Humans prize rare objects--be they paintings, coins or rare species of amphibians. This proclivity toward the hard-to-find led researchers at Paris-Sud University to hypothesize that the overexploitation of species--via trophy hunting and exotic pet collecting, for example--can lead to their extinction. (Standard economic theory predicts that extreme exploitation will not actually lead to a species's extinction, because the cost of obtaining it will be greater than its value in hand.) The authors mention the placement of an animal or plant on a conservation organization's watch list as a means in which the value of a dwindling species can ratchet upward.

Franck Courchamp, a mathematical and evolutionary ecologist, says his group's work--which appears in this week's issue of PLoS Biology--is based on an ecological process called the Allee effect, in which a species's extinction risk increases because of reduced reproductive fitness at low population. Courchamp amended an economic resource exploitation model so that as a material became more rare, its price on the market increased. He demonstrated that if the quantity of a material decreases enough, the price could become so high that it will cancel out the costs involved in locating it. Applying that reasoning to endangered species, he concludes that humans can kickoff an "anthropogenic Allee effect," which would eventually result in an "extinction vortex" of certain prized species. "What humans can do is push this intrinsic Allee effect into density zones where the Alee effect can be revealed or be unveiled," Courchamp explains. "But they cannot create it. Either the species has it or not."

Courchamp and his colleagues set out to highlight certain examples where this scenario could take place. He cites the auk, a bird that laid only one egg per year and was sought after both as food and for its feathers until it became extremely rare. Among those who began to covet them were museum curators and ornithologists, who, in an effort to preserve evidence of the flightless bird, may have helped kill it off. Trophy hunting could also result in this anthropogenic Allee effect--hunters have been known to pay as much as $400,000 for a license to hunt one wild sheep. Animals whose pelts or eggs are considered luxury items--such as spotted cats or sturgeon--could also fall prey. Traditional medicine can result in the overexploitation of animals and plants alike, as can ecotourism. Finally, the demand for rare species as exotic pets can lead to very high sums of money being shelled out for members of fast-eroding populations--a turtle from the Indonesian island of Roti and a Chinese gecko each brought in $1,500 and $2,000, respectively, in the international pet trade.

In its study, Courchamp's team claims that some wildlife traders note that declarations by conservation organizations that a species is endangered can lead to a spike in its value. Courchamp says conservationists must consider two factors when deciding whether to sound an alert regarding a disappearing species: the means by which it could be overexploited and whether a conservation network can adequately protect it. "If it is known that it is going to be a likely target for trophy hunting or traditional medicine or you cannot reasonably think that you can protect the species because resources are lacking, for example, then you need to think twice before declaring that it's rare," he warns. "On the other hand, if you don't say it's rare, then it's not going to benefit from programs of conservation or trade bans or these kinds of things."

Ginette Hemley, vice president of species conservation for the World Wildlife Fund, contends that Courchamp's findings are not applicable to the entire arena of species exploitation, but rather to the "rare, live reptiles that collectors seek out," such as those isolated to Roti, Madagascar and New Caledonia. "When we suddenly hear about a big demand for a population of a species--and oftentimes it is these rare reptiles and amphibians that these collectors go after and they are endemics and are very range-restricted--we don't publicize it until we know more about the situation," she explains. "We don't highlight that they're out there because it could increase demand."