Researchers reported a staggering decline in wildlife. Here’s how to understand it

Counting wild animals — on land and at sea, from gnats to whales — is no small feat. Most countries lack national monitoring systems.

One of the most ambitious efforts to fill this void is published every two years. Known as the Living Planet Index, it’s a collaboration between two major conservation organizations: the World Wide Fund for Nature and the Zoological Society of London. But the report has repeatedly resulted in inaccurate headlines when journalists misinterpreted or overstated its results.

The assessment’s latest number, issued Wednesday by 89 authors from around the world, is its most alarming yet: From 1970 to 2018, monitored populations of vertebrates declined an average of 69%. That’s more than two-thirds in only 48 years. It’s a staggering figure with serious implications, especially as nations prepare to meet in Montreal this December in an effort to agree on a new global plan to protect biodiversity. But does it mean what you think?

What the data does and doesn’t mean

Remember that this number is only about vertebrates: mammals, birds, reptiles, amphibians and fish. Absent are creatures without spines, even though they make up the vast majority of animal species (scientists have even less data on them).

So have wild vertebrates plummeted by 69% since 1970?

No.

The study tracks selected populations of 5,320 species, vacuuming up all the relevant published research that exists, adding more each year as new data permits. It includes, for example, a population of whale sharks in the Gulf of Mexico counted from small planes flying low over the water and birds tallied by the number of nests on cliffs. Depending on the species, tools like camera traps and evidence like trail droppings help scientists estimate the population in a certain place.

This year’s update includes almost 32,000 such populations.

There’s a temptation to think that an average 69% decline in these populations means that’s the share of monitored wildlife that was wiped out. But that’s not true. An addendum to the report provides an example of why.

Imagine, the authors wrote, we start with three populations: birds, bears and sharks. The birds decline to 5 from 25, a drop of 80%. The bears fall to 45 animals from 50, or 10%. And the sharks decrease to 8 from 20, or 60%.

That gives us an average decline of 50%. But the total number of animals fell to 92 from 150, a drop of about 39%.

The index is designed that way because it seeks to understand how populations are changing over time. It doesn’t measure how many individuals are present.

“The Living Planet Index is really a contemporary view on the health of the populations that underpin the functioning of nature across the planet,” said Rebecca Shaw, chief scientist at WWF and an author of the report.

Another important factor is the way monitored populations end up in the index. They don’t represent a broad, randomized sampling. Rather, they reflect the data that’s available. So there is quite likely bias in which species are tracked.

One controversy has been whether a small number of populations in drastic decline call into question the overall results. Two years ago, a study in Nature found that just 3% of populations were driving a drastic decline. When those were removed, the global trend switched to an increase.

The paper sparked a flurry of responses in Nature as well as additional explanation and stress testing for this year’s update. On the bright side, the authors note that about half of the populations in the Living Planet Index are stable or increasing. However, when they tried excluding populations with the most drastic changes in both directions, down and up, the average descent remained steep.

“Even after we removed 10% of the complete data set, we still see declines of about 65%,” said Robin Freeman, head of the indicators and assessments unit at the Zoological Society of London and an author of the report.

So is it still bad?

Yes. Some scientists think the report actually underestimates the global biodiversity crisis, in part because devastating declines in amphibians may be underrepresented in the data.

And over time, the trend is not turning around.

“Year after year, we are not able to start improving the situation, despite major policies,” said Henrique M. Pereira, a professor of conservation biology at the German Center for Integrative Biodiversity Research who was not involved in this year’s report. “At most, we have been able to kind of slow down the declines.”

Latin America and the Caribbean saw the worst regional drop, down 94% from 1970. The pattern was most pronounced in freshwater fish, reptiles and amphibians. Africa was next at 66%; Asia and the Pacific saw 55%. The region defined as Europe-Central Asia saw a smaller decline, at 18%, as did North America, at 20%. Scientists emphasized that far steeper biodiversity losses in those two areas likely occurred long before 1970 and aren’t reflected in this data.

Scientists know what’s causing biodiversity loss. On land, the top driver is agriculture as people turn forests and other ecosystems into farmland for cattle or palm oil. At sea, it’s fishing. There are ways to do both more sustainably.

If climate change is not limited to 2 degrees Celsius, and preferably 1.5 degrees, its consequences are expected to become the leading cause of biodiversity loss in coming decades, the report said.