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Tanzina Vega: All right. We talk a lot about humans on this show, for good reason, but we're going to take a little trip out of the human-run world and talk about our friends in the animal kingdom for a minute.

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As we all know, wild animals play all kinds of important roles in keeping the ecosystems we depend on healthy. Bats and bees and earthworms whales and sharks, they're all part of a complex environmental weave.

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It's become harder and harder for animal populations to exist at optimal levels as human activity like agriculture encroaches on their space and human-fueled climate change drastically alters their habitats. Now, a new report from the World Wide Fund for Nature shows that animal populations have fallen 68% in the past 50 years. I asked Eric Roston about this. He's a climate reporter at Bloomberg Green.

Eric Roston: The 2020 WWF Living Planet report documents a 68% decline in the population of vertebrate animals since 1970, so that includes birds and the amphibians, mammals, reptiles, and fish as well.

Tanzina: When you say we, you mean humans, of course.

Eric: I do. The report, which was written by dozens of authors in conjunction with both the WWF and the Zoological Society of London, documents that it's human consumption. The systems that make our economy go globally are really having this catastrophic effect on animal populations. The biggest challenge is addressing how we use the land. Deforestation is a huge driver of these population declines. Converting that virgin forest to agriculture not only leads to about half of the overall declines that scientists are seeing, but it's also a major contributor to climate change as well.

Tanzina: As communities get larger and larger and people are looking for places to build, we seem to be building housing and other developments on land that animals once lived on. Is that partly to blame, or is it really more industrial land use that's partly to blame?

Eric: It's both, but you're absolutely right that the whole planet is urbanizing. As a result of that, we're seeing the destruction of these forest ecosystems. There's a number of reasons why this is a problem. One of the problems that are front-of-mind is that as the forest ecosystems decline, animals that had lived within it and carried around viruses that lived just within these niches in faraway communities, they're now free to go out into the world.

As it happens, the animals that adapt most effectively to living in cities and other human communities, those are also the ones that are best at transmitting viruses, so bats and rodents. There's a number of truly problematic issues thrown in with this network of problems related to biodiversity loss. The pandemic is one of them.

Tanzina: I'm wondering, also, you mentioned some of the categories of animals. Can we talk a little bit more specifically about what kinds of animals, beyond just broader categories? Are there specific species or types of birds, for example, or other mammals that are feeling this more so than other types of animals? Are there animals that we're more familiar with?

Eric: Well, many of them are animals we're familiar with. The group of animals that have the single biggest impact are freshwater animals, that's fish and the birds that live off of freshwater communities around the world. Those have seen an 84% drop in animal populations, that's largely fish and birds, since 1970. It's the largest single decline of any particular kind of ecosystem around the world.

In addition to the problems we see from land-use change and also, there's pollution, there's pesticides from farming that ends up in places that shouldn't be and poisons communities, there's industrial metals that find their way to the sensitive communities, and, of course, there's also just overfishing and poaching of animals that should not be occurring if we're going to keep these ecosystems healthy, the scientists definitely emphasize that that's the overall goal here is to keep these ecosystems healthy.

The reasons that's important, in addition to what I mentioned a second ago about pandemics, is ecosystems, quietly and persistently, for the entire breadth of human history, have been doing the quiet, hard work of cleaning our air and cleaning our water, making sure that the soil stays where it is and that soils regenerate so that we can continue to grow food. Those ecosystem services, which we pay nothing for and also pay little attention to, are absolutely critical for economic and human health. Declines in animal populations are a true red flag for threats to these systems we rely on, without much thought to it.

Tanzina: You mentioned coronavirus, COVID-19, what effect has that had on the animal population that we understand so far? How has that played into all of this?

Eric: The scientists would say, in response to that question, that, well, the monitoring that goes into these animal populations globally, it's so onerous and takes so much time to amass the data that goes into the report that they actually closed the observation for this report, I believe something in 2016. It takes a little while to process all of the data into the aggregate levels that they see.

On a more anecdotal level, it's more evidence than anecdotal, some of the most startling images we've seen from the pandemic are the evacuation of people from cities and other built environments and the return of animals to these areas. It's not a robust, highly-desirable way for nature to heal, but it does provide a glimpse that as soon as human behavior changes on a large scale, it opens up niches for animals and ecosystems to come back and figure out new ways to live once the harm that we do to them dissipates.

Tanzina: Eric Roston is a climate reporter at Bloomberg Green. Eric, thanks so much.

Eric: Thank you.

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