Living Planet Index says wildlife has declined 69% in 50 years, but what does that mean?

We all know that wildlife populations are in decline. It’s a story that’s as old as humanity. You can almost trace the human story backward by following the trail of extinct species we’ve left in our wake. From the dodo to the woolly mammoth and countless species in between, we’ve been wiping things off the face of the planet from almost the moment we arrived on the scene. Those early efforts were slow, methodical, and carried out by hand. Modern humans, however, have turned wildlife decline into an efficient and almost invisible background phenomenon.

Over the last century or so, the level of wildlife loss has skyrocketed and getting our hands around what is being lost and how quickly has become increasingly challenging. Most animals won’t exactly line up to be counted or reply to a census, so we’re left to make estimates based on the necessarily incomplete data we have on hand.

A particular research team might have a good handle on a specific population from a specific species, but ecosystems are interconnected and interactive. Knowing how well one group of animals in one part of the world is doing, doesn’t necessarily tell us about the system as a whole. All of those disparate bits of information — studies carried out by unrelated research groups across the globe and across time — need to be brought together into a more cohesive tapestry. That’s the goal of the Living Planet Index.

Every two years the project, a joint effort between the World Wildlife Fund and the Zoological Society of London, publishes a readout of wildlife populations around the globe. It endeavors to be as comprehensive as possible but there are some fairly large limitations. The latest index was published earlier this month and it compares population data from 5,320 species, made up of 32,000 populations, dating back to 1970.

As is increasingly the case with environmental and ecological news, the results of the most recent index aren’t encouraging. The team found that wildlife populations have declined roughly 69% between 1970 and 2018, the last year the survey has data for. The reasons are fairly well understood. In aquatic ecosystems, populations are declining as a result of overfishing or overhunting. On land, population decline is largely driven by agriculture, as wild habitats are converted into farmland. For a deeper look into how the food industry negatively impacts the environment, check out Eating Our Way to Extinction, streaming on Peacock.

Just looking at that number paints a fairly stark picture. It suggests that more than two thirds of wildlife have died off over the course of 48 years, but that isn’t precisely true.

While the statistics are legitimately troubling, there is some noise in the data. The survey looks at individual populations and calculates their loss over time. Then those numbers are averaged together to get an overall number. Because of the way the survey calculates their final number, it means that populations which are doing particularly well or particularly poor can skew the data.

By way of example, let’s suppose we have two populations. One population starts with five individuals and ends with two. A 60% loss. The second population starts with 50 individuals and ends with 45. A 10% loss. We would say there was an average loss of 35%, even though only 14.54% of the total was lost. Some populations are hit harder than others and that can make getting a clear picture difficult.

A prior study looking at the data from a previously published index found that only 3% of populations were experiencing dramatic declines and driving the average up. Attempting to account for statistical anomalies, the team removed 10% of the data, accounting for those populations which were outliers on either end of the spectrum, and they still found an overall loss of about 65%.

The index is also limited by the way it gathers data. It is reliant on published scientific works, so there is some bias toward animals which are of interest to researchers. It’s a large sample size but there are necessarily some variables which are excluded. At present, the index only looks at vertebrates. That’s largely due to the availability of published data, which isn’t as strong for invertebrates, despite their outsized contribution to world wildlife populations.

Having that data would provide a more complete view of things and would undoubtedly change the numbers. It isn’t all bad news though; the authors note that roughly half of all populations are either stable or increasing. And that sounds objectively good.

The truth is, this is a more complicated problem than we can easily get our hands around. Things might not be quite as bad as they initially appear, but there’s no question that wildlife populations are dwindling. Moreover, climate change will continue to put added strain on habitats and populations as the world warms. In the end, the exact numbers aren’t all that important. It’s enough to know there is a problem, to identify the causes, and take action to mitigate the consequences. We have all of that in hand, now we just need to do the work.