Extinctions Could Have Domino Effect, Study Says

James Owen
for National Geographic News
September 9, 2004
In a study released today, researchers warn that the loss of plants and animals currently listed as threatened or endangered could have a domino effect on other species that depend on them.

The scientists estimate if the nearly 12,200 animals and plants worldwide currently listed as threatened or endangered were to disappear, another 6,300 "affiliate" species could also be lost.

"Many plants and animals have a diverse selection of insects, fungi, and other organisms associated with them that are uniquely adapted to their host," the researchers wrote in a study published in tomorrow's issue of Science.

Such specialization makes affiliate species especially vulnerable to extinction should the host species die out, the scientists say.

The researchers believe these dependent species should now be included in current extinction estimates. They add that coextinction (the loss of one species resulting from the loss of another) is a largely unexamined and potentially substantial contributor to the current global extinction crisis.

The team used a model based on known coevolved relationships between organisms such as fig trees and the fig wasps that pollinate them, and parasitic butterflies and their host ants. The analysis identified a further 6,300 endangered species.

The study also highlights at least 200 plants and animals that have already been lost through coextinction.

Co-author Navjot Sodhi says an examination of the skins of extinct animals would likely reveal many more examples of coextinct creatures.

"A parasite on the dodo could have gone coextinct if the dodo was its only host bird," said Sodhi, a biologist at the National University of Singapore.

While the plight of parasitic lice and mites are unlikely to attract outpourings of public sympathy, more charismatic insects are also at risk.

Tropical Butterflies

Sodhi recently co-authored a separate study, which focused on tropical butterflies and caterpillar food plants found in Singapore. His findings suggested that the number of extinct butterfly species there increased in line with the number of extinct plants. The Southeast Asia island has lost 95 percent of its original vegetation in the last 200 years.

"Our research shows that, out of 908 host plants and 383 butterfly species, at least 208 plants and 56 butterflies have gone coextinct from Singapore," Sodhi said.

In some cases the loss of just a single species could lead to multiple extinctions, according to the study's authors.

They give the example of the army ant (Eciton burchelli), found in Central and South America, on which no fewer than a hundred species depend, including various beetles, mites, and birds.

To prevent a host species going extinct and any potential domino effect, the researchers say entire habitats and the often complex interactions between different species need to be considered in conservation plans.

One example, also cited in the study, comes in the shape of what is perhaps Britain's most extraordinary butterfly, Maculinea arion, commonly known as the large blue.

In 1979 the species became extinct in Britain. While the insect had always been considered uncommon there, its disappearance baffled scientists.

"At [the butterfly's] remaining [habitat] sites everything looked fine to the conservationists. But it proved impossible to save," said Richard Fox of Butterfly Conservation, a Wareham, England-based nonprofit.

"We didn't know about the ants then," he said.

Not long after the butterfly disappeared in Britain, scientists revealed that the insect's lifecycle was dependent on a single species of red ant, Myrmica sabuleti.

Caterpillar Deception

Researchers discovered that on reaching a certain size, the caterpillar that becomes the large blue butterfly leaves its food plant and masquerades as a red ant larva.

"The caterpillars have special glands that produce chemicals which attract the ants and fool them into thinking [the caterpillars] are one of their own ant grubs," Fox said.

The caterpillar is taken inside the ant nest where it promptly turns carnivorous and starts devouring its hosts' eggs and young. Eventually the parasitic caterpillar pupates, later emerging as an adult butterfly.

Fox added: "The caterpillar will be collected by other [related species of] red ants, but its survival in those nests is very poor, presumably because the ants realize they haven't got a genuine ant grub."

Scientists noted that the large blue's disappearance mirrored that of the host species in regions where both insects had previously been found. They then traced the cause of the red ant's own decline to another animal—sheep.

"The ant needs hot conditions and therefore needs vegetation that's quite short so the sun gets through," Fox explained. "What seems to have happened is that the amount of grazing [by sheep] declined, and the [grass] had got slightly longer. [This] led to a massive reduction in populations of these ants."

He added: "You can't possibly conserve the large blue butterfly in Britain or anywhere else unless conditions are right for the ants."

A large blue reintroduction program, using stock from mainland Europe, is now underway at sites in southern England.

"It's as much about getting the amount of grazing and vegetation structure right for the ant as it is for the butterfly," Fox said.

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