It’s no secret that illegal wildlife hunting threatens the existence of many species, including the iconic African elephant. Shockingly, however, the magnitude of the threat has remained elusive, in large part because of the covert nature of poaching.

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Now a team of American, Kenyan and British researchers has finally put continent-wide figures on African elephant slayings, using the latest data from a combination of elephant monitoring programs.

The World Wildlife Fund estimates that 470,000 to 690,000 African elephants remain in the wild, and they are listed as vulnerable by the International Union for Conservation of Nature, although that designation has not been updated since 2008. In the new paper, the team estimates that the total population of African elephants has declined by 2 percent each year since 2010, while some individual populations have declined by as much as 7 percent annually. The analysis also revealed that, on a continent-wide scale, elephant poaching began picking up in 2008 and peaked in 2011, with around 40,000 animals killed that year alone.

Elephant poaching is primarily driven by an insatiable desire for animal parts used as trophies, medicine or food in countries such as China and Vietnam. Killing elephants and taking their tusks is a criminal act, and poachers do what they can to cover their tracks. Animals tend to be shot deep in the forest or savannah, so their carcasses often go undiscovered.

Even when those remains are found, sometimes wildlife managers don’t have the tools or training necessary to determine whether a human was behind the elephant’s death or whether the animal died of old age or natural causes. Other times, the data are never published online or reported to any central, easily accessible database.

As such, killing rates of elephants have to be pieced together by comparing rough population estimates to the number of recovered body parts—skins, tusks, bones or meat. But like guns, drugs or other illegal goods, authorities only seize a small fraction of what they estimate to be the total amount of black market merchandise.

To get around these problems, the authors of the new study, published today in the Proceedings of the National Academy of Sciences, drew upon a system launched in 2002 called Monitoring the Illegal Killing of Elephants (MIKE). Wildlife managers at 45 parks and reserves across Africa report any elephant carcasses they find during their patrols to MIKE, and they note what they believe to be the cause of death.

To fine-tune those figures, the team turned to the Samburu National Reserve in Kenya, where park managers intensively monitored elephant populations from 2009 to 2012. Because they knew the elephant populations so well, they also knew when elephants disappeared—and thus presumably were no longer alive. During that time, 20 elephants died, although they could not locate all of the carcasses.

Combining figures for the total elephant population, the number that died, the number of carcasses found and the number of elephants confirmed to have been poached, the team statistically estimated the rate of poaching, including the uncertainty surrounding that figure. Then they applied that formula to the MIKE sites and compiled those data to get a continent-wide estimation of the total killing rates based on 287 elephant populations.

While their results are not exact, the team’s figures do represent the most comprehensive data yet about elephant killings in Africa, showing that numbers are going down across the continent. As such, the new data could help inform whether or not elephants’ conservation status should be switched from vulnerable to endangered.

The researchers’ findings also hint at some useful leads for halting the decline. For example, why did the killing subside slightly after 2011? At the end of that year, China implemented ivory auction restrictions. Further studies could verify whether those two events are linked, which would suggest that similar strategies could help curb illegal killings.

The paper’s biggest message, however, is straightforward: “It is obvious that stemming the rate of illegal killing is paramount,” the authors write. “Current ivory consumption is not sustainable.”