https://en.wikipedia.org/wiki/Coextinction
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 depends on it, potentially leading to cascading effects across trophic levels. The term originated by the authors Stork and Lyal (1993) and 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 competition with their counterpart, and specialist herbivores with their food source. Coextinction is especially common when a keystone species goes extinct.
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 depends on it, potentially leading to cascading effects across trophic levels. The term originated by the authors Stork and Lyal (1993) and 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 competition with their counterpart, and specialist herbivores with their food source. Coextinction is especially common when a keystone species goes extinct.
Causes
The most frequently 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 louse was rediscovered,
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.
Recent studies have suggested that up to 50% of species may go extinct in the next 50 years.
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.
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.
Investigations have been carried out into coextinction risk among the rich Psyllid fauna Hemiptera – Psylloidea inhabiting acacias (Fabaceae-Mimosoideae: Acacia) in central eastern New South Wales, Australia. The results, 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 taxa "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 the rhinoceros stomach bot fly (Gyrostigma rhinocerontis) and its host species the endangered black rhinoceros and white rhinoceros (Diceros bicornis and Ceratotherium 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.
Consequences
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 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.
Risks
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 effect 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 choose one that can benefit an array of
other dependent species.
