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Bring in the replacements

Field experiments suggest introduced species could play understudy for extinct ones

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7:06pm, May 6, 2008
Magazine issue: Vol. 173 #17, May 24, 2008
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Missing links in ecosystems disrupted by extinctions could be restored by introducing species that perform the same function, field experiments on a remote island suggest.

But some scientists caution against the controversial process, called “rewilding.”.

Mauritius, a remote island in the Indian Ocean, lost many of its unique creatures after Europeans colonized the island in the 17th century. Besides its most famous extinction, the dodo, the island has lost giant tortoises, pigeons, fruit bats and a giant lizard, says Dennis M. Hansen, a tropical ecologist at StanfordUniversity. Those die-offs now threaten many of the island’s plants, especially the species that depend on frugivores to disperse their seeds.

Take, for example, Syzygium mammillatum, a critically endangered species of tree that grows between 2.5 and 9 meters tall and is found only on Mauritius. The fruit of this tree, rather than developing on branches, forms on the lowermost portion of its trunk, Hansen says. In one of the few pristine areas of forest left on the island — a 24-hectare conservation area that has been fenced off and weeded since it was established in 1996 — Hansen and his colleagues found no S. mammillatum seedlings or saplings more than two meters away from adult trees. At such proximity, young trees would be more susceptible to diseases or fungi that might afflict the adult tree, as well as foraging herbivores.

When the researchers fed S. mammillatum fruit to turkeys, a possible ecological analogue of the dodo, none of the thin-hulled seeds survived passage through the bird’s digestive tract. In similar tests, about 15 percent of the seeds from fruit fed to giant Aldabran tortoises passed through the reptiles unscathed. Neither of the species were native to the island.

Hansen and his colleagues then scattered some of the gut-passed seeds and covered them with a one-centimeter layer of tortoise dung in plots about 20 meters away from adult S. mammillatum trees. The scientists also scattered fruit and bare seeds at sites about one meter from an adult tree and at spots about 25 meters away.

The field tests revealed that S. mammillatum seedlings from gut-passed seeds grew taller, had more leaves and suffered less damage from natural enemies such as insects than other seedlings did. These results indicate that the Aldabran tortoises could replace the now-missing seed dispersers of S. mammillatum, thereby enhancing conservation efforts for the tree, the researchers report in the May 7 PLoS ONE.

Hansen and his colleagues “are certainly doing some very interesting and inspiring research in getting to the nitty-gritty of plant reproductive failure in devastated ecosystems,” says Anthony Cheke, an island ecologist formerly at OxfordUniversity in England who has studied ecosystems on Mauritius and nearby islands since the 1970s. The results offer hope that threatened plants, of which there are many on Mauritius, can proliferate if appropriate seed dispersers are reintroduced to the island.

Despite the apparent success of the experiments in bolstering the reproduction of S. mammillatum, however, scientists seeking to introduce one species to take the ecological place of one that has died out should proceed with caution, says Douglas J. Levey, an evolutionary ecologist at the University of Florida in Gainesville. “An exotic species is an exotic species, even if there was a similar species there before,” he notes. “There always seem to be surprises, despite good intentions, especially in island ecosystems,” he adds.

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