Keystone Species
A keystone species is an organism that has a disproportionately large effect on its ecosystem relative to its abundance or biomass; if removed, the ecosystem would change dramatically or collapse entirely. The concept was introduced by ecologist Robert T. Paine in 1969 following his experiments showing that removing sea stars (Pisaster ochraceus) from intertidal communities caused mussels to dominate and biodiversity to plummet. Keystone species can be predators, ecosystem engineers, or mutualists, and their identification is critical for conservation and wildlife management.
Examples of Keystone Species and Their Ecological Roles
| Keystone Species | Ecosystem | Role | Effect of Removal |
|---|---|---|---|
| Sea otter (Enhydra lutris) | Pacific kelp forest | Predator of sea urchins | Urchin boom → kelp forest collapse |
| African elephant | Savanna | Ecosystem engineer (creates waterholes, clears bush) | Loss of open grassland habitat |
| Gray wolf (Canis lupus) | Yellowstone | Apex predator (controls deer) | Overgrazing, riverbank erosion |
| Fig tree (Ficus spp.) | Tropical rainforest | Year-round fruiting food source | Collapse of frugivore populations |
| Beavers (Castor canadensis) | Temperate streams | Ecosystem engineer (dam-building) | Loss of wetland habitats |
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Related Terms
Biodiversity
Biodiversity refers to the variety of life on Earth at all its levels — from genes and species to ecosystems — and the ecological processes that support this variety. It is typically measured at three levels: genetic diversity (variation within species), species diversity (number and abundance of species in an area), and ecosystem diversity (variety of habitats, communities, and ecological processes). High biodiversity generally confers ecosystem stability, resilience to disturbance, and a wider range of ecosystem services such as food, medicine, and clean water for human societies.
Invasive Species
An invasive species is a non-native (exotic or alien) organism introduced — intentionally or accidentally — to a new environment outside its natural range, where it spreads rapidly and causes ecological, economic, or health damage. Invasive species are one of the leading causes of global biodiversity loss, often outcompeting native species for resources, introducing novel diseases, altering food webs, and transforming physical habitats. Their success in new environments often stems from the absence of natural predators, parasites, or competitors that regulate their populations in their native range.
Ecological Succession
Ecological succession is the process of change in the species composition of an ecological community over time, following a disturbance or the formation of a new habitat. Primary succession occurs on bare, previously uncolonised substrate (e.g., newly formed volcanic rock), beginning with pioneer species such as lichens and mosses that gradually modify the environment for subsequent species. Secondary succession occurs in areas where a community has been disturbed but soil remains (e.g., after a forest fire), proceeding more rapidly to a stable climax community because soil and seed banks persist.
Named by analogy with the architectural keystone — the central stone at the top of an arch that holds all others in place and without which the arch collapses. The ecological term was coined by Robert T. Paine in his 1969 paper in the American Naturalist.