A species that has a disproportionate effect on its environment is referred to as a "keystone species." These species, often predators or important herbivores, play a critical role in maintaining the structure and stability of their ecosystem. Their presence or absence can lead to significant changes in community composition and biodiversity. Examples include the sea otter in kelp forests and the wolf in terrestrial ecosystems, both of which help regulate populations of other species and sustain ecological balance.