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Coextinction and cothreatened refer to the phenomena of the loss or decline of a host species resulting in the loss or endangerment of other species that depend on it, potentially leading to cascading effects across trophic levels. The term originated by the authors Stork and Lyal (1993)  and it was originally used to explain the extinction of parasitic insects following the loss of their specific hosts. The term is now used to describe the 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 recent studies, up to 50% of species have been said to go extinct in the next 50 years. This may be possible due to an example of coextinction being the loss of tropical butterfly species from Singapore attributing to the loss of their specific larval host plants. To see how possible future cases of coextinction would play out, researchers have made models to show probabilistic relationships between affiliate and host extinctions across co-evolved inter-specific systems. The subjects are pollinating Ficus Wasps and Ficus, primate parasites, (Pneumocystis Fungi, Nematode, and Lice) 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(Koh et al2004).
Recent investigations of coextinction risk among the rich Psyllid fauna Hemiptera: Psylloidea inhabiting acacias (Fabaceae-Mimosoideae: Acacia) in central eastern New South Wales, Australia provide information about coextinction. The results, derived from simple criteria to name host specialists among threatened acacias in Australia’s central east, suggest that A. ausfeldii hosts one specialist psyllid species, Acizzia, and that A. gordonii hosts one specialist psyllid, Acizzia. Both psyllid species may be threatened at the same level of their host species with coextinction.
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.
"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.
Another example of species going through coextinction is rhinoceros stomach bot fly (Gyrostigma rhinocerontis) and of its host species the endangered black rhinoceros (Diceros bicornis) and the white rhinoceros (Ceratotherium simum). The female fly lays eggs behind the rhinoceros' ears or horns or neck and then the flies larvae enter the animals digestive tract and digs into the stomach lining and then are excreted out and the lifecycle restarts.
Coextinction can mean loss of biodiversity and diversification. Coextinctions can influence not only parasite and mutualist diversification but also their hosts. Arguably, parasites facilitate host diversification through sexual selection. That loss of parasites can reduce host diversification rates. Coextinction can also result in loss of evolutionary history. The extinction of related hosts can lead to the extinction of related parasites. The loss of history is likely to be greater than the loss expected, were species to go extinct at random. Furthermore, if coextinctions are clustered, it is more likely that coextinction can produce non-random trait loss. Species that are at risk of coextinction are expected to be larger because rare hosts tend to be larger and larger hosts have larger parasites. They can also be expected to have lengthy generation times or higher tropic positions. Coextinction can extend beyond biodiversity and has direct and indirect consequences from 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. In rare hosts, losses of specialist parasites can predispose hosts to infection by emergent parasites. Furthermore, relating to the consequences 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. On the other hand, if they are able to find and switch onto alternative hosts, those hosts can turn out to be humans. Either way, the loss of parasites by co extinction or the acquiring of new parasites by alternative hosts, proves to be a major issue. Coextinction can go beyond the decreased biodiversity, it can range into various biomes and link various ecosystems.
A study conducted in New Caledonia has shown that that extinction of a coral reef-associated fish species of average size would eventually result in the co-extinction of at least ten species of parasites.
The host Specificity and Life Cycle is a major factor in the risk of coextinction. Species of mutalists, 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. Also, if organisms are evolutionary flexible, then these organisms may escape extinction.
The area with that has the greatest affect of coextinction is the tropics. There is a continued disappearance in the habitat, human intervention, and a great loss in vital ecosystem 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 with forest loss other risk factors include: coastal development, overexploitation of wildlife, and habitat conversion, that also affect human well-being.
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 chose one that can benefit an array of other dependent species.
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