bioindicator

:  a species or ecological community that is so closely associated with particular environmental conditions that its presence is indicative of these conditions in a particular environment Bats are what naturalists call bioindicators: they indicate the state of the environment. If you see them, it means the air and water around you are healthy. — Michelle Barker, Sherbrooke (Quebec) Record, 31 July 2007; also :  a species, ecological community, or biological process that is typically monitored over time for changes (as in abundance or health) which are used to assess the state of a particular environment The team compared two approaches to water-quality testing: measurement of pH and dissolved oxygen levels, and the use of bioindicators (using measures of the abundance and types of stream organisms present). — Jennifer Miner et al., The Science Teacher, April 2007 Oysters, like other bivalves, are sessile organisms that are continually filtering water, and they often bioaccumulate toxins, making them ideal bioindicator species. — David C. Metzger, Journal of Shellfish Research, August 2012