Coextinction of a species is the loss of a species as a consequence of the extinction of another. The term was originally used in the context of the extinction of parasitic insects following the loss of their specific hosts. The term is now used to describe the subsequent loss of any interacting species, including predators with their prey, and specialist herbivores with their food source. Coextinction is especially common when a keystone species goes extinct.
The most often cited example is that of the extinct passenger pigeon and its parasitic bird lice Columbicola extinctus and Campanulotes defectus. Recently, C. extinctus was rediscovered on the band-tailed pigeon, and C. defectus was found to be a likely case of misidentification of the existing Campanulotes flavus. However, even though the passenger pigeon lice story has a happy ending (i.e. rediscovery), coextinctions of other parasites, even on the passenger pigeon, may have occurred. Several louse species—such as Rallicola extinctus, a huia parasite - probably became extinct together with their hosts.
In a 2004 paper in Science, ecologist Lian Pin Koh and colleagues discuss coextinction, stating
"Species coextinction is a manifestation of the interconnectedness of organisms in complex ecosystems. The loss of species through coextinction represents the loss of irreplaceable evolutionary and coevolutionary history. In view of the global extinction crisis, it is imperative that coextinction be the focus of future research to understand the intricate processes of species extinctions. While coextinction may not be the most important cause of species extinctions, it is certainly an insidious one." (Koh et al. 2004)
Koh et al. also define coendangered as taxon
"likely to go extinct if their currently endangered hosts become extinct."
One example is the near extinction of the genus Hibiscadelphus as a consequence of the disappearance of several of the Hawaiian honeycreepers, its pollinators. There are several instances of predators and scavengers dying out following the disappearance of species which represented their source of food: for example, the coextinction of the Haast's Eagle with the moa.
Coextinction may also occur on a local level: for example, the decline in the red ant Myrmica sabuleti in southern England, caused by habitat loss, resulted in the local extinction of the Large Blue butterfly, which is dependent on the ant as a host for the larvae. In this case the ant avoided local extinction, and the butterfly has been reintroduced.