Coextinction andcothreatened refer to the phenomenon of the loss or decline of a hostspecies resulting in the loss or endangerment of another species that depends on it, potentially leading to cascading effects acrosstrophic levels.[1] The term was originated by the authors Stork and Lyal (1993)[2][3] and was originally used to explain the extinction ofparasiticinsects following the loss of their specific hosts. The term is now used to describe the loss of anyinteracting species, including competition with their counterpart, and specialistherbivores with their food source. Coextinction is especially common when akeystone species goes extinct.
The most frequently cited example is that of the extinctpassenger pigeon and its parasiticbird liceColumbicola extinctus andCampanulotes defectus. Recently,C. extinctus was rediscovered on theband-tailed pigeon,[4] andC. defectus was found to be a likely case of misidentification of the existingCampanulotes flavus.[5] However, even though the passenger pigeon louse was rediscovered, coextinctions of other parasites, even on the passenger pigeon, may have occurred.[citation needed] Several louse species, such asRallicola extinctus, ahuia parasite, probably became extinct together with their hosts.[6]
Recent studies have suggested that up to 50% of species may go extinct in the next 50 years.[7] This is in part due to coextinction; for example the loss of tropical butterfly species from Singapore is attributed to the loss of their specific larval host plants.[7] To predict how possible future cases of coextinction might play out, one set of researchers made models to predict probabilistic relationships between affiliate and host extinctions across co-evolved inter-specific systems. The subjects are pollinatingFicus wasps andFicus; primates and their parasites; (PneumocystisFungi,Nematode, andLice) and their hosts; parasitic mites and lice and their avian hosts; butterflies and their larval host plants; and ant butterflies and their host ants. For all but the most host-specific affiliate groups (e.g., primate Pneumocystis fungi and primates), affiliate extinction levels may be modest at low levels of host extinction but can be expected to rise quickly as host extinctions increase to levels predicted in the near future. This curvilinear relationship between host and affiliate extinction levels may also explain, in part, why so few coextinction events have been documented to date.[7]
Investigations have been carried out into coextinction risk among the richPsyllid faunaHemiptera – Psylloidea inhabitingacacias (Fabaceae-Mimosoideae: Acacia) in central eastern New South Wales, Australia. The results, suggest thatA. ausfeldii hosts one specialist psyllid species, Acizzia, and thatA. gordonii hosts one specialist psyllid, Acizzia. Both psyllid species may be threatened at the same level of their host species with coextinction.[8]
Interaction patterns can be used to anticipate the consequences of phylogenetic effects. By using a system of methodical observations, scientists can use the phylogenetic relationships of species to predict the number of interactions they exhibit in more than one-third of the networks, and the identity of the species with which they interact in about half of the networks. Consequentially, simulated extinction events tend to trigger coextinction cascades of related species. This results in a non-random pruning of the evolutionary tree.[9]
In a 2004 paper inScience,ecologistLian Pin Koh and colleagues discuss coextinction,[10] stating
"Species coextinction is a manifestation of the interconnectedness of organisms in complexecosystems. The loss of species through coextinction represents the loss of irreplaceableevolutionary andcoevolutionary 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." (Kohet al. 2004)
Kohet al. also definecoendangered astaxa "likely to go extinct if their currently endangered hosts [...] become extinct."
One example is the extinction of many species of the genusHibiscadelphus, as a consequence of the disappearance of several of theHawaiian honeycreepers, itspollinators. There are also several instances ofpredators andscavengers dying out or becoming rarer following the disappearance of species which represented their source of food: for example, the coextinction of theHaast's eagle with themoa, or the near-extinction of theCalifornia condor after the extinctions of its primary food, the dead carcasses of North AmericanPleistocene megafauna; in the latter, the condor survived by possibly relying onbeached marine mammals.
Coextinction may also occur on a local level: for example, the decline in the red antMyrmica sabuleti in southernEngland, caused by habitat loss, resulted in theextirpation of thelarge blue butterfly (which is dependent on the ant as a host for its larvae) fromGreat Britain. In this case the ant avoided extirpation, and the butterfly has since beenreintroduced to the island.
Another example of a species that could currently be experiencing coextinction is therhinoceros stomach bot fly (Gyrostigma rhinocerontis) and its host species the endangeredblack rhinoceros andwhite rhinoceros (Diceros bicornis andCeratotherium simum). The fly's larvae mature in a rhinoceros's stomach lining, having entered the body via the digestive tract, and so are dependent on rhinoceros species to reproduce.[11]
Coextinction can meanloss of biodiversity and diversification.[citation needed] Coextinction can also result in loss of evolutionary history.[citation needed]
Coextinction can extend beyond biodiversity and has direct and indirect consequences on the communities of lost species. One main consequence of coextinction that goes beyond biodiversity is mutualism, by loss of food production with a decline in threatened pollinators. Losses of parasites can have negative impacts on humans or the species.[citation needed]
A consequence of removing specialist parasites from rare hosts is the problem of where the parasites will go once their host is extinct. If the parasites are dependent on only those species than there are parasite species that are at risk of extinction through co-endangerment.
A study conducted inNew Caledonia has shown that extinction of acoral reef-associated fish species of average size would eventually result in the co-extinction of at least ten species of parasites.[12]
The host specificity and life cycle is a major factor in the risk of coextinction. Species of mutualists, parasites, and many free-living insects that have staged life cycles are more likely to be a victim of coextinction. This is due to the fact that these organisms may depend on multiple hosts throughout their lives in comparison to simple life cycled organisms.[13] Also, if organisms are evolutionary flexible, then these organisms may escape extinction.[14][15]
The area with that has the greatest effect of coextinction is the tropics. There is a continued disappearance in the habitat, human intervention, and a great loss in vitalecosystem services. This is threatening because the tropics contain 2/3 of the all known species but they aren't in a situation where they can be fully taken care of. Along withforest loss other risk factors include: coastal development,overexploitation of wildlife, and habitat conversion, that also affect human well-being.[16]
In an effort to find a stop to coextinction, researchers have found that the first step would be to conserve the host species in which other species are dependent on. These hosts serve as major components for their habitat and need them to survive. In deciding what host to protect, it is important to choose one that can benefit an array of other dependent species.[17]
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