The Macabre Science of Animal Mass Die-Offs

Unfolding right now across swaths of Australia is an ecological catastrophe, as massive, turbo-charged fires reduce whole landscapes to nothingness. Tens of thousands of koalas had no way of escaping. Livestock lie dead in fields. Innumerable animals have perished, with many species likely pushed to extinction. The few survivors could well starve or fall victim to predators.

We’ll never know the true toll of this mass mortality event, or MME as scientists call it, but we know this: The cadavers that litter the Australian landscape are now rotting, kicking off a cascade of ecological consequences and potentially imperiling human health.

As a field, the experimental study of MMEs is quite new. Sure, researchers can scrutinize a mass die-off in the wild, for instance when a bacterial outbreak killed 200,000 saiga antelopes in central Kazakhstan in 2015. But without meticulously surveying the environment just before the event, any comparison is inherently limited. To do a controlled experiment, you’d either have to source and distribute hordes of carcasses yourself or somehow learn to predict an impending mass death.

One team of researchers has found a clever workaround to the problem, using a subject no one will miss: feral pigs. Mississippi State University ecologist Brandon Barton, forensic entomologist Abby Jones, and environmental microbiologist Heather Jordan set up plots of land and take stock of their ecosystems: the nutrients and microorganisms in the soil, insects, vegetation, and more. Cameras and microphones capture the movements of animals stalking the lands. Then it’s time for the swine.

Feral pigs have become an invasive menace in the US, breeding like crazy and tearing through ecosystems. Trappers nab them, kill them, and send them to Barton and Jones. The team then delivers them to their test plots, distributing some 15 tons of pig carcasses (about 200 individuals of varying sizes) over the course of a day.

Photograph: Brandon Barton/Mississippi State University

While they’re still fairly fresh, dealing with the carcasses is comparatively easy. Returning daily for two weeks, as the stench grows more brutal and penetrating by the day, that’s the perilous bit. “We took a lot of precautions with gallons of Purell and soap and water and protective equipment, but you still are dealing with a lot of wild animals that are harboring who knows what,” says Barton.

Maggots, for one—rivers of maggots. These infant flies play a pivotal role in the way nature recycles dead animals: They eat decaying flesh, converting it into their own tissue. They took a liking to the pig carcasses in such numbers, they formed great squirming mats on the ground. “It was one of the most incredible things I’ve ever experienced,” says Barton. “Just seeing this carpet of writhing maggots going through the forest and you think, that’s gross. And maybe it is, but those guys are basically little packets of nutrients.”

The maggots then become a source of food for a range of creatures. The researchers watched birds, hornets, beetles, and even armadillos devour them. “Everything we could think to measure, we wanted to measure,” says Barton. “Because with an experiment like this, we’ve never done one and been like, can’t wait to do the next one. The idea is to do it right the first time and not have to deal with 15 tons of dead pigs again.” (They’ve done three separate simulated MMEs thus far.)

The plants, however, are not as happy as the predators, at least not in the near term. When a pig rots on the forest floor, it floods the soil with nutrients like nitrogen—too many nutrients that overwhelm the vegetation. At first, says Jones, they’d see “barren brown land where all of the grasses and the other plants had perished.” But after 5 months, the ecosystem would adapt and “you start getting extremely green grass right around that area.” The grass directly underneath a carcass, though, wouldn’t return until the next year.

The river of maggots

Photograph: Brandon Barton/Mississippi State University

The microbiome of the soil, too, goes into shock. Jordan, the microbiologist, samples the dirt’s microorganisms, such as fungi and bacteria, before and after the introduction of pig carcasses. As a carcass decomposes, the bacteria in the body itself runs rampant, producing its signature stink and bastardizing the soil’s microbiome. “From day zero to day 365, there’s microbes from the beginning that have not completely bounced back,” says Jordan. This could interfere with an environment’s natural allotment of fungi and bacteria and how they cycle nutrients and decompose matter like leaf litter.

Still, eventually the environment will return to equilibrium, and plants will grow anew. But that period of vegetal death is a precarious time, and indeed this could be a particular problem in Australia. When a plant community emerges from the decay, is it the same as it was before? “One of the things we see over and over again is that the a lot of invasive species are really good at beating the native species to colonizing these open habitats,” says Barton.

In Australia, vast tracts have been reduced to open habitats. Even in the absence of innumerable animal carcasses, ecologists are worried that hardy, fast-growing invasive species will gain a foothold in burn scars and muscle native species out. The problem is these climate-change-driven bushfires are obliterating whole plant communities, whereas before they would burn mildly enough to leave some species standing, putting a check on the invasives.

There’s also concern about the pathogens that might thrive amid the carcasses. Usually, scavengers like raptors consume carrion and shrink the breeding grounds of demons like E. coli, anthrax, salmonella, and botulism. If the bushfires also killed off the scavengers, the risk of transmission and potential epidemics goes up. “Some of these are zoonotic, and they could impact humans,” says Jordan, the microbiologist. “In Australia, all of these carcasses are out there, and there’s got to be a method of controlling them.”

If people burn the carcasses, they could come into contact with pathogens. Flies, too, might carry pathogens into homes. Or the scavengers themselves could carry them far and wide. “It sounds scary, because the possibility is definitely there, but more studies need to be conducted,” she adds. A lot of open questions persist around how an intense bushfire influences its environment. “We know very little about mass mortality events in general, and the impact of microbes in the environment and transmission of pathogens,” says Jordan. “But we know even less about the impact of these in fire.”

The answers will come from carefully controlled experiments, like what Jordan and her colleagues are doing. They can’t come quickly enough: The killing fields are growing around the world, with dire effects on ecosystems, and potentially human health.

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