The Red List, which is compiled by the World Conservation Union (IUCN), gauges a species' risk of extinction mainly on the basis of its population size, rate of decline and geographic range. But Alexander Harcourt and Sean Parks at the University of California, Davis, argue that this is not enough. They compare an endangered species to a house that has been left unlocked. The house is vulnerable to burglary, but it only becomes threatened when there is a burglar nearby. In the same way, a small population of animals susceptible to extinction only becomes actively threatened when it is being poached or its habitat is destroyed.
Harcourt and Parks advocate modifying the Red List criteria to include local human population density. Although a large number of people nearby may not in itself be a threat, they argue that hunting, pollution and habitat destruction, for example, are all likely to increase as people encroach on wildlife. What's more, data on human density is readily available. "We have the numbers, why not use them?" says Harcourt.
To illustrate their point, the researchers reassessed 200 primate species from the 1996 Red List. They found that 17 species designated as being at relatively low risk by the Red List should now be reassigned as high priority. Two such species are Wied's tufted-ear marmoset (Callithrix kuhlii) and the golden lion tamarin (Leontopithecus rosalia) from South America. Contrary to the expectations of many, the researchers also found that two high-profile species, the gorilla and the pygmy chimpanzee, or bonobo, should be downgraded to a lower level of threat (Biological Conservation, vol 109, p 137).