Living Planet Index: what does an average decline of 68% really mean?

Summary

The Living Planet Index is the biodiversity metric that always claims the headlines. Unfortunately many of these headlines are wrong. The index is very easy to misinterpret.

The Living Planet Index reports an average decline of 68% across tens of thousands of wildlife populations since 1970. This does not tell us anything about the number of individuals, species or populations lost, or even the share of populations that are shrinking.

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 20,000 of the world’s animal populations.

“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.

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 to see how the LPI is calculated. 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 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 67%.

If we calculate the average change of these two populations – just as we would do in the LPI – 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%.¹

The common misinterpretation of this – where headlines would incorrectly report that “we’ve lost 15% 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 3285. We then lost 3615 of them, to give us only 210 in 2017. This means we actually lost 95% of the rhinos. Not 15% like the LPI might wrongly suggest. The LPI is therefore a very 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. A 15% decline would suggest that the Black rhino is not doing particularly well, but has only seen a moderate decline over several decades. The true situation could not be more different. 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.

Seeing a moderate 15% decline 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.

	Black rhino (Tanzania)	Black rhino (Botswana)	Total (Tanzanian and Botswananian black rhinos)
Population size in 1980	3795	30	3825
Population size in 2017	160	50	210
Number of animals lost since 1980	3635	-20 (gained 20 rhinos)	3615
Percentage change in population size	-96%	+67%	-95%

The Living Planet Index tells us that studied animal populations have seen an average decline of 68% since 1970

Let’s then dig into the actual results of the LPI. The above headline is the main message of the 2020 Living Planet Index report.² It tells us that from 1970 to 2016, there was an average decline of 68% across studied animal populations.

What types and how much of the world’s wildlife does the LPI cover? In the latest report it covered 20,811 populations of 4,392 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, however the tropics tend to be 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 15% of known bird species; 12% of mammals; 5% of fish; 4% of amphibians and merely 2% of known 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 32% in 2016, signifying a 68% decline.

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 2016, on average, there was a 68% decline in population size across the 20,811 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 68% 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 68% average decline hides even more drastic declines in some populations. A few percent of the populations are in a very steep decline [we look at the role of outliers in the LPI in the dropdown box at the end of this article]. 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 20,000 populations that have been reduced to a single number.

How the Living Planet Index is 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.

Let’s look at a simple example of how this might work: how would a population experiencing an extreme decline be reflected in an averaged result across several populations? Say we have five lizard populations. One of these populations has experienced a terrible 99% decline in numbers. The other four have not changed at all – they have just as many individuals as they did decades ago. When we calculate the average change across these five populations we get a 20% decline.³

This would be the headline we see in the news. But actually this result is pretty useless. It does not tell us that most lizard populations are doing fine – we probably think they are shrinking. And worse, we are blind to the great peril of the one lizard population that is quickly vanishing. It has declined by 99%, but we only see the final result of a bad, but more moderate, decline of 20%. These averages don’t allow us to prioritize our conservation efforts where they’re most needed.

There are other hypotheticals that make this clear: 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.

This is not just borne out in hypothetical examples. It’s reflected in the LPI results. In a study published in Nature, Brian Leung and colleagues looked at how the LPI was affected by extreme declines (or increases) in a small subset of the studied populations.⁵ For this study they looked at the results of the 2018 LPI Report – we’ll look at a similar analysis for the 2020 update soon. It reported a 60% average decline in wildlife populations since 1970.⁶

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.

Impact of extremes on living planet index

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.

To be clear: these analyses do not undermine the severity of global wildlife losses. Even if we remove extreme outliers we still see a significant decline. But what they do highlight is how much we miss by reporting a simple global index value. We miss the few hundred species that have seen very dramatic declines. By mixing them into a single global index they have been completely lost from the headlines. Our priorities should be on protecting these populations, but we can’t because they have been lost in the noise. And we also fail to understand what populations across the world continue to do well and why. We assume that most species must be doing poorly.

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.

Lpi outliers 2020 — Impact of removing positive or negative outliers from the Living Planet Index⁷

Explore more of our work on Biodiversity

How does the Living Planet Index vary across the world?

How the Living Planet project helps us understand how the world’s wildlife is changing