Every year, from August to November, Travis Livieri becomes nocturnal. The field biologist goes out in his truck in Conata Basin, South Dakota, armed with a spotlight in search of one of the most endangered mammals in North America: the black-footed ferret. When the light catches the reflective green shine of the ferret’s eyes, he waits for the animal to disappear into a burrow and then lays a trap at the entrance.
Once the ferret is trapped, Livieri coaxes it into a long black tube and anaesthetises it before giving it a vaccine shot. Then he takes a blue marker and draws a line from the ferret’s left ear to its right shoulder. About a month later, he returns again at night to the same location to give the ferret its booster, drawing a line from its right ear to its left shoulder. Ferrets marked with an X are safe from the plague.
Although they have been protected by the US Endangered Species Act since it was signed into law in 1973, there are only about 300 black-footed ferrets alive in the wild today, spread across about 20 sites in the western US, Canada and Mexico. Every one of them is descended from just 18 ferrets that were taken into a captive breeding programme in the 1980s, after conservationists failed to keep the last-known population alive in the wild.
Habitat loss and the widespread shooting and poisoning of prairie dogs, a herbivorous rodent that makes up more than 90% of the ferret’s diet, are both threats to the black-footed ferret. But nothing poses a greater existential threat than the bacteria Yersinia pestis – otherwise known as plague.
Yersinia pestis, which killed millions of people across Europe in the middle ages, is the ferrets’ biggest biological enemy, says Livieri, who founded the conservation nonprofit Prairie Wildlife Research in 2001. “Plague is something that can wipe out all the prairie dogs and all the ferrets pretty easily,” he says.