CONTROVERSES NUCLEAIRES !
ACTUALITE INTERNATIONALE
2005
TCHERNOBYL: "Une fantastique expérience" en faveur de la biodiversité ?!
Despite high radioactivity, plants and animals seem to be thriving.
    Dix-neuf ans après la pire catastrophe nucléaire civile au monde, la région de Tchernobyl, frappée par les radiations à la suite de l'accident d'un réacteur de la centrale nucléaire, est revenue à la vie, et son écosystème affiche "une biodiversité plus importante qu'avant le désastre", rapporte Nature
    Le magazine publie les observations de Viktor Dolyn, membre de l'Académie des sciences d'Ukraine, qui travaille sur les effets de la radioactivité sur l'environnement. Selon Dolyn, "une centaine d'espèces inscrites sur la liste rouge de celles qui sont menacées sont à présent visibles dans la zone évacuée qui couvre plus de 4.000 km2 en Ukraine, Biélorussie et Russie. Environ une quarantaine d'entre elles, dont certaines espèces d'ours et de loups, n'avait jamais été aperçue dans cette région avant l'accident." Or, si ces animaux qui figurent au sommet de la chaîne alimentaire sont présents, c'est que les animaux et les plantes dont ils se nourrissent le sont aussi.
    "Comment cela est-il arrivé, étant donné que les niveaux de radiation sont trop élevés pour que les humains puissent revenir sans danger ?" D'après James Morris, de l'Université de Caroline du Sud, qui a dirigé le groupe de scientifiques qui a mené l'étude, cela s'explique par le fait que les individus, animaux ou végétaux qui ont muté à cause des radiations meurent prématurément jeunes. Ainsi ne subsistent que les individus adultes.
    Par ailleurs, il semblerait que l'un des principaux éléments radioactifs, le césium 137, reste dans les sols plutôt que de s'accumuler dans les plantes et les animaux. "Cela pourrait signifier que la contamination de la chaîne alimentaire humaine n'est pas aussi grave qu'on le croit."
    Malgré tout, "cela ne veut pas dire que les gens peuvent vivre dans la région". Au contraire, un séjour prolongé aurait des "conséquences désastreuses" pour les êtres humains. En revanche, "il est difficile de dire ce qu'il va advenir des plantes et animaux de la région. La biodiversité va-t-elle encore s'enrichir ? Telle est la question, déclare James Morris. Dans un sens, il s'agit d'une fantastique expérience."
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    Chernobyl's ecosystems seem to be bouncing back, 19 years after the region was blasted with radiation from the ill-fated reactor. Researchers who have surveyed the land around the old nuclear power plant in present-day Ukraine say that biodiversity is actually higher than before the disaster.
    Some 100 species on the IUCN Red List of threatened species are now found in the evacuated zone, which covers more than 4,000 square kilometres in Ukraine, Belarus and Russia, says Viktor Dolin, who studies the environmental effects of radioactivity at the Ukrainian National Academy of Sciences in Kiev. Around 40 of these, including some species of bear and wolf, were not seen there before the accident.
    If animals at the top of the food chain are present, then the plants and animals they eat must also be thriving, says ecologist James Morris of the University of South Carolina in Columbia, who chaired a panel of scientists presenting the results at a meeting of the Ecological Society of America in Montreal, Canada, this week.
    "By any measure of ecological function these ecosystems seem to be operating normally," Morris told news@nature.com. "The biodiversity is higher there than before the accident."

Mutant die-off
    How has this happened, given that radiation levels are still too high for humans to return safely? Morris thinks that many of the organisms mutated by the fallout have died, leaving behind those that have not suffered problems with growth and reproduction.
    "It's evolution on steroids. There are a lot of deleterious mutations in species but these seem to be very quickly weeded out," Morris explains. Many young fish living in the reactor's cooling ponds are deformed, but adults tend to be healthy, implying that those harmed by radiation die young.
    Another factor in the ecosystem's apparent good health could be that the major radioactive elements in the region, such as caesium-137, tend to stay in the soil rather than accumulating in plants and animals, suggests Dolin. This means that contamination of the human food chain by radioactivity from Chernobyl might not be as severe as was feared.
    All this has led some people to propose that tourism to Chernobyl would help develop the area. In 2002, a United Nations report suggested that ecotourism could help plug the gap left by dwindling funds for regeneration.

A nice place to visit
    It is now possible to visit the area on holiday. But this doesn't mean that people can live there. Some 40 different radioactive elements, including strontium-90 and decay products of uranium and plutonium, were released into the exclusion zone, and it will be many hundreds of millennia before humans could move safely back, Dolin says.
    Humans spending long periods of time there would suffer a build-up of radiation that would shorten lives and raise newborn mortality. "It would be a disaster for humans," Morris says.
    Many birds are also showing the harmful effects of the fallout. Morris's colleague Timothy Mousseau found that barn swallows nesting around Chernobyl have lower survival rates, fewer eggs and are in generally worse condition than those living southeast of Kiev, away from the exclusion zone.
    It is difficult to say what will become of the region's plants and animals, admits Morris. One way to find out is to sample the genetics of populations to see whether diversity is likely to continue to increase. "What will happen here? That's the question," he says. "In a way it's a fantastic experiment."