MANY more species are at risk of extinction than we thought. That is because some species depend on another for their survival, so if one disappears, many more that have gone largely unnoticed by conservationists will follow.
There are 12,200 species at risk according to the World Conservation Union (IUCN) Red List, the definitive audit of rare, threatened and endangered species. But the true number is likely to be much higher, says a team led by Lian Pin Koh at the National University of Singapore and Robert Dunn at Curtin University of Technology in Perth, Western Australia. In addition to the Red List species, another 6300 are 鈥渃o-endangered鈥�, they say. For instance, at least three species of lice depend on the threatened red colobus monkey. If the monkey becomes extinct, so will the lice it carries. Similarly, hummingbird flower mites face extinction if either the hummingbirds they use for transport, or the flowers on which the mites depend for nectar or pollen, become extinct. Many of these dependent species might have been overlooked in the past because they are often less charismatic than their hosts (麻豆传媒, 27 March, p 40).
鈥淲e would like to see changes to the Red List so that when, for example, a mammal species is listed as endangered, all the species affiliated to that animal are also listed as being possibly at risk,鈥� says Navjot Sodhi, of the National University of Singapore, who worked on the study.
Advertisement
The team started by looking at eight well-studied groups of host animals and their affiliated species, such as primates and their parasites, seabirds and the mites and lice that live on them, and the Ficus plant which supports the Ficus wasp. For each group, they estimated the number of affiliate species that would be expected to survive the extinction of their host species. Then they estimated the number of affiliate species that would be likely to go along with their host.
They used their results to develop a model to estimate levels of co-extinctions for other animals and plants and compared their estimates with the Red List.
In addition to 399 extinct host species of plants, fish, birds or mammals on the Red List, they estimate another 200 affiliate species have already been lost. And of the 9536 endangered host species on the Red List for which there is sufficient data to run the model, another 6300 affiliate species are likely to go with their hosts. This co-endangered list includes more than 4000 beetles, and butterflies, lice and other parasites (Science, vol 305, p 1632).
The number of affiliate species for each host can vary widely. It will be important to identify those species on which particularly large numbers of others depend, Sodhi says. For instance, the army ant has at least 100 affiliate species, many of which would be lost if it disappeared.
鈥淚t is extremely important to take co-extinctions and co-endangerment into account,鈥� agrees Thomas Brooks of the Center for Applied Biodiversity Science at Conservation International in Washington DC. 鈥淭he team has done conservationists a great favour by synthesising the theory and data on the co-extinction crisis.鈥�
Craig Hilton-Taylor, an IUCN Red List programme officer in Cambridge, UK, also welcomes the research. 鈥淚t certainly does elevate the absolute magnitude of recent extinctions and potentially imminent ones.鈥� But it will not be easy to add details about dependent species to the Red List, he says, because 鈥渋n the vast majority of cases we just don鈥檛 know about a species鈥� affiliates鈥�.
For instance, when conservation biologists captured the last 25 remaining Californian condors and brought them into captivity, they deloused the birds. In doing so, they inadvertently killed off the last surviving condor lice.