Chernobyl supports 'abundant' mammal life, says new study

06 October 2015

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A scientific study has for the first time demonstrated 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 study, conducted by scientists from Belarus, Japan, the UK and the USA was published yesterday in the journal Current Biology.

The exclusion zone covers an area of about 262,000 hectares immediately surrounding the Chernobyl nuclear power plant where radioactive contamination from fallout from the accident in 1986 is highest and public access and inhabitation are restricted. The zone borders a separately administered area, the Polesie state radiation and ecological reserve (PSRER), to the north, in Belarus.

Following the 1986 Chernobyl accident, 116,000 people were permanently evacuated from the 4200 square kilometre Chernobyl exclusion zone. There is continuing scientific and public debate surrounding the fate of wildlife that remained in the abandoned area, according to the article titled Long-term census data reveal abundant wildlife populations at Chernobyl.

"Several previous studies of the Chernobyl exclusion zone indicated major radiation effects and pronounced reductions in wildlife populations at dose rates well below those thought to cause significant impacts. In contrast, our long-term empirical data showed no evidence of a negative influence of radiation on mammal abundance," the scientists wrote.

Relative abundances of elk, roe deer, red deer and wild boar within the Chernobyl exclusion zone are similar to those in four (uncontaminated) nature reserves in the region and wolf abundance is more than seven times higher, they wrote. Additionally, their earlier helicopter survey data show rising trends in elk, roe deer and wild boar abundances from one to 10 years post-accident.

The Belarus sector of the Chernobyl exclusion zone - the PSRER - covers 2165 square kilometres, half of the total area, and has similar radiation levels to the Ukrainian sector, they wrote, adding that only about 1% of the Ukrainian sector is more contaminated.

The PSRER provides a "unique opportunity" to test three key hypotheses concerning the resilience of wildlife to the world's worst nuclear accident, they wrote.

Hypotheses

The first hypothesis proposes that mammal abundances are negatively correlated with levels of radioactive contamination at Chernobyl, but this hypothesis was not supported by the new data. The mean number of tracks per 10 km (2008-2010) was assessed as a function of radiocaesium contamination density on 35 winter survey routes for elk, wolf, wild boar, roe deer, fox, and a combined category of other predatory and non-predatory mammals.

"For all species, our statistical models rejected radioactive contamination as an important predictor of mammal density within the PSRER. Although census data do not give direct information on population metrics such as reproductive success or longevity, a scenario in which depressed populations in the highly contaminated areas are supported (on a daily basis) by rapid influx and habitat utilization from less contaminated areas seems highly unlikely," they wrote.

The second hypothesis proposes that densities of large mammals are suppressed at PSRER (the Chernobyl zone) compared with those in four uncontaminated nature reserves in Belarus. Again, they found that this hypothesis was not supported by the empirical data.

"We analysed population density estimates (2005–2010) derived from winter track survey routes and published by the Belarus Ministry of Natural Resources. Similar densities of large ungulates (hoofed mammals) were observed at PSRER for elk, red deer, roe deer and wild boar compared with four uncontaminated reserves in Belarus," they wrote. "Wolf density at PSRER was seven times higher. Though there is uncertainty in estimating population density from winter track survey data, our comparison of relative density between reserves is robust because the same census methods were used in each reserve."

The rejection of this second hypothesis is supported by comparing track counts at PSRER with the Bryansky Forest reserve in Russia, 250 km away, they wrote. Elk and boar counts (from 2008–2010) were broadly similar in the two reserves. Wolf and lynx tracks were respectively 19 and 1.3 times higher, but roe deer four times lower, at PSRER.

"Interpretation of this comparison of ungulate densities among reserves is difficult owing to the much higher wolf densities but lower human pressure at PSRER compared with other reserves. Nonetheless, the data indicate that mammal densities at PSRER are not atypical of what would be expected in an uncontaminated nature reserve in the region," they wrote.

"Due to natural variation in mammal densities and habitats, this comparative analysis cannot exclude some impact of radiation on populations at PSRER, though we found no evidence of this in our data that refuted Hypothesis 1."

The third hypothesis proposes that densities of large mammals declined in the period between 1 and 10 years after the accident. Aerial survey counts of wild boar (1987-1996), elk and roe deer (1988-1996) increased significantly over time, they wrote. For boar, the significance of this decreased when they excluded two years with lower survey coverage. Increases in large non-predatory mammals from 1987-2996 were accompanied by a large increase in wolves, a likely cause of the decline in wild boar from 1993-1994, they wrote, although an outbreak of African swine fever also contributed to this decline. Before the Chernobyl accident, mammal population densities were likely depressed due to hunting, forestry and agriculture, they added.

Extremely high dose rates during the first six months after the accident significantly affected animal health and reproduction at Chernobyl, they wrote, but any potential long-term radiation damage to populations is not apparent from their trend analysis of large mammal abundances.

"Increases in elk and wild boar populations in the Chernobyl exclusion zone occurred at a time (early 1990s) when these species were undergoing a rapid decline in former Soviet Union countries owing to major socio-economic changes (which resulted in increased rural poverty and weakened wildlife management). Our data on time trends cannot separate likely positive effects of human abandonment of the Chernobyl exclusion zone from a potential negative effect of radiation (though we could detect no such negative effect in our test of Hypothesis 1). Nevertheless, they represent unique evidence of wildlife's resilience in the face of chronic radiation stress."

None of the three hypotheses postulating radiation damage to large mammal populations at Chernobyl was supported by the empirical evidence, they wrote.

"The results from these unique data will help society balance the negative impacts to wildlife from chronic radiation exposures against how the removal of humans alleviates one of the more persistent and ever growing stresses experienced by natural ecosystems."

Current Biology is a ScienceDirect publication. ScienceDirect is part of academic publishing company Elsevier.

Researched and written

by World Nuclear News

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