The term bellwether came from the practice of putting a bell on the leader of a flock of sheep. The term is now used to describe an indicator of a complex of ecological changes. Bellwether species are also called indicator species and are seen as early warning signs of environmental damage and ecosystem change.
Ecologists have identified many bellwether species in various ecosystems, including the Attwater prairie chicken(-Tympanuchus cupido attwateri ), the Northern diamondback terrapin(Malaclemys terrapin terrapin ), the Northern and Southern giant petrels (Macronectes giganteus and M. halli ), and the polar bear(Ursus maritimus ). There are many more. Problems with these species are extreme indicators of troubles within their ecosystems.
The Attwater prairie chicken is almost extinct. It once numbered in the millions in Louisiana and along the Texas coast. The Northern and Southern giant petrels, large scavenger birds of the Antarctic Peninsula, are also heading towards extinction as commercial fishing fleets head further and further south. In Massachusetts' Outer Cape, conservationists are trying to save the Northern diamondback terrapin. Troubles for the Arctic polar bear are indicating ecosystem change in the Arctic, as humans interact more and more with that ecosystem.
[Douglas Dupler ]
Walsh, J. E. "The Arctic as a Bellwether." Nature, no. 352 (1999): 19–20.