Don't give up too soon. That's one piece of advice from a comprehensive analysis of missing and extinct mammalian species. The study shows that most of these species turned up alive after only three or more thorough field searches. All told, 67 species once considered missing have since been rediscovered. "It's an encouraging number," says conservation biologist Ana Davidson, a postdoctoral researcher at the University of New Mexico, Albuquerque, who was not involved in the study. "It provides us with incentives for future searches." Of course, once a missing species is found, then hard work of conservation begins.
The study was conducted by Diana Fisher and Simon Blomberg of the University of Queensland in Australia. Fisher became interested after studying the bridled nailtail wallaby, which was believed to be extinct for decades until a small population was "rediscovered" in 1973. "I realized that there are very many reports of rediscovered species, but that no one had looked at these as a whole."
Fisher and Blomberg created a database of all 187 mammal species that have been identified as extinct or possibly extinct, then combed through the literature to find out which ones had been rediscovered. They also included what threats the species had been facing, such as habitat destruction or hunting.
What makes a species likely to be rediscovered? According to the analysis, they are species that once had large ranges and then suffered substantial loss of habitat. In contrast, species that had been heavily hunted or preyed on by invasive species were less likely to still be around. One reason for the difference is that new predators, such as feral cats, pose a constant and growing threat, whereas some mammals can manage to persist in degraded or marginal habitat. Fisher and Blomberg report  their findings in the current issue of the Proceedings of the Royal Society B. These trends likely apply to other taxonomic groups, such as birds, they suspect.
It also helps to keep looking, obviously—but only up to a point. Species are more likely to be found alive if more than two field surveys are conducted. Several species—such as the thylacine, a carnivorous marsupial, or the forest ox known as the kouprey--have been searched for many times without success, that likely indicates they really are extinct, Fisher and Blomberg suggest.
Wes Sechrest, a mammalogist who runs an organization called Global Wildlife Conservation, agrees with much of the analysis. But he points out that not all field surveys are done properly—for instance, using automated cameras—so even a series of unsuccessful searches doesn't necessarily mean the species has vanished entirely. In fact, Sechrest is involved in planning a new search for the kouprey.