The number of kangaroos, koalas, and others killed keeps skyrocketing. Here’s where the eye-popping estimate comes from.

A kangaroo jumps in a field amid smoke from a bushfire in Snowy Valley on the outskirts of Cooma, Australia, on January 4, 2020.

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As fires continue to rip through Australia, some devastating numbers are emerging: At least 24 people killed. More than 15.6 million acres torched. Over 1,400 homes destroyed. And, according to one biodiversity expert’s count, an estimated 1 billion animals killed.

That last number is staggeringly huge, and has begun to make the rounds on social media. You might be wondering: How are so many animals dying? And how do we know the number of animals killed?

The bushfires, exacerbated by climate change, have since September swept through vast swathes of Australia — we’re talking about an area bigger than Vermont and New Hampshire combined — affecting a mix of rural and suburban areas.

Many wild animals and some farm animals have been killed directly by the flames. We can see the evidence with our own eyes: Distressing images of burned kangaroos and koalas, and videos of dead animals on the sides of the roads, have circulated online over the past week.

Other animals have not been burned alive but have faced death due to the destruction of their natural environment, which they rely on for food and shelter.

Initially, the number of animals killed was put at 480 million, an estimate that came from Chris Dickman, a biodiversity expert at the University of Sydney, last week. A statement from that institution explained how he arrived at the number.

Anwen, a female koala, recovers from burns at the Port Macquarie Koala Hospital in Port Macquarie, Australia, on November 29, 2019.

Nathan Edwards/Getty Images

In 2007, Dickman co-authored a report for the WWF (formerly known as the World Wildlife Fund) on how land-clearing affects Australian wildlife in the state of New South Wales (NSW). To calculate the impact, he and the other authors first mined previously published studies for estimates of mammal population density in NSW. Then they multiplied the density estimates by the areas of vegetation approved to be cleared.

Using this simple formula, Dickman was able to calculate that approximately 480 million animals had been killed since the bushfires in NSW started in September.

Some experts suggested that estimate was too high. Sadly, there are three reasons to believe the true loss of animal life is much greater — more like 1 billion.

First, the 480 million number applied to NSW alone, and the bushfires have since spread to the state of Victoria. Second, the number included mammals, birds, and reptiles, but did not include insects, bats, or frogs. Third, the 2007 report “deliberately employed highly conservative estimates in making their calculations,” according to the statement.

That’s why Dickman now estimates the real number of animals lost in the fires is at least 1 billion.

“The original figure ― the 480 million ― was based on mammals, birds, and reptiles for which we do have densities, and that figure now is a little bit out of date. It’s over 800 million given the extent of the fires now ― in New South Wales alone,” he told the Huffington Post.

If we also count bats, frogs, and invertebrates (and given their environmental impact, there’s good reason to think we should), Dickman said it’s “without any doubt at all” that the number of animals lost tops 1 billion. “Over a billion would be a very conservative figure.”

Stuart Blanch, an environmental scientist at WWF Australia, also said 1 billion was a modest estimate given how far the fires have recently spread, according to HuffPost.

How the fires became so deadly for animals

At this point, you might be asking yourself: Can’t animals just run away from a raging fire? Can’t birds just fly away?

In many cases, particularly for birds, the answer is yes. “Certainly, large animals, like kangaroos or emus — many birds, of course — will be able to move away from the fire as it approaches,” Dickman told the BBC. But he added that “it’s the less mobile species and the smaller ones that depend on the forest itself that are really in the firing line.”

Koalas are a good example. An estimated 8,000 of them have died from the fires, ecologists say. That’s almost one-third of all koalas in NSW, which forms their main habitat.

“It may well be up to 30 percent of the population in that region [was killed], because up to 30 percent of their habitat has been destroyed,” explained Sussan Ley, Australia’s environment minister.

WWF is currently collecting donations to restore koala habitats.

Other animals may have fared better — reptiles, for example.

“Although it is hard to find estimates of how well reptiles survive fires, in similar areas of Australia the majority of these reptiles live in the soil,” said Colin Beale, an ecologist from the University of York. “Soil is a very good thermal insulator and burrowing reptiles can certainly show very low mortality even during intense fires.”

Some ecologists, including Beale, say Dickman’s estimates may be inflated. Although it’s plausible many animals have been affected by the fires, the proportion of them that actually died may be smaller.

Let’s hope so. The truth is, it’s hard for anyone to know the precise impact of the fires at this stage, not least because many animals that survive the flames will likely die later due to lack of food, water, and shelter.

Regardless of the exact numbers, this is a crisis for biodiversity in Australia, which is home to some of Earth’s most distinctive animals, like marsupials. Around 244 species of mammals are found only in Australia. What’s more, according to the University of Sydney’s statement, “Some 34 species and subspecies of native mammals have become extinct in Australia over the last 200 years, the highest rate of loss for any region in the world.”

The current loss of Australian animal life is a serious tragedy by anyone’s count. It adds to the terrible human toll: two dozen people killed, and thousands more evacuated. Fires are expected to keep raging for another month.

To help with the evacuations and firefighting, the government announced this weekend that it’s deploying the military. Experts say the deployment is on a scale not seen since World War II. As Defense Minister Linda Reynolds put it, “It is the first time that reserves have been called out in this way in living memory.”

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