On 26 April 1986, the worst nuclear accident in history occurred at the Chernobyl power station in the former Soviet Union. More than 135,000 people and 35,000 cattle living within 30 kilometres (19 miles) of the stricken nuclear reactor were evacuated and an unprecedented "zone of exclusion" was established around the site, close to the border between the Ukraine and Belarus.
Within this zone, nature was left alone to cope with the high doses of radiation released from the reactor. As a result, the exclusion zone effectively became a huge, unwitting experiment in what happens to animals and plants in the wild following catastrophic radioactive contamination.
In the immediate aftermath of the accident, when the emphasis was on the immense human cost of the fallout, few people thought about what would happen to the wildlife within the zone or how to monitor it.
Radiation is known to cause genetic mutations and there were some early suggestions that wildlife would suffer widespread birth defects and other developmental deformities. Lurid reports of mutant animals were never confirmed. Twenty years later, a clearer picture has emerged, yet scientists are still arguing about what has actually happened to wildlife within the zone.
The latest spat has taken place within the pages of Biology Letters, one of the scientific journals of the Royal Society. On the one side is Anders Moller of the Pierre and Marie Curie University in Paris. Opposing him is Jim Smith, an ecologist at the University of Portsmouth, who has carried out his own studies of the zone's animals and plants. Moller believes that the radiation damage to wildlife within the zone has been dangerously underplayed, while Smith believes that much of the evidence collected so far points to a flourishing wildlife haven created when the area was abandoned.
There is no doubt about the severity of the accident itself and the great damage it did to the people whose lives have been irrevocably blighted. In addition to the tens of thousands of people who lost their homes and livelihoods, there have been several deaths although not as many as some environmental groups have suggested. Three people died immediately as a result of the explosion and fire and a further 28 died a few weeks after the accident as a direct result of acute radiation poisoning. All were either staff of the power plant itself, or workers from the emergency services that attended the accident.
A further 19 people from the emergency services died of various causes between 1987 and 2004. There have been an estimated 4,000 cases of thyroid cancer in Belarus, Ukraine and parts of Russia, most of which are attributed to the radioactive fallout from Chernobyl. (Fortunately, thyroid cancer is treatable if caught early enough.) But there were also concerns about the longer-term genetic damage to humans, which is why the study of wildlife within the zone is so important.
Scientists have had access to limited data when it comes to assessing the true facts within the 4,000 square kilometres of the "zone of alienation". Photographs of the abandoned city of Pripyat, near Chernobyl, reveal that trees and shrubs have started to sprout through the roads and buildings. Nature has begun to reclaim what was originally lost to urban development and agriculture.
Scientists from the International Radioecology Laboratory in Slavutych have documented an increase in sightings of large animals that were rare or absent before the disaster. Several packs of wolves have appeared and they seem to have made easy meals of any stray dogs left behind by their owners. (Wolves Eat Dogs is the title of a novel based on the exclusion zone by Gorky Park author Martin Cruz Smith.)
The rare Przewalski's horse from the Russian steppe has been reintroduced, along with European bison. Beavers and boars are beginning to reshape the forest ecosystems, European lynx have been sighted and many rare birds, such as the black stork and white-tailed eagle, have returned, along with many swans and owls.
For some scientists, the sight of wildflowers growing through the cracks in the concrete roads of Pripyat and the many and varied species of larger animals within the zone are signs that something positive has come out of the disaster. These researchers believe that the detrimental effects of the radioactive fallout have been exaggerated, while the true impact of human activity has been overlooked.
"The benefit of excluding humans from this highly contaminated ecosystem appears to outweigh significantly any negative cost associated with Chernobyl radiation," said Robert Baker, a biologist at Texas Tech University, who has made more than a dozen scientific excursions into the zone. Baker believes that the diversity of animals and plants within the zone is what could be reasonably expected to be seen in a nature park dedicated to conservation. Indeed, there have been calls to turn the Chernobyl exclusion zone into what would become Europe's largest nature reserve.
But this has cut little ice with Anders Moller, a controversial Danish-born scientist who was once accused of scientific misconduct in his home country. Writing with his colleague Timothy Mousseau of the University of South Carolina, Moller says that reports of a thriving ecosystem around Chernobyl are not actually based on standardised censuses. "Surprisingly, there is little data on how the abundance of animals varies with levels of radiation, and there has been no effort to census animals in relation to local levels of radiation around Chernobyl," say Moller and Mousseau.
The two scientists carried out their own birdlife census at eight different sites within the zone. They recorded some 1,570 individual birds from 57 species and compared their observations with records of local radiation levels. They found that the higher the background level of radiation, the lower the number of birds, which they say is a clear indication of a possible cause and effect. The abundance of birds differed by as much as 66 per cent between the most contaminated sites and those with normal, background levels. And the variety of species varied by more than 50 per cent depending on radiation levels, they say.
One possible explanation they put forward is that the birds are affected by the loss of their main source of food, such as worms and insects living in contaminated soil. "The effect was differential for birds eating soil invertebrates living in the most contaminated top-soil layer. These results imply that the ecological effects of Chernobyl on animals are considerably greater than previously assumed," they say.
Previous work by the two scientists found that barn swallows in the exclusion zone tend to suffer a high level of albinism and carry a significantly higher level of "germ line" mutations to their sperm and eggs compared with barn swallows elsewhere. They have followed up this work with similar research on house sparrows and black redstarts, and they claim to have found individuals from these two species that also carry more germ-line mutations if they live within the exclusion zone compared with birds living elsewhere.
Professor Mousseau says that the findings point ominously towards what may also happen in humans. "Our work indicates that the worst is yet to come in the human population. The consequences for generations down the line could be greater than we've seen so far," he says.
Jim Smith disagrees. He questions the methodology of Moller and Mousseau, saying that their statistical analysis wrongly clumps sites of widely varying radiation dose rates under one category. He also points out that the apparent fall in numbers of barn swallows and other birds living in parts of the zone may be due to the absence of people rather than increases in levels of radiation.
"Barn swallows are commonly associated with human habitation and their population is influenced by farming practices... [Moller and Mousseau] do not apparently consider the abandonment of contaminated lands to be a potential confounding factor." Smith says that given the limitations of Moller's work, it is "premature" to extrapolate the findings to the controversial and potentially emotive issue of human health.
For their part, Moller and Mousseau ask why there has been no concerted effort to monitor the long-term effects of Chernobyl in free-living organisms and humans, despite it being possibly the biggest-ever environmental disaster. "Scientific inquiry," they say, "depends on rigorous analysis of data rather than rendition of anecdotal evidence."
But perhaps this last point is where both sides can reach agreement. Chernobyl has spawned a vast rumour industry based on anecdote rather than science. They include stories as varied as two-headed mutants breeding in the exclusion zone to the blogs of "Elena", a mysterious "girl on a motorcycle", whose claims about illicit rides through the exclusion zone were taken seriously until she was exposed as a hoax by Mary Mycio, a foreign correspondent of the Los Angeles Times and author of Wormwood Forest: A Natural History of Chernobyl.
If the true long-term effects of the world's biggest nuclear accident on the surrounding countryside are ever to be known, it has to be based on science rather than anecdote.