Here’s what a shocking new number on wildlife declines really means

Wildlife populations around the world continue dropping precipitously, according to an important but limited and often misinterpreted assessment that's issued every two years.

The declines reported by the Living Planet Index, a collaboration between two large conservation organizations, have been so steep as to feel disorienting. This year is no exception: A reduction of 73% in the average size of monitored wildlife populations in a mere 50 years, from 1970 to 2020. The previous figure was similar, a 69% decline through 2018.

But the findings do not mean that wildlife in general has dropped by that much.

This year's index was based on evidence from 34,836 local populations of 5,495 species, all of them vertebrates: mammals, birds, fish, amphibians and reptiles. Changes in tiny populations can have outsized effects on the global count because they are averaged...