A scientist from the University of Sydney estimates that Australia’s ongoing bush fire disaster has already killed at least one billion animals in Victoria and New South Wales (not including bats, insects, frogs, or fish). That number is so large that it’s hard to comprehend, so I wanted to find out what it means. I’m not sure I succeeded, but what I’ve learned along the way is as interesting as it is tragic. Let me share my work with you.

My first thought was to try to rank one billion alongside other major disasters. Is the scale of these deaths, from a single event, up there with the overall impacts of climate change? Or maybe even that asteroid that killed the dinosaurs?

The first obstacle to figuring that out is how the number is expressed. It’s an actual number of victims, and most major losses of animal life are reported instead in the number of species to go extinct—not the outright population of those species, much less the cumulative total of those populations.

So how’d Chris Dickman, the Australian ecologist, get to that number of victims? He explains that it’s based on a 2007 study Dickman co-authored that attempted to estimate the cost to animal life of bush clearing and development in New South Wales. It sought to create a calculation of the population density of mammals, birds, and reptiles, across the various habitats that exist in NSW, covering animals ranging from bandicoots to babblers, and habitat including everything from coastal rainforest to plains scrub, plus all the woodlands and forests in between.

“In that work, we estimated from published density figures of native mammals, birds, and reptiles how many animals would be at risk of being killed under state land clearing approvals,” explains Dickman in an email. “In the eight years (1998-2005) that we analysed, the state government approved 640,000 hectares of woodland and forest to be cleared. We estimated that it would have resulted in the deaths of 104 million native vertebrates. This estimate didn’t include bats, frogs, and some other groups as there were no density estimates available for them; hence 104 million was a conservative estimate.”

That methodology and its results are novel. Such population density estimates have not been created for other threatened habitats in other parts of the world. For instance, it combines the diversity of species with their mean population density in a certain type of habitat, and the area of those different habitats to estimate that 8.45 billion reptiles live in the studied area in NSW. That’s 200 reptiles per hectare (about 2.5 acres) in the studied area. It also gives us our first point of comparison: one billion animals killed in less than five months by the fires is a lot worse than the total lost to development in NSW in seven years.

To get to the one billion number, Dickman simply extrapolated his result to a larger area. “If we now take the five million hectares of land burnt in the bushfires in NSW since September 2019, and assume that all the native mammals, birds, and reptiles have been killed by the fires, the figure is 5,000,000 / 640,000 x 104 million = 812.5 million (rounded down to 800 million to be slightly more conservative) animals,” Dickman says. “If we assume that the density estimates for animals in NSW apply also in Victoria (this should be reasonable as habitats are the same or very similar), and consider that Victoria has some 1.25 million hectares of burned habitat, then we get 800 million + (1,250,000 / 640,000 x 104 million) = ca. 1,003,000,000 (i.e., > one billion). Of course, another two million hectares have burned in other states in the current fire season. We don’t have good density estimates for animals in these states that can be reliably extrapolated, so it is very safe and conservative to say that over one billion animals have been killed.”

But, that comparison between losses caused by fire and losses caused by development may be comparing apples to oranges. “Animals killed by permanent loss of their habitat are permanently removed from the future global population, but burned forests recover (mostly, I assume), and wildlife populations at least have the chance to recolonize recovering forests,” says Bradley Bergstrom, a biology professor at Valdosta State University. Some of those billion animals Australia has lost in the fires to-date may be replaced by recovering populations in the coming decades.

Dickman is less sure that will be the case. “Not all animals in the burned areas would necessarily be killed immediately and directly by the fires. Some would fly off, others would go underground, others may find a small unburnt refuge under rocks. But the severity and speed of the fires in the current fire season make it unlikely that very many will survive,” he says. “Much prior research indicates that in severely burned areas, the resulting lack of shelter, lack of food, and incursions by invasive predators (red foxes and feral cats) lead to further indirect reductions of animal numbers, in effect killing off the survivors of the flames themselves.”

I also asked Bergstrom the same question I’m asking everyone I know this week: Can you compare that number to large losses of animal life? His answers surprised me: cats and roadkill.

In 2013, researchers from the Smithsonian Institution and the United States Fish and Wildlife Service compared cat populations to predation rates, and estimated that, in the lower 48 states alone, cats kill between one billion and four billion birds, between 6.3 billion and 22.3 billion small mammals (think: mice), and hundreds of millions of reptiles and amphibians. The total populations of prey species can obviously take some predation, but bird populations in North America, for instance, have fallen by three billion, or nearly 30 percent since 1970.

And, it’s estimated that one million vertebrates (not including humans) are killed on American roads each day. So, the number of animals killed in the Australian fires is significantly less than the number killed by cats every year, in the lower 48 alone, and roughly equivalent to three years of road kill in the United States. However, these events don’t destroy habitat, and the total number of animals killed by cats and cars may not permanently subtract from the future population total, whereas portions of the areas burned in Australia may take a long time to recover, or never recover at all.

It’s interesting that when I asked Dickman for comparisons, he also cites cats. “The only comparison I can think of is the toll taken by feral and domestic cats across Australia,” he says. “In a series of studies, several of us have estimated that two billion native mammals, birds, reptiles, and frogs are killed Australia-wide each year by cats. Of course, cats don’t have the same effects on animal habitat as do the fires, so populations of prey are not necessarily wiped out. The bushfire and feral cat (and red fox) interaction is a real double-whammy for wildlife, though.”

Bergstrom had one other point that’s relevant here: habitats like eucalyptus forests in Australia are adapted to, or even dependent on, fire. Other types of habitat may not be, and the results of them burning could be permanent as a result. This is true in some areas impacted by the Australian fires and potentially in one other major instance of climate change-worsened fire killing a disturbingly huge number of animals: the Amazon rainforest.

In a horrifying series of events that shocked the world last year, over 906 thousand hectares of the Amazon rainforest burned. That’s substantially less than the over seven million hectares that have burned in Australia in the last five months. But, if anywhere has greater biodiversity and denser animal populations than Australia, it’s the Amazon. I have not been able to find any large-scale population density estimates for the Amazon, but we can still scribble some numbers down on the back of an envelope. That Australian animal population density puts the number of birds per hectare in NSW’s rainforests at 33. If we assume that number holds true for the Amazon (I’m sure this is wildly inaccurate), then nearly 30 million birds alone would have been killed there last year. There’s zero chance that number is anywhere near to the real total, but it does help apply some context to the scale of that tragedy. And it’s relevant to this discussion because it’s thought that ecosystem will not recover from the fires like some of Australia’s will, and may instead convert to savanna, leading to the permanent loss of the animal species it supports.

Another scientist I spoke with offered an example of a different ecosystem permanently altered by human-caused fire, with a subsequently enormous loss of animal life: our own. “It has long been argued that the advanced hunting technology of the Clovis peoples was fundamentally caused by the dire necessity to kill increasingly rare prey,” says Luis Ruedas, a biologist from Portland State University.

The Clovis people existed from between 13,300 and 12,800 years ago. The relatively short duration of that civilization could be due to their hunting methods, which saw them start massive fires in order to hunt the megafauna that lived on this continent back then—now-extinct species like the mammoth and mastodon. In addition to driving those species to extinction, the fires started by the Clovis may have also destroyed the vast swaths of open savanna sparsely covered in few trees that covered most of this continent. As plant life began to recover from the Clovis’ fires, those trees no longer found their growth and spread limited by presence of massive herbivores, and North America transitioned to the dense forest cover we know in many parts of the country today. There’s no definitive estimate of the total megafaunal populations, but human-caused fire (from humans with only stone age technology) caused the mass extinction of a huge number of large animals in a very short time period, remaking an entire continent in the process. The potential parallels to Australia are haunting.

I also spoke with Noah Greenwald, Endangered Species Director at the Center for Biological Diversity. The comparison he suggested to Australia’s losses? Bison. We managed to kill nearly 30 million of that species in just 50 years during America’s westward expansion. At its peak in the 1870s, bison hunting was killing 5,000 animals a day. Just 325 ultimately survived, a population that’s now recovered to at least 250,000.

Greenwald offers perhaps the best context for animal deaths in the Australian fires that I’ve found. “One billion is a large number, and it is an unqualified tragedy,” he says. “But this is just one more day in the ongoing extinction crisis.” It’s unique that researchers were able to put such a fine estimate on the number of animals killed in the fires, but the outright number of animal deaths is not unique. It’s estimated that up to half of all animal species on earth could go extinct by the middle of this century due to a combination of habitat loss, climate change, pollution, and other human-caused factors. The Australian fires are just one more event along the path to our planet’s sixth mass extinction. What’s causing this one? Us.