“In the last 50 years, Earth has lost 68% of wildlife, all thanks to us humans” (India Times)

“Humanity has wiped out 60% of animal populations since 1970, report finds” (The Guardian)

“We've lost 60% of wildlife in less than 50 years” (World Economic Forum)

These are just three of many headlines covering the Living Planet Index. But they are all wrong. They are based on a misunderstanding of what the Living Planet Index shows.

I sympathize with the journalists. Interpreting this metric is hard. I’m sure I’ve made similar mistakes in the past: using the terms ‘decline’, ‘lost’, and ‘fall’ interchangeably in biodiversity discussions. Combine this with the complexities of ‘populations’, ‘species’ and ‘extinctions’, and it gets complicated pretty quickly.

Before reporting on the Living Planet Index we should understand what it actually tells us about the world’s wildlife. We should also be aware of the misconceptions and pitfalls of using this index to capture the changes in more than 34,000 of the world’s animal populations.

What does ‘average decline’ actually mean?

The Living Planet Index (LPI) measures the average change in the number of individuals across the world’s animal populations. A ‘population’ is defined as a species within a geographical area. So, despite being the same species, the African elephant in South Africa and Tanzania are counted as different populations.

Let’s take a look at an example. This will not only show us how easily this figure can be misinterpreted but also why we should be careful when assuming that it gives us an accurate picture of what’s really happening to all wildlife populations.

We’re going to take a real-life example of two populations of the Black rhino: one in Tanzania and one in Botswana. In the first row of the table, we see their population size in 1980: there were 3795 rhinos in Tanzania, and only 30 in Botswana. In the following decades intense poaching in Tanzania has plunged its population to critically endangered status: in the second row we see that by 2017 there were only 160 rhinos left. Things in Botswana actually improved over time: its 30 rhinos increased to 50.

The difference between their population size in 1980 and 2017 is shown in the third row: this represents the number of animals lost over time. And in the final row, we see the percentage change in population size for each.

The Tanzanian rhinos obviously did not fare well: they lost 96% of their population. The group in Botswana did much better. In fact, their numbers increased by 67%.

If we calculate the average change of these two populations we get a value of -15%. Take the average of the change in Tanzania (-96%) and Botswana (67%). This means the Black rhinos saw an average decline of 15%.

Here, things get slightly more complex, because the actual LPI is calculated in a similar way but with one difference. It doesn’t just take the mean change across populations (called the arithmetic mean), it takes the geometric mean.¹ The geometric mean across these two populations is -74%.²

Let's keep that in mind. I'll continue to also explain this using the arithmetic mean because it's easier to follow and understand. But it's the geometric mean that the LPI uses. The numbers are different, but their implications are similar.

The common misinterpretation of these numbers – where headlines would incorrectly report that “we’ve lost 15% [or 74%] of animals” – is shown in the far right column. There we’ve summed these numbers to show the two populations combined. In 1980, the total number of animals was 3825. We then lost 3615 of them, to give us only 210 in 2017. This means we actually lost 95% of the rhinos. The LPI is therefore a different measure from the number or percentage of individual animals lost.

But this also highlights an even greater danger when reporting the LPI. By averaging these two populations we’ve ended up pretty clueless about the status of either of them. Either a 15% [or 74% using the geometric mean] decline would give a skewed understanding of the situation. The Black rhino in Tanzania has lost 96% of its rhinos and has become critically endangered. On the other hand, something is going right in Botswana because its numbers have increased. Both of these outcomes would be missed.

This might mean we don’t prioritize the Tanzanian black rhino when we really need to. And we might lose out on an important lesson from Botswana on how to increase numbers in critically endangered populations. This is why a more population-specific approach to conservation is needed, as I discuss in the dropdown box at the end of this article.

The Living Planet Index tells us that studied animal populations have seen an average relative decline of 73% since 1970

Let’s then dig into the actual results of the LPI. The above headline is the main message of the 2024 Living Planet Index report.³ It tells us that from 1970 to 2020, there was an average decline of 73% across studied animal populations.

What types and how much of the world’s wildlife does the LPI cover? In the latest report it covered 34,836 populations of 5,495 species across the world. It only covers vertebrate species – mammals, birds, fish, reptiles and amphibians.

It includes a large number of populations from each world region. In the latest report, the authors significantly increased the number of studies that were included in languages other than English. The number of included populations from Asia and the Pacific increased by almost one-third relative to the previous report. For Africa, coverage increased by 45%.

This is great progress. However, the tropics are still underrepresented relative to Europe and North America. This is not ideal, considering the tropics are home to the greatest diversity of species and is where wildlife is most threatened.

This reveals two further limitations. First, it only covers a tiny percentage of species: Only 16% of known bird species; 14% of mammals; 6% of fish; and 3% of amphibians and reptile species. It’s hard to say how representative the available data is: it’s often the case that the species we are most concerned about (deservedly) get the most attention in the research. Second, many taxonomic groups are not included at all – nothing on insects, fungi, coral or plants. This is largely due to data availability – it’s easier to count bears than ants. Still, we should be wary of generalizing these results to all life on Earth. [In our FAQ on the Living Planet Index I look at the geographical and species breakdown of data availability, and where it’s sourced from].

The global LPI is shown in the chart, where the value in 1970 is indexed to 100%. As we see, this has fallen to 27% in 2020, signifying a 73% decline.

Click to open interactive version

To be clear once again: the LPI does not tell us the number of species, populations or individuals lost; the number of extinctions that have occurred; or even the share of species that are declining. It tells us that between 1970 and 2020, on average, there was a 73% decline in population size across the 34,000 studied populations.

Effective conservation means we have to look past the average

To tell the real story on biodiversity, we have to be conscious of how the headlines are communicated. Losing 73% of the world’s wildlife within decades would be devastating. Thankfully it isn’t true.

But, this shouldn’t detract from the fact that the loss of many wildlife populations is deeply concerning. Unfortunately, averages are not particularly helpful in understanding what and where these populations are. When we look at more detailed analyses of the LPI we find that actually this 73% average decline hides even more drastic declines in some populations.

If we want to protect our most endangered species, these are the ones we need to prioritize. The average index unfortunately hides these populations from view.

We need to not only be careful with how we report the headline index itself. But we also need to be aware of what it does, or more importantly, does not tell us about how global wildlife is changing. As I cover in a related article, the Living Planet database itself is an incredible resource that allows us to dig deeper into the individual stories of the 34,000 populations that have been reduced to a single number.

Is the Living Planet Index sensitive to outliers?

An average isn’t helpful – we need to know where and what animal populations are in greatest danger. Some are doing much worse than the headline suggests, but they get lost in the averages. This means we can’t focus our efforts on protecting the species that need it the most.

We should also be careful about what the average actually tells us. It can, quite easily, be sensitive to outliers: populations that have seen a dramatic decline or increase.

We'll look at a simple example: let’s say we had an ecosystem where one population saw a decline of 99%, and 393 other populations each increased by 1%. Our final result would report an average decline of 50%.⁴ This is the difference between what we’d call a ‘catastrophic decline’ (a decline across most or all species) versus a ‘cluster decline’ (where it is a very specific set of species that are struggling). Our approach to tackling either of these scenarios would be very different.

After the publication of the 2018 Living Planet Index report, researchers published a follow-up study in Nature where they looked at how the LPI was affected by extreme declines (or increases) in a small subset of the studied populations.⁴ For this study they looked at the results of the 2018 LPI report. It reported a 60% average decline in wildlife populations since 1970.⁵

By looking at the population data underlying the LPI they found that this 60% average decline was driven by extreme losses in a small subset of populations. If you excluded the 2.4% most-strongly declining populations – which was 356 out of 14,700 – the result reversed from a 60% average decline to a slightly positive growth. In other words, 2% to 3% of populations were doing extremely badly, but it appears that most species were doing okay.

In the chart we see how the 2018 LPI result would have been affected by excluding the most extreme-negative populations. In red we see the final headline result of the report – a 60% average decline across the 14,700 populations. But as we exclude the most extreme negative populations, first 120 then 238 populations, we see that this average decline reduces significantly. Then, when we exclude the 356 most-severe populations, not only does the average decline reduce, it actually turns into a net positive. The abundance across these populations was, on average, increasing.

But, extreme positive outliers can also have a strong impact on the final result. In the 2020 update of the LPI they updated this analysis to show the impact of removing extreme negative, extreme positive, and both outliers combined. This is shown in the chart. Again we see the impact of removing negative extremes: removing the most severe 5% of populations turns the average 68% decline into an average increase. But removing positive extremes also has a large impact. When we remove even a few percent of the populations growing the most, we see an average decline of greater than 70%. Finally, if you remove both negative and positive outliers together we still see a significant average decline, although less than the reported LPI of 68%. If we remove the most extreme 10% of populations we get a 42% average decline since 1970.

In response to this criticism to the sensitivity of the LPI to outliers, the authors tested this sensitivity in the 2022 report. To do this they removed 2.5%, 5% and 10% of the most extreme declining and increasing populations, and recalculated the index each time. While they got slightly different results from the 73% average decline reported, the overall trend is very similar in each one.

This suggests that the final result is not mostly driven by the combination of extreme declines and increases. It might be the case that populations that have experienced very large increases cancel out the impact of very large declines. That would mean the overall result is not affected much when both are removed.

Regardless of how much of the final result is driven by extreme outliers, this discussion highlights the continued issue of trying to summarise global biodiversity trends into a single index number.

What is more useful for conservation is looking at clustered declines: cutting through the average to identify where and what types of populations are seeing dramatic declines. Leung et al. (2020) highlighted several clusters of species that have struggled: reptile, amphibian and mammal species across Latin America; Indo-Malayan freshwater birds and amphibians; Arctic mammals; and both marine and freshwater fishes across most of the world’s environments.

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Hannah Ritchie (2022) - “Living Planet Index: what does it really mean?” Published online at OurWorldinData.org. Retrieved from: 'https://ourworldindata.org/living-planet-index-decline' [Online Resource]

@article{owid-living-planet-index-decline,
    author = {Hannah Ritchie},
    title = {Living Planet Index: what does it really mean?},
    journal = {Our World in Data},
    year = {2022},
    note = {https://ourworldindata.org/living-planet-index-decline}
}

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