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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.”

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