Merely declaring that a species is endangered could be enough to sign its death warrant, thanks to the human penchant for collecting rarities.

Economists have thought that over-exploitation is unlikely to drive species to extinction, because numbers will dwindle to the point where it becomes unprofitable to continue harvesting them. However, that view ignores the possibility that enthusiasts may be willing to pay exorbitant prices for rare specimens of endangered animals.

This is just the incentive unscrupulous hunters need to track down the last few individuals.

Animals command much higher prices if they are listed as endangered. Bahaba taipingensis, a species of fish nearing extinction because of its use in Chinese traditional medicine, is now so coveted that a single swimbladder can sell for as much as $64,000.

In the 1990s, more than 100 boats continued to search for the fish despite total annual catches of fewer than half a dozen individuals.

Endangered species like the Dumeril's Boa, native to Madagascar, could become extinct if its value as an exotic pet goes up as a result of its status as a rare, narrow-range reptile.