A keystone species is an organism that plays a significant role in its environment. Like the keystone in an arch, this species holds the ecosystem together. If removed, the structures it supports will collapse. Different environments can have entirely unique makeups and a keystone species in one environment may not be one in another. Identification of such species as important, as they can be a critical factor to consider when developing conservation initiatives and other projects.
Typically, the population of a keystone species is relatively small, with the animals, plants, or other organisms making up a small percentage of the overall biomass. The influence exerted by the organisms is disproportionate to their overall numbers in the natural environment. This can make them difficult to single out and identify as keystone species, as scientists may not realize the importance of a seemingly minor population of plants or animals until it is too late.
Many species considered keystones in their environments are predators. While predator populations tend to be small, they are critical members of the ecosystems they live in. Predators keep prey numbers within reasonable limits, reducing the risk that prey will overrun the environment. When prey such as deer become too numerous, the natural balance can be disturbed. The animals continue multiplying, strip the environment of all edible plants, and eventually starve. With predators, their numbers are kept in check, allowing fewer animals to survive but conserving resources so that they are all healthy.
Another example of a keystone species is a natural engineer like a beaver. Beavers literally shape the environment around them by constructing dams. These dams have a profound impact on the natural world, creating habitat and a source of fresh water. Other engineers can shape plant populations, as well as the natural environment. Other keystone species are mutualists, organisms that maintain mutually beneficial relationships with other organisms.
When a keystone species is disturbed, it can have a rippling impact. As the numbers decline, the function they were serving is no longer performed. Interconnected systems may start to fall apart as the environment struggles to adjust to the loss of the keystone species. A common problem emerges when animal and plant populations that are normally kept at low numbers by prey start to multiply, driving out weaker organisms and decreasing biodiversity. The decrease in diversity in an ecosystem can leave it very vulnerable to disease, climate change, and other events.