The term ecological indicator is increasingly being used in connection with environmental monitoring and with reporting on the state (condition of health) of the environment. In ecology, the term ecological indicator has a fairly precise meaning. However the term is being used more and more in a very general or generic sense to refer to data, information, indicators, and indices.
Ecological indicators are biological assemblages or taxa that by their presence or condition indicate something about the environment. For example, the presence or absence of patches of plants or bare soil can be used as indicators of the state of the health of the landscape. Such variables have been used as indicators of rangeland health in North America.
In Western Europe, for example, lowland heathland communities (low-growing ericoid shrubs) are indicators of low-nutrient, acid soils. By way of contrast, the assemblages of grasses and herbs on chalk soils are made up of specific mixes of species. In water monitoring studies, some freshwater invertebrate communities have been described as indicator communities.
These indicator communities have important applications in ecological monitoring. For example, some freshwater invertebrate communities have been the basis of some ecological monitoring programs. The community structure and the species present have also been bases for the classification of rivers and lakes. For example, a software package aptly called RIVPACS (the river invertebrate prediction and classification system) is based on the assumption that the presence of certain taxonomic groups depends on certain physical and chemical variables.
There are both indicator communities and indicator species. Ecological indicator species are those species that by their presence or condition or numbers tell us something about the state of the environment. This of course assumes that the condition of the environment affects the distribution, state, and numbers of the particular species. Lichens have long been used as indicator species. Particularly well-known freshwater indicator species are species of fish and species of macro-invertebrates that are intolerant to high levels of organic load or pollution. Salmon are fish that are well-known ecological indicators of the health of rivers. Species of mayflies and caddis flies are examples of macro-invertebrate indicator species.
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