It is a species that had a disproportionally large impact on its environment when compared to its population. They play a critical role in maintaining the stability of an ecological community. It affects many other organisms in the ecosystem, and helps to decide the types and numbers of various other species of the community. An example is the grizzly bear in many forest ecosystems.$^1$
If a keystone species is removed, an ecosystem may experience may undergo a dramatic shift even though that species was a small part of the ecosystem through bio mass or productivity measurements.$^1$
Protection of keystone species can bring overall stability(balance) and biodiversity to the ecosystem.$^1$
If a keystone species like the grey wolf is removed from the ecosystem, the population of their prey(ex. elk) would exponentially increase. This will have a severe effect on the rest of the organisms in the rest of the ecosystem.$^1$
The purple sea star is a keystone predator. They prey on mussels that would dominate the ecosystem. It allows for greater biodiversity within its habitat.$^1$
The American alligator is another keystone species, found in the southeast. They prey on fish, amphibians, reptiles, birds, and mammals. It is also an example of a keystone modifier (or) ecosystem engineer.$^1$
The black-tailed praire dog is another example of a keystone modifier. It maintains the grassland ecosystem. This benefits other grassing species like cattle. They burrows aerate compacted soil and allow water water to pernitrate deeper into the ground. Other species use these burrows for shelter. They are also an important food source.$^1$
The red-napped sap sucker is a keystone modifier. It excavates nests in the aspen trees. These nests form shelter for many other bird species.$^1$
Pacific salmon are an example of keystone prey. They play a vital role in the Pacific Northwest ecosystems. They directly benefit many birds, mammals, reptiles, and amphibians through ecosystem nutrient loading(food chain, food web).$^1$
The black-bellied salamander is another keystone prey species. It provides a large amount of protein biomass for predators in stream ecosystems.$^1$
Hummingbirds are the most notable example of keystone mutualists. They are also known as a link species.$^1$
The quaking aspen canopy is an example of a keystone host species. They create an open canopy that harbors diversity on the forest floor. It attracts many insects not found any where else in the ecosystem.$^1$
There are generally four categories of keystone species: $^1$ $^3$