In the newest offering within the multiverse of Marvel films, the Avengers superhero team is up against its biggest challenge yet: saving life, across all the galaxies, as we know it. When the cosmically powerful villain Thanos succeeded in attaining the infinity stones at the end of Avengers: Infinity War, he eradicated half of all life in the universe with the snap of a finger.

Like 18th-century scholar Thomas Malthus , Thanos believed that the amount of life in the universe was unsustainable and would eventually destroy itself by consuming all resources. In 1798, the philosopher wrote an essay claiming that populations grow much faster than their food sources, and if growth remained unchecked it would eventually lead to societal collapse. Malthus’ opposition to improving the lives of the poor (who he feared then might have more children) provides a clear template for Thanos’ motivations.

Malthus’ theories were proven wrong, as humans have managed to scale food production along with population. Today, many scientists worry about the effects of population growth on greenhouse gas emissions and climate change, though the variables involved are numerous and researchers are still studying the potential impacts .

Even if the remaining heroes triumph and reverse Thanos’ devastating action in Avengers: Endgame, as they are (spoiler!) likely to do in their fight against the Mad Titan, the question remains: What would the ecological wreckage from such an extinction event actually look like on Earth?

Smithsonian.com spoke to a group of scientists to learn what would really happen to our planet if a mad supervillain were able to wipe out half the life here with the snap of his fingers.

A Broken Heart… and an Upset Stomach?

In addition to all the humans and animals that would meet their untimely ends, Thanos’ reality-altering snap would destroy some of the smallest lifeforms that are a fundamental part of human health—our own gut microbes. How would humans fare if we suddenly lost half of the microbes that keep us healthy?

“[The microbiome] is a complex ecosystem of organisms that includes bacteria, but also viruses as well as fungi,” says Zuri Sullivan , an immunologist at Yale University. This microscopic ecosystem serves three main functions. First, it helps the host digest food by breaking down complex molecules. “Humans have a pretty limited ability to digest complex plant material, so we rely on these commensal bacteria in our microbiome to break down complex carbohydrates that we get from eating plants,” Sullivan says.

Our microbiomes also help teach our immune systems to differentiate between dangerous and harmless bacteria. The immune system has to learn when it should mount an all-out attack on deadly pathogens, and when it should refrain from overreacting to benign molecules, which is what happens when a person has an allergic reaction. And finally, the microbiome helps defend us directly against pathogens as well.

While these are all vital functions for human health, microbiologist Nicholas Lesniak at the University of Michigan doesn’t think Thanos would instantly make everyone sick with his snap. “We’re talking about halving, and we’re talking about billions of cells, so going from two billion cells to one billion cells,” he says. “But then they have a doubling time of hours, so in a matter of hours we’ve already overcome that hit.” While some of us might get an upset stomach for a bit, our microbiomes are pretty good at bouncing back.

Insecticidal Maniac

On a slightly bigger scale, the next major concern would be insects. Although a trailer for Avengers: Endgame opened with Thanos retired to life as a farmer, he might not even be able to farm without half of the world’s pollinating insects.

“It would be very chaotic, and I don’t even know how you would snap your fingers in an ecologically sustainable way,” says entomologist May Berenbaum at the University of Illinois Urbana-Champaign. “You would have problems with all the ecosystem services that insects are responsible for, including removing dead bodies or pollination services.”

Pollination is a key role of insects, and declines in pollinator species are a major concern around the world . The loss of these bugs has downstream effects for fruit growers and honey aficionados alike. But insects are also important cleanup crews, dealing with materials like corpses or dung that other animals can’t break down.

“There’s a whole community of dung-feeding insects, and when this community is absent, then you end up knee deep in dung,” Berenbaum says.

A situation like this actually happened to Australia in the 1890s . Colonists brought non-marsupial mammals like sheep to the continent, and the local dung beetles couldn’t digest their different poop. The accumulation of feces and accompanying flies caused a massive problem until a Hungarian ecologist named George Bornemissza recognized the cause and started importing dung beetles that could process the waste. Thanos’ snap could cause a similar situation worldwide.

Where Thanos Would Have the Most Impact

For the bigger species of the world, such as large mammals and other carnivores, Thanos’s snap could prove most devastating. With half the world’s life gone, small animals like rats could come to inherit the Earth while larger species simply die out. In the Cretaceous–Paleogene extinction event, for example, when an asteroid hit the Earth 66 million years ago and helped snuff out the dinosaurs , some 75 percent of all species were lost, but small, rodent-like mammals managed to survive and adapt.

University of Pennsylvania paleobiologist Lauren Sallan , who studies mass extinctions, says that because larger species tend to have fewer offspring and breed more slowly, they would fare poorly after the snap. “After a mass extinction, what I’ve found in the past is that it’s the smaller species that tend to breed fast are the source of future diversity,” Sallan says.

Of course, it would take a long time for even small animals to bounce back. According to Sallan, it takes between 20 to 30 million years to recover from a mass extinction. “It’s all because the ecosystems are kind of churning over and everything is shaking out according to what the individual groups are doing and how they’re responding to these new conditions,” she says. In the immediate aftermath, Sallan thinks a 50 percent loss of life would probably lead to most ecosystems collapsing entirely.

In a multiverse wherein the remaining Avengers can’t reverse Thanos’s destruction, the universe likely wouldn’t recover for millions of years. But on the bright side, Sallan says “I think humans would figure out a way to [survive], provided that not all of the ecosystems collapse.”