this post was submitted on 09 Oct 2026
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Explain Like I'm Five

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ELI5: What does "a 73% average decline in wildlife populations since 1970" mean? (ourworldindata.org)
submitted 1 day ago by Artisian@lemmy.world to c/explainlikeimfive@lemmy.world
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I understand that it does not mean

this doesn’t mean 73% of populations are in decline; in fact, around half of the studied populations were in decline, while half were either increasing or stable.

Maybe I should ask why the LPI report uses such a weird number instead?

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[–] threeonefour@piefed.ca 2 points 22 hours ago (1 children)

Maybe I should ask why the LPI report uses such a weird number instead?

Take your population of 10,000 rhinos. It drops by 90% to 1,000. The next year it increases by 90% to 1,900. If you did a normal average of the changes, you'd get (-90%+90%)/2 = 0% change. That's a useless number because there has obviously been some major changes between the two years.

If you use a geometric average you'd get sqrt(0.1 × 1.9) ~= 0.436 or roughly a 56% average decrease. This is more meaningful than saying the average change was 0%.

However, you can't use that number to work out how many rhinos there are. It's used to measure an overall trend. If the average decrease gets smaller, it would mean more populations are increasing and less populations are decreasing. Overall, that's good. That's what the number is meant to be used for.

If the average decline next year is 68% then that'd mean we're doing something right because last year the average decline was 73%. What are we doing right? Which populations are better? Which are worse? You can't tell from the average. It's just a general marker of the overall trend.

[–] Artisian@lemmy.world 1 points 17 hours ago

But if I take a population that dies out, any population that dies out, in my sample, then the geometric average becomes 100% loss. There are definitely cases where the geometric average also gives a weird answer.

(I'll also note that their headline number changes quite a bit year-to-year, as they expand the dataset and fiddle with how important they treat each part.)

I strongly agree with the base dataset - I just don't understand why this is the best indicator. If we care about total population, we could just compute with the sum. If we care about how many populations are stable/growing, those numbers are also easy to get from the same data.

[–] Artisian@lemmy.world 1 points 1 day ago* (last edited 1 day ago)

My own attempt, but I'm confused.

There might be many herds of each species, all scattered in different places. The report counts all the animals in each of these herds over time, and looks at how those change. Consider some rhino's with two herds that are far apart. One of them is a giant herd, with 10000 rhinos, while the other is tiny with just 50. If the giant herd mostly dies out, and drops to 1000, while the tiny herd gets 5 times bigger to 250, then the average percent change is (-90% + 500%)/2 = +205%, even though almost all the rhinos died.

Except this is wrong; they actually take the geometric average, which I think would be sqrt( 0.1 * 5) = 0.25, a 75% decline?

Now, swap which populations do well. the 10000 rhinos become 50000 strong, and the 50 rhinos shrink to just a family or two of 10. The relative percent changes are the same; 500% for one, 10% for the other, so we would still report a 75% decline.