Un reportage du biologiste Timothy Mousseau montre plus de 20 ans après les conséquences de la catastrophe de Tchernobyl sur la faune et la flore. Son travail s’est basé notamment sur l’observation des toiles d’araignées dont la structure a été modifiée.
Source : l’article du New York Times avec de magnifiques photographies : http://www.nytimes.com/2014/05/06/science/nature-adapts-to-chernobyl.html?_r=0
At Chernobyl, Hints of Nature’s Adaptation
NOVOSHEPELYCHI, Ukraine — The clicking sound from Timothy Mousseau’s radiation detector slowly increased as he walked through the forest here, 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, Dr. Mousseau said, for this area not far from Novoshepelychi, one of hundreds of villages that were abandoned after radioactive fallout from the 1986 reactor explosion at the plant 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 resident of the United States 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,” Dr. Mousseau said. “It’s a perfect laboratory setting for us.”
Dr. Mousseau, a biologist at the University of South Carolina, 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. After the nuclear meltdowns at Fukushima, Japan, three years ago he has conducted similar research there, too.
In dozens of papers over the years Dr. 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 last month, showed something new. Some bird species, they reported in the journal Functional Ecology, appear to have adapted to the radioactive environment by producing higher levels of protective antioxidants, with correspondingly less genetic damage. For these birds, Dr. 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 on April 26, 1986. They were exposed to lethal doses, in many cases in just a few minutes, and their organs and tissues were so badly damaged that they 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 Dr. 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.
Dr. 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, though not barn swallows or robins — to thrive in the zone. However, it remains to be seen whether these species are truly thriving, Dr. Mousseau 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.
Like almost all of the studies by Dr. Mousseau and his colleagues, the latest one takes advantage of the unique circumstances of the Chernobyl exclusion zone as a real-world laboratory. “Nature is a much more stressful environment than the lab,” Dr. Mousseau said. Abnormalities and other effects of radiation are seen at much lower radiation levels than in lab-based studies, he said.
Radiation levels in the zone also vary considerably from place to place because weather patterns during the accident and its aftermath affected the intensity of the fallout.
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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,” Dr. Mousseau said. “This gives us the ability to compare hot and cold areas that are similar ecologically in most ways except for radiation.” Those comparisons have generally shown a lower abundance of birds and rodents in the more radioactive areas.
On this day, while collaborators from Finland were inspecting traps and collecting mice for genetic studies, Dr. Mousseau was gathering data for another project. He photographed spider webs.
Dr. Mousseau wonders if radiation exposure might have affected spiders’ orderly web-spinning abilities, just as caffeine and other substances are known to do. So he’ll have students back in South Carolina systematically analyze the photographs — he’s taken more than 200 — for signs of disorder.
Dr. Mousseau said that for years he pursued the Chernobyl research largely out of personal interest. But that changed after the nuclear accident at the Fukushima plant in Japan in 2011. While the two accidents were different — at Chernobyl the reactor exploded, while at Fukushima cores melted down and there was an explosion outside the reactor itself — the result was basically the same: radioactive contamination over a wide area.
“No one really expected there’d be another nuclear disaster on this scale,” he said. “But it’s clear now after Fukushima that all of this has some broad relevance.”
Dr. Mousseau has expanded his work to include similar studies in Japan — he’s made about 10 trips there. Already, he said, he is seeing some Chernobyl-like effects in the contaminated area around the Fukushima plant, but he needs to gather data for at least a few more years before he can be confident about the impact.
“If we find the same sort of dose response in both places,” he said, “that provides incredible strength to the hypothesis that it is indeed radiation that is leading to these negative impacts.”