A keystone species is an organism that has a disproportionately large impact on its natural environment relative to its abundance. Ecologist Robert T. Paine coined this term in 1969, likening it to the wedge-shaped stone at the top of an archway that locks other stones into position; if removed, the arch could collapse. These species play a defining role in maintaining the structure and health of an ecosystem, affecting other organisms and influencing the types and numbers of species within a community.
Immediate Ecological Shifts
The removal of a keystone species triggers immediate changes within an ecosystem. When a keystone predator is removed, for instance, populations of its primary prey species experience rapid growth. This increase in herbivore numbers can lead to the overconsumption of vegetation, altering plant communities. Conversely, if a keystone species that serves as a primary producer or food source disappears, other species that directly rely on it face immediate food scarcity, leading to starvation.
These direct impacts can quickly degrade habitats, as the physical structures or food sources maintained by the keystone species diminish. Such changes in resource availability and population dynamics disrupt existing community structures and species interactions.
System-Wide Cascading Effects
The immediate ecological shifts caused by the removal of a keystone species initiate broader, indirect consequences known as trophic cascades. In a top-down trophic cascade, the removal of an apex predator allows its prey populations to flourish, which can decimate the next lower trophic level, like plant life. This “domino effect” reshapes food web dynamics, impacting species not directly interacting with the keystone species.
Beyond direct feeding relationships, keystone species also influence habitat structure and nutrient cycling. Some species, categorized as ecosystem engineers, physically modify their environment, creating or maintaining habitats for many others. Their disappearance can lead to the loss or degradation of these modified habitats, affecting dependent species and altering the physical landscape. Changes in plant composition due to altered grazing patterns can also impact the quality and quantity of organic matter returned to the soil, disrupting nutrient cycles like carbon, nitrogen, and phosphorus. This can reduce soil fertility, impacting plant growth and the species that rely on those plants. The cumulative effect is a shift in species dominance, resulting in reduced biodiversity as formerly suppressed species become dominant, outcompeting others.
Fundamental Ecosystem Reorganization
Over a longer timeframe, the consequences of keystone species removal can lead to a fundamental reorganization of the entire ecosystem. The disruptions initiated by their absence can cause an ecosystem to transition into a completely different state, a “state shift.” This new state may bear little resemblance to the original ecosystem in terms of species composition, physical structure, and ecological processes.
Such profound changes result in a loss of biodiversity, as many species are unable to adapt to the altered conditions or the absence of resources and interactions previously facilitated by the keystone species. The ecosystem may become less resilient to further environmental disturbances, making it more vulnerable to invasive species or climate changes. The intricate web of life that once characterized the ecosystem can unravel, leading to a simplified, less stable, and less productive environment.
Case Studies of Removal Impacts
The impact of keystone species removal is evident in various ecosystems around the world. One example involves sea otters in the North Pacific’s kelp forests. Sea otters prey on sea urchins, which are herbivores that graze on kelp. When sea otter populations were reduced due to hunting, sea urchin populations exploded. These urchins consumed kelp, transforming kelp forests into barren “urchin barrens” and leading to a loss of habitat and biodiversity for marine species.
Another case is the reintroduction of gray wolves to Yellowstone National Park in the mid-1990s, following their earlier eradication. Before reintroduction, the absence of wolves, an apex predator, led to an overpopulation of elk. These abundant elk overgrazed willow and aspen trees along riverbanks, which degraded riparian habitats and reduced plant diversity. With the wolves’ return, elk behavior changed, and their numbers were controlled, allowing vegetation to recover. This recovery subsequently benefited other species, such as beavers, and altered the physical course of rivers due to increased bank stability.
African elephants also serve as keystone species in savanna ecosystems. They act as “ecosystem engineers” by pushing over trees and consuming shrubs, which helps maintain open grasslands and prevents savannas from becoming dense woodlands. Elephants also dig for water in dry riverbeds, creating waterholes used by numerous other wildlife species, and disperse seeds through their dung, aiding plant growth. Their removal would lead to savannas being overtaken by woody vegetation, reducing habitat for many grazing animals and impacting water availability for the broader ecosystem.