In nature, mass mortality sometimes happens. More than 200,000 saiga antelopes in Kazakhstan drop dead in a matter of weeks; 337 dead whales wash up in a remote fjord in southern Chile; some 300 reindeer in Norway are felled by a single bolt of lightning— all that has happened since 2015. There’s evidence such spectacular displays of death are increasing in frequency due to climate change.
“These are cataclysmic events that result in ecological chaos,” says Texas A&M entomologist Jeffrey Tomberlin. And yet, “we really have no idea how they’re impacting the environment.”
The problem is the die-offs are unpredictable. Once one has happened, scientists can’t go back in time to make the baseline measurements that would allow them to say how exactly an ecosystem has been changed by a sudden jolt of animal carcasses.
The solution, Brandon Barton of Mississippi State University and his colleagues decided, along with Tomberlin, was simple: If they couldn’t predict a mass mortality event, they would make one happen.
For that the team needed an immense mass of dead animals. Luckily, wildlife biologist Marcus Lashley of Mississipi State had connections with people at state and federal agencies who are responsible for combatting a wildlife pest that currently plagues Mississippi and many other states.
A few phone calls later, the dead feral pigs started streaming in.
Where to Put Them?
The plan was to place varying amounts of pig corpses in a series of 20-meter-square plots, and to collect before-and-after data on everything from soil chemistry and microbes to vultures and coyotes.
But as the team collected and froze the incoming pigs, they struggled with a second problem: where to actually do the experiment. ”Nobody wants you to put thousands of pounds of dead carcasses on their property and watch them rot,” Lashley says.
Serendipitously, he received an email from the university administration urging researchers to make use of university-owned forest land for research. Lashley sent in the team’s pig proposal. “Though the request was a little unusual,” says Wes Berger, Mississippi State’s associate director of research, the university gave it the green light.
By the beginning of July 2016, the team had amassed about 4,000 pounds of pig carcasses, collected their baseline data, and were ready to begin. Then the telephone rang again. “I never though I’d be so excited about a phone call saying I have a ton—a literal ton—of dead pigs for you,” Barton says.
So they added another 2,000 pounds into their design, and on July 5th, with the help of some technicians graciously loaned to them by colleagues, they dragged 6,000 pounds of dead pigs into their study plots and left them to rot.
Rivers of Maggots
Almost immediately, camera traps recorded dozens of vultures descending on the piles of pigs. Sticky traps to collect insects had to be changed daily because “you couldn’t have stuck another one on there,” Bartons says. The writhing maggot swarms on the carcasses were several inches deep.
Vultures and maggots were to be expected, of course, but the intensity of the response awed the researchers. “We were completely unprepared for what happened,” Barton says.
Collecting the microbial data during this active decomposition phase was like playing Twister, says Heather Jordan of Mississippi State. “You have to not fall, try not to step on the pigs, try not to step in the adipocere”—aka corpse wax, it results from decomposition of fats—“which was everywhere, this muck and soup and slime, and bend down and get into that internal microbial community with spiders and larva and all kinds of nasty flies everywhere.”
The biological response was so extreme that the researchers had to abandon some of their sampling methods. Measured quantities of leaf litter, whose decomposition rate they planned to clock precisely, “got soaked in pig gunk,” Lashley says, which “made it pretty hard to interpret the results.” Traps for ground-dwelling insects were lifted out of the ground by maggots and “rode the river of maggots down the hill.”
The flies attracted predators, including anoles—a kind of small lizard—and bald-faced hornets. “You’d see the hornets dart off, grab a fly, then land on the bird netting and sit there and eat it,” Barton says. It was “one of the coolest things we saw.”
And after the wriggling streams of maggots abandoned the skeletonized carcasses, and buried themselves in the dirt to pupate, herds of nine-banded armadillos shuffled into the area. They tore up the ground to get at the maggots. The sum of all that disturbance made the soil “weird to walk on,” says Barton, its texture had changed so much. It also wrecked the plant communities, allowing new species to colonize the area.
Even now, more than a year later, the sites remain ecologically scarred. “Will they ever go back to normal? Probably not,” Barton says.
A Downside to Culling Feral Pigs?
It was the constant presence of vultures that creeped out Jordan, the microbiologist. “I’d look up and there were at least fifteen or twenty roosting in the trees,” she recalls.
On the other hand, half the plots had fencing and netting to exclude vultures and other large carrion eaters—and in those plots, the pig carcasses took longer to disappear, and there were a lot more flies. “Those vultures are doing us a favor,” Barton says, by keeping down populations of flies that can transmit disease.
The researchers shared their preliminary results at this year’s Ecological Society of America annual meeting and recently in the journal Ecology—but the research is far from over. Jordan is still processing samples to track how the soil microbial community changes over time.
For Tomberlin, the experiment has highlighted the environmental impact of culling feral pigs. The pigs are invasive species themselves, and they do considerable damage, but the paradox, Tomberlin says, is that killing them on a large scale may actually promote the spread of other invasive species.
“Invasives tend to like environments that are unstable,” he explains, “and if you create this environment that's quite chaotic, you could be creating opportunities for them to have a foothold.”
“Like so much in ecology, the consequences of these events are not isolated to the event, and cascade through the ecosystem in very unexpected ways,” says Barton.
Though not much is left of the three tons of dead pig they started with, the researchers plan to continue monitoring their experimental plots until they’re indistinguishable from the surrounding forest—which may be never, they note. “We’re going to measure this for the rest of our careers,” Lashley predicts.
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