The argument that was put forth was that the lack of human activity allowed populations to rebound and for animals to engage in more natural behavior. The conversation we ended up having was exploring what's important to look at. Overall population growth? Animal activity? The health of individual animals? One of the really interesting questions asked was, if animals in the wild tend to have shorter life spans than their counterparts in captivity (due to threats of injury and illness, trouble finding food, etc.) how much should radiation be considered an issue if animals might die of other causes before radiation poisoning and cancer ever set in? Mind you, my friend and I aren't scientists and we were just shooting the breeze, so we were more exploring questions and ideas than hard data. That said, actual researchers are looking into these things. I wouldn't be surprised if the answer in the end is "Well, it's a bit of a mixed bag." It has been 30 years since the world's worst nuclear accident, yet it is still not clear how badly the local wildlife has been affected by the radiation