How does the Living Planet Index (LPI) work?

Robin Freeman

​​Head of Indicators & Assessment Unit at ZSL (Zoological Society London), and co-project lead on the Living Planet Index (LPI), Dr Robin Freeman explains what indicators like the LPI measure, what they do not, and how they fit into the wider range of tools which tell us how ecosystems and biodiversity are changing. 

Understanding the status and trends of wildlife populations is critical in understanding the health of the ecosystems on which we rely. It is also important in identifying those animals and places that are most in need of conservation action. The Living Planet Index (LPI) measures the state of the world’s biological diversity based on vertebrate population trends from around the world. It now tracks the abundance of almost 21,000 populations of mammals, birds, fish, reptiles and amphibians. For two decades it has used the trends that emerge as a measure for changes in biodiversity. The building blocks for this indicator are wildlife population datasets gathered from almost 4,000 sources. It is used to inform WWF’s Living Planet Report which in 2020, revealed a 68% decline in the LPI between 1970 and 2016: a statistic that cast a spotlight on biodiversity loss around the globe. 

New research published recently highlights the sensitivity of the LPI to extreme declines and increases in populations prompting discussions about how metrics like the LPI capture changes in biodiversity. These discussions have highlighted the sensitivity of the LPI to extreme increases as well as declines (Loreau et al. 2022), which we have discussed in an earlier blog, describing the importance of historical baselines in measuring biodiversity (Mehrabi and Naidoo 2022), and the importance of those species populations that are in significant decline (Murali et al. 2022). Whether indices like the LPI can measure abundance itself is another interesting question. Importantly, the LPI does not measure abundance (Puurtinen et al. 2022), instead it looks at the average change in relative abundance, which we discuss below. 

 

How do we measure biodiversity change?  

Ecological systems contain many interacting species all impacted by changes in the environment they live in, but also in the species they interact with. It is fair to say that there is no single metric that successfully captures all changes in the status of a natural system. What indicators like the LPI do is to measure one particular and important aspect of these changes: how wildlife populations are changing (their average change in relative abundance).  Other indicators like the Red List Index, Biodiversity Intactness Index all capture other important aspects of biodiversity change and should be considered in combination as a holistic picture of biodiversity change.

 

What is relative abundance and why is it important? 

Indicators like the LPI focus on changes in the relative abundance of species populations. In other words, they are trying to capture how wildlife populations are changing over time on average. Some of those populations may contain many individuals, some very few, but it is the average relative change we are trying to measure accurately here, rather than the total change in absolute numbers of individual animals      .  

The LPI and the Wild Bird Index for example, both measure the geometric mean of relative abundance. Geometric means are routinely used for tracking growth rates, like population growth or interest rate. A number of papers have considered how different indicators might best capture changes in biodiversity, and have concluded that the geometric mean of relative abundance is a particularly useful method of capturing important aspects of biodiversity change. 

Often, these metrics are misunderstood as trying to measure abundance itself. Changes in overall abundance may be very important for some aspects of biodiversity (e.g. some ecosystem services), but how individual populations are changing can reflect their overall health and likelihood of disappearing. We also do not often have real abundance data of wildlife populations, perhaps only a proxy (number of nests) or a previously calculated index (where the number of animals itself is presented like an LPI). The LPI gives us an accurate measure of changes in relative abundance of wildlife populations. This difference can be highlighted by some simple examples: 

Firstly, if we have four populations that double from 100 individuals to 200 individuals, the LPI would capture that doubling (by indicating an LPI value of ‘2’ for that second year). 

 

  Population 1 Population 2  Population 3  Population 4     
Year 1  100  100  100  100  Total Year 1  400 
Year 2  200  200  200  200 Total Year 2  800 
Change  2 2 2 2 LPI 2

 

Similarly, if we have four populations that halve from 100 individuals to 50 individuals, the LPI would capture that halving (by indicating an LPI of ‘0.5’ for that second year). 

  Population 1 Population 2  Population 3  Population 4     
Year 1  100  100  100  100  Total Year 1  400 
Year 2  50 50  50  50 Total Year 2  200 
Change  0.5 0.5 0.5 0.5 LPI 0.5

It is important to note here that the LPI value for doubling (‘2’) is a greater absolute difference from stability (‘1’) than the LPI value for halving (‘0.5’). However, the proportional changes are in fact equivalent. 

This is highlighted when we consider 4 populations that all have 100 individuals in year 1 and two of those populations halve (to 50) and two double (to 200 individuals). Here, we may have increased the number of individuals (from 400 to 500), but the LPI would give a value of 1 (indicating no average change in relative abundance). This is because these proportional changes are considered equivalent. For the populations that have halved, they would need to double to recover and if the increasing populations returned to their original size, they would have halved. 

  Population 1 Population 2  Population 3  Population 4     
Year 1  100  100  100  100  Total Year 1  400 
Year 2  200 50 200 50 Total Year 2  500
Change  2 0.5 2 0.5 LPI 1

 

Changes in proportions vs changes in numbers 

This can be further highlighted by looking at population trends graphically. Let’s consider three hypothetical species populations over the last 50 years, each starting with 1000 individuals in 1970. By 2020, one population has increased to 2500 individuals (250% increase, Pop1), one has declined to 580 individuals (-42% decline, Pop 2) and one has declined to five individuals (-99.5% decline, Pop3) 

a coloured data graph

One way to consider the change in these populations is to take the average number of individuals over time (‘Arithmetic average). This accurately captures how the number of individuals may have changed, but overestimates the average proportional change across these populations. Two of these populations have declined, one considerably, but this isn’t captured well. In this figure, the geometric mean (‘LPI’) may appear to be underestimating the average trend, but this is because the population trends are displayed in a way that can mask an important aspect of proportional change.  

The smallest a population can become is 0, but it can (in theory) grow infinitely. The space between 0-1 is actually equivalent to the space from 1 – infinity. This is much more clearly displayed if we present the figure logarithmically.  Here the extreme decline in the third population is displayed more accurately. Such a strong proportional change (a decline of -99.5%) would require that population to increase by almost 10000% to recover. Capturing the importance of these relative changes is best done using geometric means and indicators such as the LPI. 

In summary, understanding how biodiversity is changing is critical. Particularly as human impact on our ecosystem continues. Biodiversity is complex and multifaceted and thus requires a suite of different tools to effectively measure its change. Indicators such as the Living Planet Index are a useful part of this suite. Indicators such as the Red List Index, Biodiversity Intactness Index, Species Habitat Index and Living Planet Index together can give us a picture of how ecosystems and biodiversity are changing. At present, many of these continue to show continued global declines.  

For further insight, a blog delving into the Living Planet Index can be read here:

Find out more


References: 

http://ww2.rspb.org.uk/our-work/conservation/conservation-projects/details/192786-global-wild-bird-indicators 

Lamb et al (2009) Ecological Indicators (10.1016/j.ecolind.2008.06.001); van Strein et al (2012) Ecological Indicators (https://doi.org/10.1016/j.ecolind.2011.07.007); Santini et al (2017) Biological Conservation (https://doi.org/10.1016/j.biocon.2016.08.024). 

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