This text is taken from the Courrier de la Planète of November 6, 2022. To subscribe, click here.
It’s hard to paint an accurate picture of the health of the world’s wildlife with a single brushstroke. However, as is often the case, it is in the details of the painting that the devilishly important elements are hidden.
When Brian Leung, a biology professor at McGill University, began digging into global data on vertebrate populations a few years ago, he expected to see widespread decay there.
“I also read the media. Originally, I thought all bands would be declining: some more, some less,” he says. However, over the course of his approach, this specialist in statistical analyzes found himself surprised several times: most populations of vertebrates are “stable”.
There is no question of denying the seriousness of the situation: certain groups of vertebrates, such as amphibians, are undergoing major declines. Vertebrates from certain regions of the world, such as those from the Indo-Pacific zone, are particularly affected. Close to home, think of the migratory caribou, whose herd on the George River has declined by 99% since the 1990s.
However, there is also a rebound in animal populations in certain regions of the world, particularly in Europe. Over the past fifty years, for example, groups of birds and mammals have shown statistically significant growth in northern Eurasia.
The raw material examined by Mr. Leung is the Living Planet Database. It brings together the observations of biologists from around the world who record the size of animal populations from year to year. The latest edition counts 32,000 populations of 5,230 different species.
“Substantial declines are underway, it’s true,” summarizes the researcher. But it should also be noted that in some places the situation is improving. It is important to be aware of this, particularly when developing international agreements on biodiversity. If nothing we had done in the past had improved the situation, that would be worrying. »
A difficult index to interpret
Every two years, the World Wide Fund for Nature (WWF) uses the Living Planet Database to produce its “Living Planet Index” (LPI). The latest version reports an “average decline of 69% of populations [de vertébrés] since 1970”. The Aichi Biodiversity Targets, adopted at COP10 in 2010, use GPI as one of their indicators of progress.
The interpretation of LPI is far from obvious. It is based on a geometric mean of trends in animal populations. Let’s take two populations of 100 individuals: let’s say that one doubles, the other decreases by half. The geometric mean will remain the same. However, there are 250 animals in all, or 25% more than originally.
Mr. Leung’s work, published in November 2020 in the journal Nature, show that a handful of “extremely declining” populations largely determine the catastrophe depicted by the LPI. These differ statistically from the prevailing trend. If they are removed from the calculation, the overall trajectory turns into a slight increase.
The chagoun vulture is one of those species that pull very hard on the average. This scavenger, once numbered in the millions in India, is now nearly extinct due to an anti-inflammatory drug it ingested while feeding on cattle carcasses.
“It’s a good addition to the literature,” said Philippe Marchand, a former professor of biostatistics at the University of Quebec in Abitibi-Témiscamingue, of the study by Leung and his colleagues. The article shows that the index is “very sensitive to extremes”.
More fundamentally, this advance puts into perspective the usefulness of tools that claim to represent all of the world’s wildlife with a single number. “How do we represent the living? asks Mr. Marchand. In practice, if the caribou decreases by half, this loss is not compensated by a doubling of the deer. »
According to the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), a quarter of the species belonging to the groups of animals and plants assessed by scientists are threatened, on average. The IPBES is the equivalent of the IPCC for biodiversity.
For the McGill University team, the next step is to link trends in animal populations to phenomena in the environment. Does pollution, climate change, the presence of invasive species or the creation of protected areas change the trajectory taken by populations?
“Protecting 30% of the earth’s surface by 2030 is a grand undertaking,” said Mr. Leung. […] Even if we do not achieve this objective, it will probably improve the situation. That said, how such a goal really translates to biodiversity is another question. »