The biodiversity crisis is a math problem. Unlike most math problems, however, this is one where getting hung up on the precise numbers can lead you astray. Maybe 1 million species are at risk of extinction. Or if you’re going by species that scientists have specifically identified as threatened, it’s 42,100. But neither of these is exactly right. At least we can agree that extinction rates are 1,000 times higher than historical averages. Or is it 100 times higher?