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The clicking sound from Timothy Mousseau’s radiation detector slowly increased as he walked through the forest in Ukraine, a few miles west of the Chernobyl nuclear power plant.

As he stopped to examine a spider web on a tree branch, the display on the device showed 25 microsieverts an hour. That is typical, he said, for this area not far from Novoshepelychi, one of hundreds of villages that were abandoned after radioactive fallout from the April 26, 1986, reactor explosion rendered a large part of this region uninhabitable.

The levels of radioactivity here are far below those still found in parts of the deteriorating shelter that covers the destroyed reactor – a shelter that by 2017 will itself be covered by a huge arch that is intended to eliminate the threat of further radioactive contamination.

But the levels in this lowland glade, where acacias and Scotch pines are interspersed with the occasional tumbledown barn, are higher than normal. In 10 days here, a person would be exposed to as much background radiation as a typical U.S. resident receives from all sources in a year. That makes it off-limits except for short forays, but a good place to study the long-term effects of radiation on organisms.

“This level of chronic exposure is above what most species will tolerate without showing some signs, either in terms of how long they live or in the number of tumors they have, or genetic mutations and cataracts. It’s a perfect laboratory setting for us,” said Mousseau, a biologist at the University of South Carolina. He has been coming to the contaminated area around Chernobyl, known as the exclusion zone, since 1999. The list of creatures he has studied is long: chiffchaffs, blackcaps, barn swallows and other birds; insects, including bumblebees, butterflies and cicadas; spiders and bats; and mice, voles and other small rodents.

In dozens of papers over the years Mousseau, his longtime collaborator, Anders Pape Moller of the National Center for Scientific Research in France, and colleagues have reported evidence of radiation’s toll: higher frequencies of tumors and physical abnormalities like deformed beaks among birds compared with those from uncontaminated areas, for example, and a decline in the populations of insects and spiders with increasing radiation intensity.

But their most recent findings, published in the journal Functional Ecology, showed something new. Some bird species appear to have adapted to the radioactive environment by producing higher levels of protective antioxidants, with correspondingly less genetic damage. For these birds, Mousseau said, chronic exposure to radiation appears to be a kind of “unnatural selection” driving evolutionary change.

Ionizing radiation, like that produced by cesium, strontium and other radioactive isotopes, affects living tissue in several ways, among them by breaking strands of DNA. A high enough dose – many thousands of times higher than the levels in the forest – can cause sickness or death. That is what happened to several dozen technicians and firefighters at the Chernobyl plant when the Unit 4 reactor exploded. They were exposed to lethal doses, in many cases in just a few minutes, and died within weeks.

Relatively low doses of radiation, however, even over a long time, may have little or no effect. But lower doses can cause genetic mutations, leading to cancers and other physical problems that may show up over longer periods and affect breeding and longevity. Studying the effects on animals and insects can lead to a better understanding of the impact on people as well.

Some researchers have challenged the studies by Mousseau and his collaborators, arguing that it is difficult to show that radiation levels in the exclusion zone, which covers about 1,000 square miles, have had much noticeable effect. There have also been anecdotal reports of abundant populations of some animals in the zone, suggesting that the lack of human activity there has led to the area becoming a haven for wildlife.

Mousseau dismisses the idea that the zone is some kind of post-apocalyptic Eden. But the latest study has given him pause, he said, because it shows the kind of adaptations that may allow some creatures – chaffinches and great tits in this case – to thrive in the zone. However, it remains to be seen whether these species are truly thriving, he said.

The findings also suggest that in some cases, radiation levels might have an inverse effect – birds in areas with higher radiation exposure may show greater adaptation, and thus less genetic damage, than those in areas with lower radiation levels.

Not far from here is one of the hottest areas in the zone, the so-called Red Forest, where radiation levels were so high in the days after the accident that Scotch pines turned red and died.

“But 10 to 15 miles from here, there are areas that are absolutely clean,” Mousseau said. “This gives us the ability to compare hot and cold areas that are similar ecologically in most ways except for radiation.”