Wolves have been disappearing mysteriously in Sweden. Between 1999 and 2009, 18 of the animals—or about 17% of the individuals that researchers have actively followed—have gone missing; the global positioning system (GPS) collars used to track them suddenly blinked off, and the wolves never reemerged. Researchers suspected poaching, but it's been hard to determine how much of a toll such clandestine kills have taken. Now, by using a new mathematical analysis, scientists have estimated that poaching accounts for half the deaths of Scandinavian wolves, potentially stymieing the rare predator's recovery.
As recently as the 1970s, not a single wolf lived in Norway or Sweden, says Guillaume Chapron, a conservation scientist at Grimsö Wildlife Research Station at the Swedish University of Agricultural Sciences and a co-author of the new study. DNA evidence has shown that those carnivores living in the region today descend from a single male-female pair that made the treacherous trek from Finland in the early 1980s and a second male that arrived in 1991. Packs have grown steadily from those three founders; in 2009, Sweden and Norway were home to 263 wolves.
To keep an eye on their numbers, Scandinavian researchers fitted 104 wolves with GPS collars between 1999 and 2009. When a GPS blip goes dead, conservationists with the Scandinavian Wolf Project SKANDULV go looking. Some teams circle wolf territories in helicopters, whereas others set out on skis or snowmobiles to follow tracks and locate scat for DNA testing. If these extensive searches turn up nothing, as happened with 18 wolves that disappeared over the past 10 years, Chapron and his colleagues suspect foul play. "We cannot find any other mortality cause that would destroy the wolf and the radio-tracking collar other than poaching," he says.
But bodies still haven't turned up for any of the lost wolves. Chapron suspects that poachers disposed of their remains and the GPS collars to cover up the crimes. So instead, the researchers turned to ecology to show the extent of poaching in Scandinavia. Chapron and his colleagues projected how fast the Scandinavian wolf packs should have grown between 1999 and 2009. Had wolves died only from known causes—illnesses, speeding cars, and a few cases of confirmed poaching—numbers would have grown from 74 animals to nearly 700. But in 2009, researchers counted fewer than 300 wolves in Sweden and Norway, the team reports online today in the Proceedings of the Royal Society B.
Poachers didn't kill 400 wolves directly but took out an unknown number of males and females that would've otherwise been able to breed and multiply. "You cannot really explain the population counts," Chapron says. "You need an extra source of mortality." In other words, hidden poaching.
Chapron says that in Scandinavia, much of the conflict between wolves and people arises from an unlikely source: moose hunts. Hunting dogs are a big part of these outings, a national pastime in Sweden. But wolves often maul or kill dogs when they get too close, and that gets hunters angry, he says.
Regardless of the motive, illegal kills account for about 50% of total wolf deaths in Scandinavia , Chapron and colleagues estimate. In two-thirds of those cases, poachers seem to be killing and ditching the evidence without anyone knowing. Such "cryptic poaching" takes a whopping toll on the population, and it's one that has gone unrecognized until now.
The difficulty in estimating poaching's toll is that people rarely fess up, even in regions lacking Sweden's strict law enforcement, says Julia Jones, a conservation scientist at Bangor University in the United Kingdom, who was not involved in the study. Adopting psychological tools drawn from interview-based studies of prostitutes and drug addicts, she investigates the illegal hunting of lemurs in Madagascar and poaching of other endangered animals. But Chapron's team went the other direction, one that she considers solid. "They're using purely ecological and biological data ... to infer something about a social behavior."
What works for Scandinavian wolves, however, may not work for other carnivores, Jones cautions. Wolf numbers in Sweden and Norway are small, and the predators don't stray far. Still, Chapron would like to try the analysis out on other big predatory animals, including, perhaps, North American wolves and grizzlies.
*This story has been updated to include the Grimsö Wildlife Research Station in the affiliation of Guillaume Chapron. A previous version listed him as a conservation scientist at the Swedish University of Agricultural Sciences in Riddarhyttan.