Nearly 30 years ago, in April 1986, the Chernobyl nuclear power plant in the Ukraine failed spectacularly when a series of power surges led to catastrophic explosions and fires. The result was the biggest nuclear accident in history, with fallout seriously contaminating an estimated 100,000 square kilometres (38,610 square miles), an area larger in size than the country of Portugal.
It is estimated that exclusion zone, which extends 30 kilometres (19 miles) in all directions, will be unsafe for human habitation for another 20,000 years to come.
That hasn't stopped some people from returning home; and animals are even less biddable than humans. According to a paper published this week in the journal Current Biology, wild animal populations in Chernobyl are thriving. Red deer, roe deer, elk, wild boar and wolves have all returned in force to the mostly abandoned zone.
This, the authors say, doesn't mean that the radiation is harmless.
"It's very likely that wildlife numbers at Chernobyl are much higher than they were before the accident," said co-author Jim Smith of the University of Portsmouth in the UK. "This doesn't mean radiation is good for wildlife, just that the effects of human habitation, including hunting, farming, and forestry, are a lot worse."
Before the radiation could spread, most domestic animals were evacuated along with humans. However, some were left behind. According to the 1991 International Chernobyl Project Technical Report, horses and cows died from thyroid complications due to radiation, while others were stunted. Birth defects in animals in affected regions rose dramatically in the years following the disaster, too; and sheep across Europe have been affected by radiation, possibly from eating fungus that has absorbed radioactive caesium.
Within the exclusion zone itself, wild animal populations have grown. The team has studied the Belarussian section of the Exclusion Zone over a long period of time. In the winter months between 1987 and 1996, helicopters flew over the zone to count the animals. From 2008 through 2010, the team collected ground data, looking at tracks in the snow to record mammal activity.
This data was then compared to uncontaminated nature reserves in Belarus. What the team found is that elk, red deer, roe deer and wild boar numbers were similar in the exclusion zone to the numbers found in uncontaminated nature reserves. Moreover, elk and roe deer numbers have been increasing consistently since 1987, with roe deer numbers increasing tenfold in the period between 1987 and 1996.
Wolf numbers, on the other hand, have massively increased. Wolf populations in the exclusion zone are seven times higher than wolf populations in uncontaminated nature reserves. This could at least partially be attributed to the lack of human hunters.
The animal populations aren't just interesting for their own sake. They can also be used to measure the effects of long-term radiation exposure, something that isn't possible in a lab setting -- at least not on such a large scale and across so many populations. Last year, for example, a study of 152 birds from the exclusion zone found that the animals were adapting to radiation levels.
Other data that has proven valuable is the wild boar tests high for radiation compared with other animals. This is because truffles and mushrooms, which absorb radioactive isotopes at a high rate, are the pigs' favoured food. It has been an ongoing problem in Germany, where the radioactivity levels render the boar meat unfit for human consumption.
The thriving wolf population is indicative of the rich ecosystem too. If a predator population is healthy, it means that the prey population is also healthy. This makes the wolves a good key indicator for the overall health of the ecosystem within the exclusion zone.
"These results demonstrate for the first time that, regardless of potential radiation effects on individual animals, the Chernobyl exclusion zone supports an abundant mammal community after nearly three decades of chronic radiation exposures," the paper's abstract reads.
Animal survey work in the Chernobyl exclusion zone continues. In the last few years, the TREE consortium, which assisted in this research, set up webcams in 84 locations around the zone to observe wildlife behaviour. So far, the cameras have captured images of elk, lynxes, wolves, hares, otters, pine martens, horses, weasels and the first confirmed sighting of a brown bear in the Ukrainian Chernobyl exclusion zone. Next year, they plan to fit tracking collars to selected animals to closer observe their behaviour patterns.