American Scientist

The unprecedented wildfires that raged across Australia for several months during the most recent Southern Hemisphere summer not only destroyed human lives, but also killed billions of animals—perhaps even trillions when arthropods are added to the toll of birds, mammals, and reptiles caught by the flames.

Burning is not the only cause of death in this catastrophe. Many animals outran the flames only to die in fences and roads by the thousands. Others may live, for now, but without their homes, food, and water, they are likely to succumb to the elements soon.

Unfortunately, animal die-offs of massive proportions are becoming more frequent. Global change—which includes human-caused changes in climate, land use, fire regimes, and other factors—may largely be to blame for the increased frequency and intensity of mass mortality events across all kinds of animals—cataclysmic occurrences where the instantaneous death rate of a given population is well above the expected level for a specific location.

These natural and man-made circumstances converged in 2015 when unseasonably warm and humid weather allowed a normally benign bacterium to wipe out 200,000 saiga antelope in Kazakhstan in just a few weeks. Similarly in Norway, a single lightning strike killed 323 reindeer in 2016, and another 200 starved last year after unseasonable rain left an impenetrable layer of ice covering their food. And, for five years in a row, tens of thousands of starved sea birds have washed up in Alaska. These are just a few recent examples.

So what happens when everything dies all at once?

Experimenting with Mass Die-Off

Our research group studies the ecological consequences of mass mortality events. Because these events are unpredictable and can occur anywhere in the world, they are logistically challenging to study. Even if we could hop on a plane and travel to a distant mass mortality event quickly, we wouldn’t have data on initial conditions and our sample size would be just one, so drawing conclusions would be nearly impossible.

To overcome this hurdle, we have collaborated with wildlife biologists Marcus Lashley and David Mason of the University of Florida to take on the dirty task of simulating mass mortality events in large experiments with thousands of kilograms of carrion.

In spring 2019 we began our largest experiment to date, deploying nearly 14 metric tons of carcasses in Oklahoma. Our first challenge was finding a large—and ethical—source of carcasses. We partnered with professional trappers who were removing feral pigs, which are an invasive species with the potential to damage crops, spread disease, and negatively affect wildlife. These wild boar were trapped for conservation purposes and normally would have been left at the trap site or transported to a designated location to decompose. Rather than having their carcasses go to waste, we were able to put them to scientific use.

We wanted to see how increasing amounts of carrion affect decomposition in ways that may alter ecosystems or promote the spread of pathogens. Before the carcasses arrived, we identified plots and sampled the initial soil nutrients, microbes, plants, insects, and wildlife. This provided important baseline information that wouldn’t be available during a real-world mass mortality event.

Mass mortality events produce more carcasses than scavengers can keep up with. Instead of disappearing quickly, carcasses become breeding grounds for bacteria and insects.

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The real work began after the carcasses arrived. Pigs have a stinky reputation for a reason, and feral swine—especially several tons of dead animals—are not a pleasant sight or smell. We carefully placed each carcass, which averaged about 30 kilograms, in the predetermined plots. Some plots received a single pig to represent a “normal” death event; others got 12 carcasses to represent a mass mortality event. Furthermore, some of the plots were fenced to exclude scavengers or herbivores. At the end of the day, we had placed nearly 14 metric tons of carcasses across 24 different plots.

Quickly, the plots were buzzing with flies, and vultures circled above. We monitored the decomposition during the first few days, which are by far the most gruesome. Scavengers shredded some carcasses, dragging their remains deep into nearby forests. Unscavenged carcasses bloated with gas before bursting open and revealing millions of writhing maggots inside. We used sticky traps and video cameras to monitor use by arthropods, documenting more than 40 families that visited the sites in just the first few days.

During this time we documented decomposition rate, monitored insect and scavenger visitors, and collected microbial samples to detect disease-causing bacteria. Over the next several weeks, the microbes, insects, and scavengers did their job, and soon nothing remained but bones and fur. We will monitor this experiment for several years to identify the long-term ecosystem consequences of mass mortality.

Effects of a Mass Mortality Event 

Our group’s field, lab, and theoretical investigations consistently reveal that mass mortality events affect ecosystems in two main ways.

First, the sheer magnitude of hundreds or thousands of individuals removed from the ecosystem means that their roles in the environment are lost too. In Kazakhstan, the 200,000 dead antelope mean 200,000 fewer herbivores on the landscape. The same applies to the electrocuted or starved reindeer in Norway.

In Australia, as much as half of the koala population in some areas have been killed by the wildfires. Besides being a national symbol and source of ecotourism, koalas are important to the ecosystem as one of the few animals that can consume and recycle nutrients from eucalyptus plants. The widespread death of koalas means a significant break in the food chain—nothing is left to eat eucalyptus.

Similarly, the mass mortality of small mammals, rabbits, and kangaroos means that little prey will remain for predators such as dingoes, who may struggle to avoid starvation in the barren landscape. Fires also kill less charismatic species, such as insects and bats, both of which are important for pollination, and their loss may represent a challenge for post-fire plant communities. Without these and other animals present to perform their ecological jobs, Australia’s ecosystems will undoubtedly change.

Second, the large number of rotting carcasses caused by a mass mortality event will have their own environmental impacts.

Although some of the animals in Australia will be consumed by fire and their bodies quickly transformed into ash, those that meet their fate outside of the flames will begin to decompose. Under normal circumstances, carrion triggers scavengers to flock to the carcasses, consuming the flesh and recycling the nutrients into the ecosystem.

However, the ongoing mass mortality of kangaroos, koalas, and other large animals will produce more carcasses than the scavengers—eagles, dingoes, and a species of reptiles known as goannas—can keep up with. Instead of disappearing quickly, carcasses will likely become breeding grounds for bacteria and insects. This prospect is worrisome, because many of these microbes may be pathogens that affect people, wildlife, and livestock, and the flies can transport pathogens great distances. Indeed, in previous experiments, our simulated mass mortality events produced enough flies to cover the ground in a river of maggots as they crawled out from the carcasses and dispersed in the nearby forests.

Our work has also revealed that mass mortality events can have long-lasting effects by poisoning soil and restructuring plant communities. As carcasses decompose, they release gases and spill cocktails of liquefied remains, acidic body fluids, and microbes that the soil absorbs. When this happens en masse, the toxicity can kill plants, including trees. Our unpublished data repeatedly show that mass mortality events alter the soil microbiome and soil nutrients. How long these effects can last is unknown.

Getting Ecosystems Back on Track

The outcomes of mass mortality events on ecosystems are complex, but one aspect has been consistent across our multiple studies: Healthy scavenger populations mitigate the effects of mass mortality events.

Scavengers such as vultures, coyotes, and dingoes are among the most persecuted groups of animals worldwide, yet they provide critical ecosystem services. When scavengers were present in our experiments, the carcasses were consumed or dragged away quickly, producing fewer maggots and flies, leaching fewer chemicals into the soil, and lessening the impact on the plants and ecosystem.

Although Earth’s ecosystems may not be able to avoid future mass mortality events, an obvious priority is to maintain the biodiversity we have—including the scavengers that clean up the mess.

In Australia, dingo, eagle, and goanna populations are likely to benefit from the ample carrion provided by these fires. Unfortunately, inflated scavenger numbers may cause more problems. When the carrion eventually disappears, these overabundant scavengers may be forced to seek food in populated areas, resulting in conflict and attacks on people and domestic animals. Furthermore, many of Australia’s scavengers are nonnative species, so there is rising concern that these carcasses may allow the invasives to strengthen their foothold while the ecosystem is in this vulnerable state. Such indirect consequences of these fires are difficult to anticipate.

What is certain, however, is that the ecosystem that emerges after the smoke clears will be dramatically different.

This article is adapted from a version on The Conversation, www.theconversation.com.