Indicator Species

Indicator species are plants and animals that, by their presence, abundance, lack of abundance, or chemical composition, demonstrate some aspect of the character or quality of an environment.

For example, in places where minerals occur in the soil, indicator species of plants can show patterns of pollution or they can be used in prospecting for potential ore. Indicator plants can 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. Copper mosses have been used by prospectors as botanical indicators of surface mineralizations of this metal in Scandinavia, Alaska, and Russia.

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 and 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. bra-chiatus, and S. niger, only occur at a few sites. Streptanthus glandulosus, S. hesperidis, and S. polyga-loides 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. 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 that are only found in 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.

Indicator species can also be used as measures of environmental quality. For example, many species of lichens are sensitive to toxic gases, such as sulfur dioxide and ozone. The growth of these organisms has been used 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.

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 sewage worms or tubificids (Tubificidae), for example, it 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.

In some cases, the absence of a species is indicative of environmental change or contamination. For instance, if the nymphs of stoneflies mentioned are absent from a stream where they would normally be expected to reside, it might indicate a lack of oxygen 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 coalmining is prevalent and can affect nearby watersheds.

Much research is being done to accurately establish which species of plants and animals can act as sentinels of particular environmental contaminants. Through the use of indicator species, it is hoped that potential environmental problems may be easily, and quickly, identified.

See also Ecological monitoring; Water pollution.

Bill Freedman