A spike in poaching has tipped the African elephant into decline, a new study finds. As many as 40,000 elephants were killed in 2011, a 3% loss to the total number of elephants on the continent. The research extrapolates from a survey of elephant deaths to provide estimates for many less known populations. “Some of the assumptions in the paper are quite conservative, and it is possible that the real situation is worse than indicated,” says Chris Thouless of the World Wildlife Fund in Windhoek, who wasn’t involved in the new work.

No one knows exactly how many elephants remain in Africa—perhaps a half million—because many areas have not been surveyed recently, and the methods are imprecise. In general, elephants have flourished in relatively well-protected parks in southern Africa, but they have been slaughtered in the lawless forests of central Africa. These populations "are on the front end of the spear,” says wildlife biologist George Wittemyer of Colorado State University, Fort Collins. “It’s been a disaster." Still, many biologists hoped that births in the southern parks outnumbered the deaths elsewhere.

To quantify the trends across the continent, Wittemyer and colleagues turned to a survey of elephant deaths run by the Convention on International Trade in Endangered Species. The program, called Monitoring the Illegal Killing of Elephants (MIKE), has collected data on carcasses in 45 places to investigate the role of the ivory trade in elephant poaching. But it is difficult to convert these data into demographic trends, Wittemyer says. Not all carcasses are found by the surveys, for example, and cause of death can’t always be established.

Wittemyer and colleagues calibrated the survey data with their detailed research on an elephant population in Samburu National Reserve in central Kenya. Focusing on the 12 MIKE sites with the best data, they then extrapolated across 306 elephant populations in Africa. All told, between 35,000 and 47,000 elephants were poached in 2011, the worst year in the 10-year record, they report online today in the Proceedings of the National Academy of Sciences. A similar result came from a different method of estimating illegal killing, which used a statistical model incorporating factors related to poaching, such as corruption and poverty.

The news is not good for African elephant populations. By using a model of population growth, the team found that the continental total of the animals declined by 8% in 2011 due to poaching. “Before this [study], we haven’t had anything that gave us numbers,” Wittemyer says. “We had people speculating.” The model confirmed that elephant populations in eastern and southern Africa were in good shape until 2008, but then started to decline. Now, about 75% of the populations across the continent are shrinking. The real-world impact could be even more dire than the model predicts, Wittemyer says, because poachers target the largest adults, whose deaths decrease birth rates and disrupt social networks.

Fiona Underwood, a statistician and visiting researcher at the University of Reading in the United Kingdom who wasn’t involved with the research, suspects there is much uncertainty in the estimate of the extent of poaching and the consequent impact on populations, which is “unfortunately a fair reflection of the quality of available data,” she says. But that is no cause for inaction, says Justin Brashares, a conservation biologist at the University of California, Berkeley, who also wasn’t involved in the study. “We must work with uncertainty in these situations because the potential consequences of inaction are severe.”

The rate of poaching across Africa seems to have declined to about 5% last year, which is still unsustainable for the population, Wittemyer says. He credits investments in fighting poachers and hopes that efforts to reduce demand in China for ivory are starting to help. But “we’re still seeing massive killing of elephants across the continent,” he says. “It’s not even close to being over.”