"the average population has declined by 60%. If you have two populations, white rhinos (8 animals) & pigeons (a billion animals), and four of each of them die, then "the average population" has declined by more than 50% but the total number of animals has declined by 0.0000007%"
The best explanation I've seen is: https://twitter.com/TomChivers/status/1057210578429755393
"the average population has declined by 60%. If you have two populations, white rhinos (8 animals) & pigeons (a billion animals), and four of each of them die, then "the average population" has declined by more than 50% but the total number of animals has declined by 0.0000007%"