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Some species are highly sensitive to toxic chemicals or changes in temperature or humidity, and others are particularly sensitive to changes in soil or water pH. Such species, when monitored regularly, can function as indicator species, analogous to water-salinity meters or pH readers. In 1989, for example, the Exxon Valdez oil spill in Prince William Sound, Alaska, devastated marine habitats and wildlife, and created conditions that demanded systems of monitoring to determine the extent of the problem and the effects over time of ameliorating efforts. In this case, mussels were sought, and their abundance and distribution were used to indicate the extent and change in environmental pollution.
The concept of indicator species has been welcomed by managers of protected areas because it allows the focusing of conservation efforts through the monitoring of particular species. The assumption is that an increase or decline in abundance of an indicator will point to similar changes in other species within that community of organisms. For example, declines in amphibian populations have led calls for frogs and toads to be recognized as indicators of wetland health, and many research studies have since resulted on amphibian diversity and ecology in the tropics. The concept of indicator species has its opponents too. Doubts were expressed by some biologists when the threatened northern spotted owl Strix occidentalis was designated an indicator species by the United States Forest Service. The objection was that the species did not satisfy the condition of strongly ecological association with other species. Ecosystem health may mean different things to different people, species richness being critical for some, while others believing in strict protectionism. Yet others, such as tribal people resident in or near a protected area, would see a forest full of edible fruit, seed, and root as a healthy ecosystem. The question of what exactly needs to be monitored and indicated has many interpretations; hence, the application of the concept of indicator species to conservation biology has been troublesome.
Certain species have been studied in detail and are widely accepted as environmental indicators. Prime examples are rock lichens, such as the endangered rock gnome lichen Gymnoderma lineare, known to be sensitive to heavy metals, ozone, and sulphur dioxide, a common air pollutant in industrial and automobile emissions. In aquatic systems, mollusks, which tend to concentrate toxic pesticides and heavy metals in their tissues, are considered indicator species, and include clams, oysters, and snails. Disease and organ failure resulting from toxins leads to a decline in the abundance of mollusks, indicating high levels of pollutants in the water. In Southeast Asia, butterflies, sensitive to changes in the structure of their habitat caused by logging, have been studied as indicators of biodiversity and forest disturbance. Some biologists also believe that monitoring the status and abundance of butterflies can indicate whether communities of plants and animals will remain stable or start to collapse by losing species over a period of time. The proposition that a single species or even a group of species from the same taxonomic group can indicate the richness or diversity within a whole ecological community is less and less promising. Instead, biologists are searching for indicators of species richness that are effective within geographic and taxonomic limits. Recent emphasis on the conservation of complete landscapes is a challenge to the rationale and necessity of discovering and monitoring indicator species for biodiversity conservation..
- John Hellawell, Biological Indicators of Freshwater Pollution and Environmental Management (Elsevier Applied Science Publishers, 1986);
- Reed F. Noss, “Indicators for Monitoring Biodiversity: A Hierarchical Approach,” Conservation Biology (v.4, 1990);
- Richard B. Primack, A Primer of Conservation Biology (Sinauer Associates, , 2004);
- Daniel Simberloff, “Flagships, Umbrellas, and Keystones: Is Single-Species Management Passe in the Landscape Era?,” Biological Conservation (v.83, 1998).