Ecological indices are expressed as numbers or scores that have been derived or transformed from quantitative data. For example, the Trent biotic index of water quality is derived from a mix of the presence or absence of certain indicator species and the number or diversity of taxa (or groups) or organisms present. One of the most well-researched indices is the Common Bird Index that was established in the UK by the British Trust for Ornithology. This index came about because of a desire to monitor the state of bird species and because it was impractical to count every individual.
Some ecological indices have been developed as a way of expressing the conservation needs of a species. For example, some red data books list species in certain categories based on a threat number: the higher the number, the greater the threat. The index in this case is based on the combination of several measurements and on subject assessments of conservation actions.
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