Indicator species are plants and animals that, by their presence, abundance, lack of abundance, or chemical composition, demonstrate some distinctive aspect of the character or quality of an environment.
For example, in places where metal-rich minerals occur at the soil surface, indicator species of plants can be examined to understand the patterns of naturally occurring pollution, and they can even be a tool used in prospecting for potential ore bodies. Often, the indicator plants accumulate large concentrations of metals in their tissues. Nickel concentrations as large as 10% have been found in the tissues of indicator plants in the mustard family (Alyssum bertolanii and A. murale) in Russia, and a concentration as large as 25% occurs in the blue-colored latex of Sebertia acuminata from the Pacific island of New Caledonia. Similarly, Becium homblei, related to mint, has been important in the discovery of copper deposits in parts of Africa, where it is confined to soils containing more than 0.16 oz/lb (1,000 mg/kg) of copper, because it can tolerate more than 7% copper in soil. So-called copper mosses have been used by prospectors as botanical indicators of surface mineralizations of this metal in Scandinavia, Alaska, Russia, and elsewhere.
Plants are also used as indicators of serpentine minerals, a naturally occurring soil constituent that in large concentrations can render the substrate toxic to the growth of most plants. The toxicity of serpentine influenced soils is mostly caused by an imbalance of the availability of calcium and magnesium, along with the occurrence of large concentrations of toxic nickel, chromium, and cobalt, and small concentrations of potassium, phosphorus, and nitrogen. Serpentine soils are common in parts of California, where they have developed a distinctive flora with a number of indicator species, many of which are endemic to this habitat type (that is, they occur nowhere else). A genus in the mustard family, Streptanthus, has 16 species endemic to serpentine sites in California. Three species have especially narrow distributions: Streptanthus batrachopus, S. brachiatus, and S. niger, only occur at a few sites. Streptanthus glandulosus, S. hesperidis, and S. polygaloides maintain wider distributions, but they are also restricted to serpentine sites.
Indicator plants also occur in many semiarid areas on soils containing selenium. Some of these plants can accumulate this element to large concentrations, and they can be poisonous to livestock, causing a syndrome known as "blind staggers" or "alkali disease." The most important selenium-accumulating plants in North America are in the genus Astragalus, of the legume family. There are about 500 species of Astragalus in North America, 25 of which can accumulate up to 15 thousand ppm (parts per million) of selenium in foliage. These species of Astragalus can emit selenium-containing chemicals to the atmosphere, which gives the plants a distinctive and unpleasant odor.
Sometimes indicator species are used as measures of habitat or ecosystem quality. For example, animals with a specialized requirement for old-growth forests can be used as an indicator of the integrity of that type of ecosystem. Old-growth dependent birds in North America include the spotted owl (Strix occidentalis), red-cockaded woodpecker (Picoides borealis), marbled murrelet (Brachyramphus marmoratus), and pine marten (Martes americana). If the area and quality of old-growth forest in some area is sufficient to allow these indicator animals to maintain viable populations, this suggests something positive about the health of the larger, old-growth ecosystem. In contrast, if a proposed forest-harvesting plan is considered to pose a threat to the populations of these species, this also indicates a challenge to the integrity of the old-growth forest more broadly.
Indicator species can also be used as measures of environmental quality. For example, many species of lichens are very sensitive to toxic gases, such as sulfur dioxide and ozone. These "species" (actually, lichens are a symbiosis between a fungus and an alga) have been monitored in many places to study air pollution. Severe damage to lichens is especially common in cities with chronic air pollution, and near large point sources of toxic gases, such as metal smelters.
Similarly, aquatic invertebrates and fish have commonly been surveyed as indicators of water quality and the health of aquatic ecosystems. If a site has populations of so-called "sewage worms" or tubificids (Tubificidae), for example, this almost always suggests that water quality has been degraded by inputs of sewage or other oxygen-consuming organic matter. Tubificid worms can tolerate virtually anoxic water, in contrast with most of the animals of unpolluted environments, such as mayflies (Ephemeroptera) and stoneflies (Plecoptera), which require well-oxygenated conditions.
Often, the lacking presence of an indicator species is indicative of environmental change or contamination. For instance, the nymphs of stoneflies mentioned above, if absent from a stream where they would normally be expected to reside, might indicate a lack of oxygenation or the presence of a pollutant. Caddisfly larvae, mayfly nymphs, and stonefly nymphs are often used to evaluate water quality and the presence of acid mine drainage in western Pennsylvania, where coal mining is prevalent and can affect nearby watersheds.
Another current example involves frogs and salamanders as indicator species. Populations of amphibians are declining on a global scale. Their decline is thought to be an indicator of tainted environments. Therefore, the numbers of amphibians worldwide are being closely monitored. In a related example, the eggs of certain bird species are tested for the presence of organic pesticides.
Much research is being done by governments to accurately establish which species of plants and animals can act as sentinels of particular environmental contaminants. Here, the indicator species shows directly the persistence of hazardous chemicals in the environment. Through the use of indicator species, then, it is hoped that potential environmental problems may be identified before they result in irrevocable damage.