A biological indicator is a 'detector' revealing the existence of complex conditions resulting from a group of biotic and abiotic factors, difficult to measure individually. Bioindicators could range from infracellular level to communities or ecosystems. They have to be carefully qualified according to their levels. An indicator should quantify information, be scientifically credible, give response to changes in time and/or space, based on data that can be collected within realistic space and time limits, be limited in number, and be adaptable to future developments. How to select indicators is certainly the most difficult and controversial problem. It is necessary to use species or taxa that can be measured (numbered). When several indicators are used to interpret these phenomena, they could be combined in a 'biotic indices' form. An indicator must have particular requirements with regard to a certain amount of physical, chemical, and biological variables. Biological indicators and biotic indices must allow us to characterize the state of an ecosystem and bring to the fore, as early as possible, its modifications, natural or provoked.
It is no longer contested that benthic organism indicators have many advantages:
1. They are relatively nonmobile and therefore useful for studying the local effects of physical and chemical perturbations.
2. Some of these species are long-lived.
3. Their taxonomy is relatively easy, as their quantitative sampling.
4. There is extensive literature on their distribution in specific environment and on the effects of various stresses that they could suffer.
A 'characteristic species' means a species linked to a particular biocenotic structure (referred otherwise as 'community' or 'biotic assemblage'). An 'indicative species' will mark the presence of a particular factor, biotic or more often abiotic, within a given environment. These indicative species intervene rather in the functioning than in the structure of the community. A 'sentinel species' is a particular species which by its presence or its relative abundance would have a role of 'warning' possible imbalances in the surrounding environment or distortions in the functioning of the community. Based on these premises, an annelidian index of pollution used in northern Mediterranean and in southern English Channel has been proposed.
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