There are certain animals whose presence or absence has a substantial effect on their surrounding environment. These animal species are called keystone species. Keystone species are crucial to the balance of an ecosystem.
Keystone species usually take the form of predators which keep potentially harmful herbivores from decimating the population of crucial plant species. A good example would be the sea otter, which keeps the population of sea urchins in check so that they cannot destroy the kelp forests which provide a habitat for various species. Another example would be the jaguar, which prevents the population of small mammals from exploding, which would throw of the balance of the forest’s ecosystem.
There are also keystone species which do not operate by keeping herbivores in check. These species engineer the environment and change the habitat in which other species live. The beaver makes dams which change rivers and streams into ponds and lakes, and provide a habitat for amphibians and certain insects. The prairie dog creates networks of tunnels which impact the environment in various ways.
Whether changing their habitat or keeping balance within a food chain, the absence of certain keystone species could throw the entire surrounding ecosystem seriously off balance.