u/itsaworkalt on r/foodforthought :

So when I see a stunning statistic like that I'm always a bit dubious. I'm doing some further digging but here's where you should look rather than this random article.

The report published by the World Wildlife Fund, which is very readable.

A technical breakdown of the Living Planet Index which is what has actually reduced by 60%. One potential issue I see is on page 9. Figure 4 shows where the data was collected from. As expected, this lines up pretty closely with a population density map

, but with some additional overweight on wealthy parts of the world over poorer parts. They do attempt to weigh things to overcome this, but I think it may be a fatal flaw in the data. They are overwhelmingly looking at parts of the world which have the most human intervention. So the stat might actually be "areas of the world that are most affected by humans have had a 60% drop in biodiversity in the last 45 years". Still not great, but less stunning than the headlines. This report simply doesn't have the data available to make any other claims.

More info on the LPI is here and it is described as:

The Living Planet Index (LPI) is a measure of the state of global biological diversity based on population trends of vertebrate species from around the world.

So basically to seems to me like what really is going on is there is this index that the WWF puts together called the LPI. This index has dropped by 60%, but that does not mean that 60% of Earth's wildlife has died. It means that this closest approximation we have for biodiversity has dropped by 60%.