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An indicator species is any biological species that defines a trait or characteristic of the environment. For example, a species may delineate an ecoregion or indicate an environmental condition such as a disease outbreak, pollution, species competition or climate change. Indicator species can be among the most sensitive species in a region, and sometimes act as an early warning to monitoring biologists.

Indicators of range

As an indicator species of a range, an environmental ecoregion is typically defined. The Chitymomma is an Agave that regionally helps define the Chihuahuan Desert of Northern Mexicomarker and the Southwestern United States. In Central America, the Guatemalan magnolia elevationally defines the limits of the cloud forests of mountains.

For paleoclimates, an extant species may be an indicator of a former climate condition. The Discus macclintocki snail defines a former Ice sheet region of the northern Midwestern United States.

In some cases entire groups of fauna and flora may be an indicator of range. Invasive species that enter an ecoregion advance at rates dependent on environmental conditions such as temperature, food supply and physical barriers. An example is the spread of the Africanized bee as it enters southern North America.

Specialized uses

Prospecting has been a minor use of indicator species. Folklore may lead to recent attempts to use some speramarker, and Desert trumpet. In Canada, forestry surveys routinely look for wild mink and the Pileated Woodpecker to get an idea of how ecologically successful the region is, and whether they have to take measures to prevent destroying their habitat.

Indicators of environmental condition

Recent examples of North American species affected by environmental changes are the American Dipper and the Gray Jay. The American Dipper is a bird that requires a habitat of clear, mountainous streams, and can be displaced by siltation from land development, land-wasting runoff and forest fire runoff. The Gray Jay has become less common in southerly (warmer) parts of its range, apparently because its food supply has been affected by rising temperatures due to global warming.

Many indicator species of the ocean systems are fish, invertebrates, periphyton, macrophytes and specific species of ocean birds (like the Atlantic Puffin). Amphibians are also common indicator species, as they may have become repositories of bioindicator chemicals, or of ecological conditions relating to global warming, air pollution chemicals, newly extant diseases (fungus), or environmental pressure on the ecosystem, which affect the population numbers, and the quality of the individuals.

Lichens are indicators of air quality. They are particularly sensitive to sulfur dioxide, a gas emitted from exhaust and industrial fumes, and so are rarely found in large cities and towns or by roads. Filamentose, fruticose and foliose varieties are particularly sensitive. Their presence indicates air very low in sulfur dioxide. Crustose, leprose and squamulose varieties are more tolerant of poor air.

Frogs can be indicators of polluted stormwater runoff. Farm fertilizers often contain large amounts of phosphorus and nitrogen. These chemicals boost algae production which in turn boost the population of certain flatworm parasites. These parasite attack frogs at their larval stage (tadpoles), causing them to develop deformities in adulthood. Deformities include missing or extra limbs.

Several species are recommended as standard indicators for water pollution in rivers and streams, including Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies). The absence or abundance of these species has been used to develop the EPT Index to quantitatively measure the water quality of streams and rivers.

Definitions

Lindenmayer et al. suggest 7 alternative definitions of indicator species:

  1. a species whose presence indicates the presence of a set of other species and whose absence indicates the lack of that entire set of species;
  2. a keystone species, which is a species whose addition to or loss from an ecosystem leads to major changes in abundance or occurrence of at least one other species
  3. a species whose presence indicates human-created abiotic conditions such as air or water pollution (often called a pollution indicator species)
  4. a dominant species that provides much of the biomass or number of individuals in an area
  5. a species that indicates particular environmental conditions such as certain soil or rock types
  6. a species thought to be sensitive to and therefore to serve as an early warning indicator of environmental changes such as global warming or modified fire regimes (sometimes called a bioindicator species)
  7. a management indicator species, which is a species that reflects the effects of a disturbance regime or the efficacy of efforts to mitigate disturbance effects.


Type 1, 2, and 4 have been proposed as indicators of biological diversity and types 3, 5, 6, and 7 as indicators of abiotic conditions and/or changes in ecological processes.

Indicator Species for Ancient Woodland in England

Indicator species for ancient woodland in England need to be shade tolerant and slow colonisers. Plant species include Common wood sorrel, Wood Anemone, Wild Daffodil, Golden Saxifrage, Wild Garlic and in the East of England and Lincolnshiremarker, Common Bluebells.

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