Ecosystem health is often measured by the presence, absence, or abundance of an indicator species in a particular habitat type. An indicator species is a species that has such a narrow range of ecological tolerance that their presence or absence is a good indication of environmental conditions. Their presence does not provide an indication of ecosystem health but a rough indication that the basic ecosystem components necessary to support the species in question are present. Examples of indicator species include the northern spotted owl in old-growth forests of the Pacific Northwest and certain types of insect larvae in aquatic sediments.
Some species are known to have a disproportionately large role in determining the overall community structure within an ecosystem. These species are called keystone species. Removal, addition, or changes in local populations of keystone species can have significant impacts on the functioning of ecosystem processes, predatory relationships, and overall long-term stability. In many ecosystems keystone species cannot be easily defined because the basic knowledge of species requirements is so poorly understood. The role of keystone species is not clearly understood until removal of the species, either artificially or by natural means, has occurred. Consequently, the use of keystone species as monitors of environmental health has limited use in most ecosystems. A classic example of a keystone species is the sea otter (Enhydra lutris). The sea otter, preys on sea urchins in large numbers. When sea otter populations were removed by trappers and fishermen sea urchin populations increased dramatically, which led, in turn, to overgrazing of algae and kelp. Entire kelp beds were consumed, which caused declines in important commercial fish species that were dependent on the kelp beds. When sea otters were reintroduced, the kelp beds recovered. Obviously an adverse effect from contaminant exposure on a sea otter population could have significant consequences on an entire ecosystem. Another example of a keystone species is the starfish (Pisaster ochraceus), which preys on intertidal invertebrates of rocky habitats along the North American Pacific coast. Experiments where starfish were removed demonstrated their controlling influence on mussels, barnacles, snails and other invertebrates.
Other types of keystone species are important as habitat modifiers. The beaver is an example of a species that can affect the dominant vegetation to such an extent that the entire ecosystem is changed. Beavers remove trees in stream and pond habitats, which can change the surface water and light conditions present in aquatic environments. Flooding can occur in relatively dry areas adjacent to water bodies that changes the mix and abundance of plant species in an area.