An ecosystem is a local environment with its complex community of organisms functioning together as a unit. An ecosystem can be damaged by natural disasters or by human activity. If the ecosystem has a limited variety of species, then destroying even one species of plant or animal can change the ecosystem forever. A resilient ecosystem, however, is able to restore itself to its original, pre-disturbance condition. Resilience in an ecosystem is possible when the ecosystem is populated by a large variety of species: if one species is eliminated by a disturbance, other species are able to take the place of that species, which allows the ecosystem to avoid long-term change.