Wait, Have We Really Wiped Out 60 Percent of Animals?

By Ed Yong, a staff writer at The Atlantic, where he covers science. Excerpt:

"Since Monday, news networks and social media have been abuzz with the claim that, as The Guardian among others tweeted, “humanity has wiped out 60 percent of animals since 1970”—a stark and staggering figure based on the latest iteration of the WWF’s Living Planet report."

But that isn’t really what the report showed.

The team behind the Living Planet Index relied on previous studies in which researchers estimated the size of different animal populations, whether through direct counts, camera traps, satellites, or proxies like the presence of nests or tracks. The team collated such estimates for 16,700 populations of mammals, birds, reptiles, amphibians, and fish, across 4,000 species. (Populations here refers to pockets of individuals from a given species that live in distinct geographical areas.)

That covers just 6.4 percent of the 63,000 or so species of vertebrates—that is, back-boned animals—that are thought to exist. To work out how the entire group has fared, the team adjusted its figures to account for any biases in its data. For example, vertebrates in Europe have been more heavily studied than those in South America, and prominently endangered creatures like elephants have been more closely studied (and have been easier to count) than very common ones like pigeons.

Ultimately, they found that from 1970 to 2014, the size of vertebrate populations has declined by 60 percent on average. That is absolutely not the same as saying that humans have culled 60 percent of animals—a distinction that the report’s technical supplement explicitly states. “It is not a census of all wildlife but reports how wildlife populations have changed in size,” the authors write.

To understand the distinction, imagine you have three populations: 5,000 lions, 500 tigers, and 50 bears. Four decades later, you have just 4,500 lions, 100 tigers, and five bears (oh my). Those three populations have declined by 10 percent, 80 percent, and 90 percent, respectively—which means an average decline of 60 percent. But the total number of actual animals has gone down from 5,550 to 4,605, which is a decline of just 17 percent."