Thirty-five years after the meltdown at the Chernobyl nuclear power plant in northern Ukraine reports often portray the area as a paradise for wildlife. Photos show foxes roaming the buildings of abandoned towns and bison and wild horses flourishing after people were permanently evacuated. But to some scientists, nature isn’t doing as well as it seems.

In fact, a debate roils in the scientific literature about the health of the microbes, fungi, plants, and animals that live around Chernobyl. Some scientists have documented thriving wildlife now that people have left, suggesting that lingering radioactive contamination doesn’t pose a significant threat. But other scientists have found mostly negative effects of radiation on the health and abundance of creatures, from birds to mammals, with many populations smaller in more heavily contaminated areas. This controversy has only sharpened in recent years.

Ultimately, “there’s a grain of truth in all of these studies,” says David Copplestone, a radioecologyst at the University of Stirling in Scotland. The question is with interpretation. At the heart of the debate is not so much whether chronic radiation has any effect on living things, but at what dose the effects become significant. Radiation levels around Chernobyl have plummeted since the initial accident, but creatures that have reclaimed the area remain chronically exposed to low levels.

Figuring out whether this radiation causes harm — and if so, how and how much — is critical to understanding not just how the largest nuclear accident in history changed the environment, but also how chronic, low-level radiation affects living things generally. Examining why scientists reach different conclusions, and how recent research shines new light on the debate, gets us closer to the answer.

Immediate fallout

When a safety system test at one of the Chernobyl power plant’s reactors went badly wrong in April 1986, explosions unleashed a fiery plume of debris and radioactive atoms, or radionuclides, into the air that, over several days, may have emitted several hundred times more radiations than the atomic bomb dropped on Hiroshima. More than two dozen first responders died within months after rapidly absorbing doses of up to 13,400 millisieverts (a sievert is a unit of radiation absorption; normal background radiation levels are usually around 1.5 to 3.5 millisieverts a year.)

Over subsequent decades, thousands of children and adolescents who likely absorbed somewhat lower doses developed thyroid cancer, a cancer type that, fortunately, most tend to survive.

Flora and fauna also suffered in the initial aftermath. A 600-hectare patch of pine trees died, along with many resident mammals and invertebrates in the area. The area with the trees’ skeletal remains is now called the Red Forest. The 1,600-square mile exclusion zone around the plant has remained largely devoid of people since the accident.

As time passed and the most dangerous radionuclides decayed, the zone became less inhospitable. The radionuclide iodine-131, for instance, vanished by the summer of 1986, leaving slower-decaying ones like cesium-137 and strontium-90 scattered unevenly across and within the zone’s soils, vegetation, fungi, and animals. Today, radiation levels are generally below what would induce acute radiation sickness and range from 0.4 millisieverts per hour in the Red Forest — thousands of times higher than background levels and dangerous to live in — to levels even lower than typical background radiation.

“It’s not black and white.”

Normal background radiation levels aren’t usually harmful, because living things have biological mechanisms in place to prevent and repair small levels of damage, explains Kathryn Higley, a health physicist specializing in radioecology at Oregon State University. But it’s still a mystery exactly when and how harm starts to accrue for different species as radiation levels increase. The patchwork radiation landscape around Chernobyl creates an ideal setting to study this question.

But studying Chernobyl’s radioactive ecosystem also poses a mammoth challenge. Though scientists know how radiation affects individual cells — by causing DNA mutations or a type of molecular damage called oxidative stress, for instance — it’s proved hard to predict how that affects whole animals over their lifespans, Higley says. And it’s especially tricky to parse these effects — if they exist at all at very low doses — in the messiness of real-world ecosystems. Yet that, she says, is “the area of real interest right now.”

A statistical meltdown

When the first reports on wildlife in the Chernobyl zone came to international attention in the early years of this century, scientists described it as flourishing in the absence of people. But two evolutionary biologists who teamed up to study the area’s birds around that time, Anders Møller of University Paris-Saclay in France and Timothy Mousseau of the University of South Carolina, presented a different picture. Their surveys showed that certain bird species tended to have more genetic mutations, smaller brains, and less viable sperm in sites with higher radiation levels. And in 2007, they counted 66 percent fewer birds — and 50 percent fewer bird species — in highly radioactive places compared to background-level sites.

In dozens of studies, the pair also documented that, with higher radiation levels, there were significantly lower numbers of soil invertebrates and a lower abundance of certain insect species and such mammals as hares and foxes. Working with collaborators in Finland, they also documented a range of health effects in bank voles.

The pair documented a steady correlation between radiation levels and effects, such that the higher the radiation levels, the more pronounced the effects were. This relationship held even at radiation levels below what scientists had thought capable of causing harm, they observed.

“It was a huge surprise for us to see all of these pretty straightforward and large consequences of this kind of exposure across multiple species,” says Mousseau, who wrote about his and others’ Chernobyl research in the 2021 Annual Review of Ecology, Evolution, and Systematics.

Some other scientists have criticized much of the pair’s research for various reasons, one being caution about Møller’s work due to earlier scientific misconduct allegations. (Møller says the misconduct investigation was inherently flawed, for reasons he outlined in a statement; a French investigation didn’t find evidence of deliberate fraud, and the work in question doesn’t include the radiation studies.)

Another common critique is that Møller and Mousseau may have underestimated the radiation doses that the creatures they studied had been exposed to because they didn’t account for radionuclides that animals had ingested or inhaled. When other scientists reanalyzed Mousseau’s data on a dozen mammal species, they found that radiation had indeed caused declines in abundance, but only at higher doses than the pair had originally reported.

But some other research teams have not found significant radiation effects on the genetic diversity or abundance of certain animals around Chernobyl. In one widely publicized 2015 survey of a Belarus area near the power plant, a team of scientists determined that the numbers of elk, roe deer, and wild boar were similar to those in radiation-free nature reserves in the region. No matter what the consequences of lingering radiation might be, there were massive benefits to people leaving.

“Everything is so uncertain.”

A later report also found no evidence that radiation reduced the density of mammal populations across the landscape, even in highly contaminated parts of the zone. Neither study rules out the possibility that radiation has negative effects on individual animals but hasn’t affected population size, says wildlife ecologist James Beasley of the University of Georgia, who was involved in both studies. “If there were any effects, they just weren’t sufficient to suppress the population growth in those animals.”

Beasley and Tom Hinton — a now-retired radioecologyst formerly at Fukushima University’s Institute of Environmental Radioactivity — have also been sampling for DNA mutations in the region surrounding the Fukushima Daiichi nuclear power plant in Japan, which experienced a less severe meltdown and radiation release after an earthquake and tsunami in 2011. Nearby mice had certain genetic aberrations in the immediate aftermath of the accident. But at least some animals didn’t show lasting effects.

By 2016, when radiation levels had fallen substantially, Hinton, Beasley, and their colleagues didn’t find any signs that radiation was causing DNA damage in the cells of rat snakes and wild boar. This was despite the fact that the animals were absorbing radiation doses similar to those for which Hinton sees effects in Mousseau’s Chernobyl data.

“I have yet to be able to duplicate anything that Møller and Mousseau have published,” Hinton says.

The stubborn discrepancies have caused some members of each camp to become distrustful of the other’s conclusions, and on some occasions, the debate has turned personal. In 2015, the International Union for Radioecology, a nonprofit group of radiation scientists, invited researchers from both sides to a meeting in Miami, striving to reach a consensus. But the conversation became so heated, “they started hurling insults at each other,” recalls McMaster University radiobiologist Carmel Mothersill, the IUR’s treasurer.

The only conclusion they could reach was that “everything is so uncertain in the low-dose region that you can’t attribute anything definitively to the radiation dose.”

The debate is still unresolved. Scientists from each camp list several reasons for the discrepancies around Chernobyl, including research methods, statistical techniques, and the possibility that environmental factors other than an animal’s direct radiation exposure explain the observations. For instance, the Red Forest, where many of Mousseau and Møller’s study sites are, has high radiation levels but is also relatively barren of vegetation.

That makes it hard to say for sure whether the animals there are suffering from radiation or simply poor habitat, Copplestone and his colleagues have argued. (Some of Copplestone’s research is funded through a program partly supported by a nuclear waste disposal company.)

Mousseau, for his part, notes that some of the studies that contradict his own were conducted in a patch of Belarus that is wilder and less developed than the exclusion zone in Ukraine, where wildlife probably recovered from the disaster more quickly and would be expected to flourish. And Olena Burdo, a radioecologyst at the Kiev Institute for Nuclear Research, thinks that foreign scientists who visit the exclusion zone only infrequently might be neglecting subtle changes to the ecosystem brought on by events like wildfires or floods. These, she’s learned, can change how radionuclides — and animal populations themselves — are distributed across the landscape.

Mousseau doesn’t doubt that some species in the least contaminated parts of the zone are doing well and maybe even better than in outside areas due to the absence of people. And other scientists agree that there are some effects of radiation in the hottest parts of the zone, but only for certain species; some of Copplestone’s recent laboratory research, for instance, hints that bumblebees — which, like most invertebrates, were previously thought to be quite resistant to radiation — experience the toll of radiation at levels comparable to those within the Red Forest.

The debate is largely in the gray area in between: At what radiation levels does significant harm kick in, and for which species? Since different species may respond very differently to radiation, “it’s not black and white,” says radioecologist Christelle Adam-Guillermin of France’s Radioprotection and Nuclear Safety Institute. Even when animals exposed to extremely low radiation doses show signs of harm, it’s up for discussion whether the troubles can be definitively attributed to radiation itself.

“It’s really difficult to have a sharp conclusion,” she says.

“We need to stop generating studies which are scratching the surface.”

Another factor may contribute to the confusion: It’s possible that many of the ill health effects observed in Chernobyl’s wildlife don’t necessarily result from the radiation they’re currently absorbing but are, instead, inherited from their ancestors who survived the fallout from the initial blast in 1986. Scientists know from laboratory studies that even when small bursts of radiation have no immediate effect on cells, or the cells they give rise to when they divide, cells generations later — the cellular grandchildren, as it were — sometimes develop mutations, die, or fail to multiply.

Without inheriting mutations directly, these cells may inherit a greater potential to develop mutations. Researchers suspect this is due to changes in the epigenome — small molecules attached to DNA that influence genetic activity — which can be passed down through generations.

Indeed, in one experiment, Belarussian scientists captured two pregnant bank voles living near the Chernobyl plant and kept them in a radiation-free lab. Remarkably, bone marrow cells of the voles’ offspring showed just as many genetic mutations as those of voles living in highly contaminated areas, even though they themselves were never exposed to strong radiation, the team reported in 2006. Using statistical models to estimate this ancestral dose from the past, Mothersill, with Mousseau and others, has concluded that mutations in Chernobyl’s birds today might well be partly caused by their ancestors’ experiences with the blast. (Both Mothersill and Mousseau note that ongoing radiation could still pose additional stress.)

If the theory holds up, scientists have been overlooking a potentially very powerful influence on the biology of creatures around Chernobyl: that any health impacts they are suffering may have little to do with the doses they’re exposed to in their own lifetime, but result from what their ancestors experienced. This, Mothersill believes, “could reconcile the people that find very dangerous effects and the people who find no effects.”

Added to that is the complication that animals in the zone have probably moved around since 1986, or even come in from outside the zone. The zone, in other words, might be a disorderly hodgepodge of individuals whose ancestors may or may not have been exposed to a given level of radiation, making it difficult to parse out any radiation-related trends.

In another twist, some animal and plant populations might be faring well today because they’ve adapted to the radiation. European tree frogs around Chernobyl are much darker than frogs found outside the area, according to unpublished research by Germán Pereda of the University of Oviedo in Spain and his colleagues. He hypothesizes that since the accident, they have evolved to produce more melanin in their skin to protect themselves from radiation. (Mousseau says he has seen no convincing evidence that animals in the region have adapted to higher radiation levels.)

In the same vein, scientists have observed that certain cells of bank voles produce heightened levels of antioxidants, which could help to protect against radiation-induced toxicity. And researchers in Ukraine and the UK have spotted signs that some birch pollen and evening primrose seeds have become better at repairing DNA damage since the late 1980s.

But to truly grasp how life responds to nuclear disasters, scientists will need to dig deeper. Most of the studies to date have relied on correlations between radiation levels and wildlife health. Instead, researchers should be designing experiments that can more definitively ascertain that radiation is indeed causing the observed effects, says evolutionary biologist Anton Lavrinienko of the University of Jyväskylä in Finland, who collaborated with Mousseau on the vole studies.

“We need to stop generating studies which are scratching the surface,” he says. “This is something that we’re trying to change.”

Until then, blissfully ignorant of the fiery debate around them, wildlife in the Chernobyl exclusion zone will continue to do what they do best: burrowing, hunting, flying, and mating in their secluded, radioactive sliver of the Earth. Radioecologists will follow this experiment for decades, and — hopefully — they’ll eventually agree on the results.

This article originally appeared in Knowable Magazine, an independent journalistic endeavor from Annual Reviews. Sign up for the newsletter.