When referring to the ways in which wildlife can impact their environment, the term keystone species is typically used (Mills et al. 1993). A keystone species is a plant or animal species that can have a vast impact on their environment by regulating or altering the habitats' ecological community. Typically, keystone species are apex predators, such as wolves, lions, or sea otters. Through predation, predators keep their prey species populations in check, whether it is by regulating the population size or influencing prey behaviors. In the United States, many top predators (i.e. keystone species) were once extirpated from large tracts of land, resulting in changes in that habitat. In some places, the changes were as benign as an increase in prey numbers, while some areas the result was a composition change of plants and animals. When keystone species are removed from the ecological equation, the associated changes are known as a trophic cascade.