What’s a conservationist to do when the biggest threat to the survival of a species is another rare species? In California’s eastern Sierra Nevada, endangered bighorn sheep could be pushed to extinction by the threatened mountain lions that like to eat them. Not all mountain lions have a taste for mutton — champions of the bighorns advocate tracking and killing those that do. But defenders of the big cats are outraged at this suggestion; they counter that humans should stand back and let nature take its course. Others argue that humans have already messed with the natural state of things — for example, removing the grizzly, the lions’ natural predator — and now’s no time to stop.