an ecosystem.
Types of Keystone Species
One way of classifying keystone species is according to the way they impact other species:
- Predators: These species help control the populations of their prey, which in turn has cascading effects on the ecosystem. Examples include the sea otter, which maintains kelp forest ecosystems by preying on sea urchins, and the gray wolf, whose reintroduction to Yellowstone National Park changes elk behavior and increases vegetation.
- Mutualists: These species provide essential services to other species in their ecosystem, forming symbiotic relationships that benefit both parties. Examples include pollinators, such as bees, which are essential for the reproduction of flowering plants, and the oxpecker, which feeds on parasites found on large mammals like buffalo and giraffes.
- Ecosystem Engineers: These species create or modify habitats, shaping the physical environment and influencing the distribution of other species. Examples include the beaver, which creates ponds and wetlands through dam construction, and the coral polyps that form coral reefs, which provide habitat for a diverse array of marine organisms.
Keystone species are often discussed in conjunction with other categories of species:
- Flagship species: Flagship species are charismatic and well-known species that are often used to raise public awareness and support for conservation efforts. Examples include the giant panda, African elephant, and Bengal tiger.
- Umbrella species: Umbrella species are those with large habitat requirements whose conservation indirectly protects many other species that share the same habitat. Examples include the northern spotted owl in the Pacific Northwest and the Florida panther.
- Foundation species: A foundation species exerts a significant influence over an ecosystem by creating or maintaining a habitat. For example, corals form reefs, which are the habitat for countless species of marine life.
- Ecosystem engineer: An ecosystem engineer is a species that directly or indirectly creates, modifies, or maintains habitats, influencing the distribution and abundance of other species. This term overlaps with the concept of keystone species, but not all ecosystem engineers are necessarily keystone species.
- Indicator species: An indicator species is one which acts as an early warning sign of ecosystem change. For example, oysters are an indicator species in a coastal marine ecosystem.
Importance of Keystone Species
Keystone species play a vital role in maintaining the structure, function, and stability of their ecosystems. Their removal or decline can have far-reaching consequences, leading to the loss of biodiversity, alteration of ecosystem processes, and even ecosystem collapse. Identifying and protecting keystone species is essential for effective conservation and management strategies.
Limitations of the Concept
Despite its importance, the keystone species concept has some limitations. It can be challenging to identify keystone species, as their impact may not be immediately apparent or easily measurable. Additionally, ecosystems are complex and dynamic, and the role of a species within an ecosystem may change over time or under different conditions. Also, the conservation of an ecosystem requires a holistic approach that goes beyond the protection of individual species.
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- Caro, Tim (2010). Conservation by proxy: indicator, umbrella, keystone, flagship, and other surrogate species. Washington, DC: Island Press. ISBN 9781597261920.
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- Paine, R. T. (1969). “A Note on Trophic Complexity and Community Stability”. The American Naturalist. 103 (929): 91–93. doi:10.1086/282586