Neither.

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%"

Out of the 16700 populations tracked - spanning ~4k species - their average decline since 1970 is ~60%.

From the article:

> Between 1970 and 2014, the latest data available, populations fell by an average of 60%.