Published online 12 March 2010 | Nature | doi:10.1038/news.2010.120

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A quantitative way to decide whether to keep on conserving a species.

How do we know when a species really is as dead as a dodo? Science Photo Library

Declaring a species extinct and so not worthy of conservation money is not a trivial matter. It's a decision that scientists working for the International Union for Conservation of Nature (IUCN) have to make when drawing up their Red List of Threatened Species, for example. Now, a team of Australian researchers are trying to bring a more rigorous approach to this area with the help of the dodo.

It may sound ridiculous to work out whether conservation dollars should be spent on the dodo (Raphus cucullatus), but the question illustrates a problem for biologists: determining when you should give up on a species.

Cut off conservation money too soon and you could drive a species teetering on the brink into extinction. Keep conserving an animal long after it has last been sighted and you risk wasting money that could keep another endangered animal in the game.

"Declaring extinction is a difficult decision with large risks and large uncertainties," says Tracy Rout, based at the University of Melbourne's School of Botany in Australia, who led the new study reported in Conservation Biology1. "There are many cases where species that have thought to have been extinct have been rediscovered — so-called Lazarus species."

Here, going, gone

The IUCN's Red List declares an animal extinct only "when there is no reasonable doubt that the last individual has died".

What this means in practice is unclear, says Rout. Her work sets out a framework for making this decision and applies it to three species: the still-with-us mountain pygmy possum (Burramys parvus), the possibly extinct ivory-billed woodpecker (Campephilus principalis) and dead-as-a-dodo R. cucullatus1.

"In terms of funding allocations, decisions are currently made in an arbitrary way, varying from agency to agency and species to species," says Rout. "Our method would help make any implicit judgements of value more explicit."

The mountain pygmy possum — still worth saving. Biosphoto / J. L. Klein & M. L. Hubert / Still Pictures

Rout and her colleagues outline a model that uses sightings of endangered species — either as the number of surveys that have failed to spot the species or the time since it was last seen — to calculate the probability it is extant and then applies this to a decision-making framework that weighs the cost of continued conservation with the cost of stopping prematurely.

The probability that the dodo lives on somewhere is about 3.07 x 10-6. According to the new model, it would only be worthwhile to spend money conserving the dodo if the value of the conservation management — the value of the species assigned by conservationists multiplied by the decrease in the probability of extinction — is more than 17 million times the cost of management and monitoring.

By contrast, the chances of the mountain pygmy possum being present on Mount Buller in Australia are around 0.55. As a previous study has valued this population at Aus$200 million–Aus$450 million (US$180 million–US$410 million), it would be optimal to conduct up to 66 more surveys for the animal without finding it before declaring it extinct. At the moment, these surveys cost around Aus$15,000 per year.

The case for the ivory-billed woodpecker is complicated by controversial claims that it was seen in 2004, 2005 and 2006. These triggered huge conservation spending (see 'Still looking for that woodpecker').

Using only the widely accepted sightings of the woodpecker, Rout calculates it would have been optimal to declare extinction in 1965. Using the more controversial recent sightings makes 2032 the year that the species should be declared extinct if there are no more seen before then.

Let's get quantitative

Rout's paper is indicative of a growing trend in some parts of the conservation field, says David Roberts, a conservation biologist at the Durrell Institute of Conservation and Ecology, at the University of Kent in Canterbury, UK.

The ivory-billed woodpecker — a difficult case. J. Cancalosi / Ardea

"At the moment you can look at the IUCN Red List criteria and all the others are quantitative, whereas declarations of extinction are very much qualitative. There's this huge level of uncertainty," says Roberts, who supplied Rout with some of the data on the ivory-billed woodpecker.

"There's a big move towards more incorporation of decision-making theory and economics into conservation decision-making," she adds. "It will at least make it more transparent why we're working on a particular species."

Andrew Solow, director of the Marine Policy Center at the Woods Hole Oceanographic Institution in Massachusetts, says that papers of this sort are beginning to make an impact on conservation practices. Solow, who co-authored a paper with Roberts on the dodo's extinction2, adds, "The resources are scarce and any time you can allocate them better it's a good thing."