ALMAD:
The Living Planet Index measures the average change in abundance across monitored wildlife populations. When it reports a 73% decline, this means that on average, the populations being tracked have 73% fewer individuals than they had in 1970 (Our World in Data) (Our World in Data) (the baseline year).
What it measures:
Data from nearly 40,000 populations of more than 5,200 species (ZSL)
Each population's trend is calculated by tracking how the number of individuals changes over time
These trends are then aggregated using a weighted geometric mean to produce the overall index
Think of it this way: if you monitored 100 deer in a forest in 1970, and that same population now has 27 deer, that's a 73% decline for that one population. The LPI averages these types of changes across thousands of populations.