Delving into the Living Planet Index: a single metric for a complex problem
Simple, summary statistics are a great way to get people’s attention and demonstrate the seriousness of an issue. However, simple metrics can also mask different trends, be based on biased data and can be misleading if not reported correctly, as we have previously reported in Research the Headlines.
WWF, in collaboration with the Zoological Society for London, recently released their Living Planet Index (LPI): this shows the change in abundance of animal populations over time. The 2016 LPI was based on trend data from 14,152 populations from 3,706 vertebrate species. This is a large dataset and the overall trend is clear and compelling: multiple threats, including habitat loss and degradation, climate change, and direct hunting, contributed to a decline of 58% in animal populations over the last forty years.
What the media said
The media coverage of the LPI, its limitations and interpretation have been patchy. Firstly, few reports highlighted the fact that the LPI is measuring change from 1970 to 2012 and how this could influence our interpretation of the findings. The reason the LPI focuses on the past half century is a lack of data before 1970 (as described in the original publication describing the index). However, it does mean that the baseline for measuring change is an environment already modified by human impacts like large-scale deforestation and commercial fisheries. For example, one Alaskan salmon fishery declined from an average of 2 million fish caught per year in 1910 to 500,000 fish caught per year in the 1970s; this decline led to disruptive changes in the ecosystem that could impact the wider environment. By the time the LPI began to track changes in animal abundance, this population could have already declined by 75%. So, the LPI could be underestimating the overall change in animal abundance, or it could be that the recent decline shows a better or worse trend than previous time periods.
The second key point is that the LPI is focused on populations of animals, rather than species. This fact was highlighted well by some news agents but not others: headlines that said two thirds of animals will be lost in the next 20 years weren’t telling the whole story. The report predicted two-thirds of animal populations would be lost from 1970 to 2020, but the relative abundance of these populations and the number of lost species this represents is not clear.
The focus on populations is again due to the availability of data: populations are one of the most commonly studied units in wild species. The definition of ‘population’ is a bit fuzzy in biology, and can be dependent on what animals you study. Generally, it can be thought of as a group of animals that is somewhat independent from other groups of the same animal. This might be shown by specialised adaptations to their environment, or it might be due to a kind of isolation between groups; such as a deep trench preventing coastal animals moving around. The key idea is that these groups of animals often have distinct fates that are dependent on the unique set of threats they face. So, biologists need to monitor and manage discrete population separately.
It is also important to highlight that different populations of the same species can have very different trends in abundance, but this is not shown by the LPI. Let’s take the example of humpback whales. Commercial whaling greatly reduced the number of humpback whales, but many populations have been increasing rapidly since the moratorium on hunting in the 1980s. In fact, humpback whales as a whole species are now listed as ‘Least Concern’ by the International Union for the Conservation for Nature (IUCN), the international body that decides such things. It is not such a happy picture everywhere, though. Populations in Oceania and the Arabian Sea are listed as ‘Endangered’ by the IUCN because they do not seem to be recovering as well as other areas. The LPI will include trends in abundance for humpback whale populations that are increasing and recovering from whaling, and those that are not, into a species-wide average.
A third key point is that there might be a bias in the LPI data towards populations that are declining. Spending on conservation is often prioritised towards monitoring populations that are in trouble, rather than those that are stable or increasing. This means the research the LPI is based on is often focused on populations that are declining. In addition, the data are based on trends rather than absolute abundance. So, a small and rapidly decreasing population could have the same impact as a large and stable one on the LPI. Other biases in the underlying data, such as geographic coverage, are discussed by the University of St Andrew’s Professor Steve Buckland in a recent episode of the BBC podcast ‘More or Less‘.
Finally, the LPI mixes species and groups of species with different trends into one, just as it does with populations with different trends. This criticism was covered well by those media outlets that sought comment from independent scientists, as recommended by Research the Headlines.
Overall, having a single metric like the LPI will focus the public’s attention on the challenge of preserving biodiversity in the modern world. However, understanding what the metric is actually reporting needs consideration of how it is estimated and the potential biases in the underlying data.