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. …

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.