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A.P. Møller et al. / Ecological Indicators 24 (2013) 75–81

similar reduced fecundity has been reported for humans (Yablokov
et al., 2009). Males living in contaminated areas have shown a failure to produce sperm, or if producing sperm then mainly inviable
sperm (Yablokov et al., 2009). Short-term physiological effects ofradiation may have negative consequences for adult survival, especially in the sex that invests the most in reproduction (Møller et al.,2005a; Yablokov et al., 2009).
Mutation effects due to radioactive contamination are
widespread, although more difﬁcult to study due to their long-term
nature that can extend across generations. Mutations arise fromDNA damage that is not repaired thereby causing a reduction inﬁtness (Eyre-Walker and Keightley, 2007). Radioactive contamination has been known to be mutagenic for almost a century(Nadson and Philippov, 1925). Extensive studies of a diverse arrayof plants, animals and other organisms have shown increased
levels of mutations by a factor 2 to 20 around Chernobyl (review
in Møller and Mousseau, 2006), and a recent meta-analysis has
shown strong effects of radiation on mutation rates arising from
Chernobyl in a study based on 151 estimates from 45 studies of
30 species (S. Randic, unpublished data). A high rate of deleterious
mutations can increase the frequency of disease in humans (Crow,
1997, 2000) and therefore also most likely in other organisms.
Mutations are costly because each deleterious mutation equals
a “genetic death”, although selective deaths may simultaneously
reduce the mutational load because multiple mutations are lost
simultaneously (Muller, 1950). Alternatively, truncation selection
or quasi-truncation selection simultaneously eliminate multiple
mutations if the probability of death is a direct function of thenumber of deleterious mutations. Because slightly deleterious
mutations may remain in populations for an extended numberof generations (in Drosophila on average 80 generations), there
is a high probability of such mutations being transmitted during
their long-term persistence in a population, resulting in mutation
accumulation and mutational load, particularly if mutation effects
are recessive (Crow, 1997).
The objectives of this study were to test whether differencesin the number of generations that have passed since exposure toradioactive contamination have had an inﬂuence on the relationship between the abundance of animals and radiation. We used
extensive census data on diurnally active animals around Chernobyl and Fukushima to test (1) whether the abundance of animals
was more severely depressed in areas with higher levels of radiation (i.e. Fukushima) or (2) whether the reduction in abundance was
the greatest in the area that had exposed animals to radiation fora larger number of generations (i.e. Chernobyl). We should expect
radiotoxic effects in both sites, while mutations would only accumulate across generations in Chernobyl, where most organisms
have been chronically exposed to radiation for many generations.

2. Methods
2.1. Study sites
The censuses were conducted within the Chernobyl Exclusion
Zone or adjacent areas on the southern and western borders witha permit from the Ukrainian authorities and in areas in southern Belarus around Gomel during the breeding seasons 2006–2009(Fig. 1A). A total of 254 points in 2006, 235 points in 2007, 237
points in 2008 and 159 points in 2009 were located at ca. 100 mintervals within forested areas (excluding successional stages ofsecondary forest due to abandoned farming (these areas are still
almost exclusively open grassland)).
We conducted similar censuses at a total of 300 sampling
points in forested areas west of the exclusion zone around the
Fukushima Daiichi power plants in 2011 (Fig. 1B). At least one local

Fig. 1. Location of census areas around (A) Chernobyl, Ukraine and Belarus and (B)Fukushima, Japan. Census points in Fukushima are located in six areas. Note thatlevels of radiation in Fukushima have only been recorded at a limited number ofsites.
Adapted from Shestopalov (1996) and http://www.nnistar.com/gmap/fukushima, a
map generated by the Japanese Ministry of Education, Culture, Sports, Science andTechnology (MEXT) and local government ofﬁcials.

ornithologist participated in the censuses in Japan to conﬁrm theidentity of some difﬁcult bird species. All sampling sites were identiﬁed using GPS coordinates.
2.2. Animal censuses
The point count census method provides reliable information
on relative abundance of birds (Møller, 1983; Bibby et al., 2005;

A.P. Møller et al. / Ecological Indicators 24 (2013) 75–81

Voříšek et al., 2010) and other organisms. It consists of counts
lasting 5 min during which the number of spider webs, and thenumber of individual grasshoppers, dragonﬂies, bumblebees, butterﬂies, cicadas, amphibians, reptiles and birds seen or heard wererecorded. APM (wearing a radiation protection suit in the mostcontaminated areas in Chernobyl) conducted these standard point
counts during 29 May to 9 June 2006, 1 to 11 June 2007, 29 May to 5
June 2008, 1 to 6 June 2009 in Chernobyl and surroundings and during 11 to 15 July 2011 in Fukushima. The fact that one person madeall counts eliminates any variance in results due to inter-observer
variability.
We directly tested the reliability of our counts by letting two
persons independently perform counts, and the degree of consistency was high for both species richness and abundance (details
reported by Møller and Mousseau, 2007a).
Mammal censuses were conducted using 161 line transects eachwith a length of 100 m along roads in a large number of areas in
the Chernobyl exclusion zone and just outside the zone during 3–4
February 2009. Mammals were identiﬁed from footprints in fresh
snow using a standard ﬁeld guide (Bang et al., 2007). See Møller
and Mousseau (2011) for further details.
2.3. Confounding variables
Abundance estimates can be affected by numerous confounding
variables (Voříšek et al., 2010), and, therefore, it is important to control such variables statistically to assess the underlying relationshipbetween radiation and species richness and abundance. We classiﬁed habitats (agricultural habitats with grassland or shrub [either
currently or previously cultivated], deciduous forest, or coniferous forest) and estimated to the nearest 10% ground coverage by
herbs, shrub, trees, agricultural habitat, deciduous forest and coniferous forest within a distance of 50 m from the census points. We
recorded altitude to the nearest foot, using a GPS. Weather conditions can affect animal activity and hence census results (Voříšek
et al., 2010), and we recorded cloud cover at the start of each point
count (to the nearest eighth), temperature (degrees Celsius), andwind force (Beaufort). For each census point we recorded time of
day when the count was started (to the nearest minute). Because
activity may show a curvilinear relationship with time of day, forexample, with high levels of activity in the morning and to a lesser
extent in the evening for birds (Voříšek et al., 2010), and higher
levels of activity for thermophilic vertebrates and invertebrates,
we also included time squared as an explanatory variable.
Møller and Mousseau (2011) have previously used estimates ofnatal dispersal distance (the distance between the site of emergence/birth to the future site of reproduction (Clobert et al., 2001))
for different taxa to explain the slopes describing the relationshipbetween abundance and level of background radiation. We usedtheir estimates reported in Appendix A in Møller and Mousseau(2011). We estimated mean abundance of different taxa as themean number of individuals recorded at the census points.
2.4. Background radiation
We measured radiation in the ﬁeld and cross-validated thesemeasurements with those reported by the Ukrainian Ministryof Emergencies. After having ﬁnished the 5 min census we measured radiation levels at ground level directly in the ﬁeld at eachpoint where were censused invertebrates using a hand-held
dosimeter (Model: Inspector, SE International, Inc., Summertown,
TN, USA). We measured levels two to three times at each site
and averaged the results. We cross-validated our measurements
in Ukraine against data from the governmental measurements
published by Shestopalov (1996), estimated as the mid-pointof the ranges published. This analysis revealed a very strong77

positive relationship (linear regression on log-log transformed
data: F = 1546.49, d.f. = 1252, r2 = 0.86, P < 0.0001, slope (SE) = 1.28
(0.10)), suggesting that our ﬁeld estimates of radiation provided
reliable measurements of levels of radiation among sites.
Measurements at Fukushima were obtained using the same
dosimeters which were cross-validated with readings with adosimeter that had been recently calibrated and certiﬁed to beaccurate by the factory (International Medcom, Sebastopol, CA,
USA). We also made a cross-validation test at Fukushima by comparing our own measurements using the Inspector dosimeter withmeasurements obtained at the same locations with a TCS 171ALOKA. Again, there was a very strong positive relationship (linear
regression on log–log transformed data: F = 2427.97, d.f. = 1, 20,
r2 = 0.99, P < 0.0001, slope (SE) = 1.120 (0.023)).
There were signiﬁcant differences between Fukushima andChernobyl in the nature of the disaster as well as the type and theamounts of radionuclides that were released to the environment.
Currently, for terrestrial ecosystems, in Fukushima cesium-134 andcesium-137 predominate (Kinoshita et al., 2011), while in Chernobyl cesium-137, strontium-90, various isotopes of plutonium,
and americium-241 are found at biologically signiﬁcant levels
across the landscape (Voitsekhovich et al., 2007), and differential
sensitivity to these mixtures could account for some of the differences in biological responses observed between sites.
Cesium 137, with a half-live of about 30 years, decays by beta
emission primarily to a meta-stable isomer of barium-137, whichis responsible for the gamma emissions of this isotope (Baum et al.,2002). Thus if ingested, cesium-137 will generate both beta andgamma doses for living organisms. Cesium-134 that has a half-lifeof about 2 years is exclusively a beta emitter and is thus mainly aconcern if ingested.
Strontium-90, with a half-life of about 29 yrs, is almost a purebeta emitter.
Most isotopes of plutonium are alpha emitters and are thus primarily of concern if ingested. However, plutonium-241, which ispresent to a signiﬁcant degree in the Chernobyl region, has a halflife of about 14 yrs, and decays via beta emissions to americium-241(half life of 432 years), which in turn decays via alpha emissions
to neptunium-237, with gamma emissions as an additional byproduct.
Although handheld Geiger counters are likely to provide reliable measures of background contamination levels of radionuclides
for gamma sources, and to a lesser degree for beta emitters, if
the Geiger detector is in close proximity to the source, characterization of alpha emitters usually requires more complex
measurement methods that are usually only tractable in a laboratory setting due to the short transmission distance of alpha particles
in air.
Given the different characteristics of radionuclides in the environment at Fukushima and Chernobyl, ﬁeld measurements ofcontaminant levels are likely to underestimate biologically relevant radiation levels in Chernobyl when the main exposure
pathway is via ingestion. Similarly, background radiation measures in the areas of Chernobyl closest to the reactor (e.g. the
Red Forest) are very likely to underestimate biologically relevant doses given the abundance of alpha emitting actinides (e.g.
plutonium isotopes) that were differentially deposited in thisarea.
2.5. Statistical analyses
We analyzed the levels of radiation at the census points using
Welch ANOVA for samples with unequal variance. Normal quantile
plots were used to illustrate the spatial distribution of radiation.
Radiation level and abundance were log10 -transformed, while
ground coverage with farmland, deciduous and coniferous forest,

78

A.P. Møller et al. / Ecological Indicators 24 (2013) 75–81

grass, bush and trees was square root arcsine-transformed. We
included these variables in addition to temperature, cloud cover,wind, time of day and time of day squared. We also included radiation level squared to account for non-linear relationships between
species richness and abundance, respectively, and radiation. We
developed statistical models to assess the relationship between
abundance (response variables) and radiation, assuming a Poisson
distribution of abundance, after inclusion of the potentially confounding variables, as implemented in the statistical software JMP(SAS Institute Inc., 2000).
We quantiﬁed the relationship between abundance of different
taxa and level of radiation by estimating the slope of the relationship between abundance and log10 -transformed radiation. These
slopes were used for subsequent analyses. We tested whether meanslopes differed from zero using a one-sample t-test. Finally, wetested if slopes for different taxa could be explained by estimates ofnatal dispersal distance (derived from Møller and Mousseau, 2011)and mean population density at census points.
3. Results
3.1. Radiation levels
Radiation levels at the census points at Fukushima ranged from0.55 to 30.76 ␮Sv/h, mean (SD) 5.15 ␮Sv/h (3.05), N = 300 census points, and from 0.01 to 379.70 ␮Sv/h, mean (SD) 0.88 ␮Sv/h(13.50), N = 898 census points at Chernobyl, following backtransformation of the log-transformed values. The frequency
distribution of radiation levels at census points in Chernobyl wasmuch more skewed than that from Fukushima, with many census
points with low levels and a few having very high levels (Fig. 2).
The mean values at census points were signiﬁcantly higher at
Fukushima (Welch ANOVA F = 265.92, d.f. = 1, 1129.3, P < 0.0001),
while the variance at census points was larger at Chernobyl (Levene’s test: F = 199.16, d.f. = 1, 1196, P < 0.0001).
3.2. Abundance of animals and level of radiation
The abundance of birds, butterﬂies and cicadas decreased signiﬁcantly with level of background radiation at Fukushima, whilethat was not the case for bumblebees, dragonﬂies and grasshoppers(Table 1). Spiders were the only taxon that increased in abundancewith increasing level of radiation (Table 1). The abundance of all
investigated taxa decreased with level of background radiation at
Chernobyl (Table 1). The mean of the slopes describing the relationship between abundance and level of background radiation
for different taxa was statistically signiﬁcant for Chernobyl (mean(SE) = −0.059 (0.015), N = 10 taxa, P = 0.0041), while that was not thecase for Fukushima (mean (SE) = −0.040 (0.106), N = 8 taxa, P = 0.32).
Analyses that included all environmental variables provided similar
conclusions to those reported in Table 1 (Electronic Supplementary
Material Table 1). The slope of the relationship between abundance
and radiation for different taxa was strongly positively correlatedwith the slope after adjusting for potentially confounding environmental variables (F = 33.98, d.f. = 1, 6, r2 = 0.85, P = 0.0011, slope(SE) = 1.668 (0.286)), and the two sets of slopes differed signiﬁcantly(paired t-test, t = 2.24, d.f. = 7, P = 0.03).
Census data from Chernobyl and Fukushima were used to test if
abundance was related to background radiation, if abundance differed between the two sites, and if the effect of radiation differed
between sites (reﬂected by a signiﬁcant interaction between radiation and site). The analyses showed a signiﬁcant overall radiation
effect on abundance of all taxa with the exception of dragonﬂies
and spiders (Table 2). There were signiﬁcant effects of area on
abundance of all taxa with the exception of birds (Table 2). Finally,

Fig. 2. Normal quantile plots of radiation levels (␮Sv/h) in (A) Chernobyl and (B)Fukushima.

there were signiﬁcant differences in radiation effect between areas
for four out of six taxa (Table 2). These differences in radiation
effect between areas are shown for spiders, butterﬂies and birds
in Fig. 3. Therefore, radiation effects on abundance differed signiﬁcantly between Fukushima and Chernobyl for a majority of taxa.
We found no evidence of the slope of the relationship between
abundance of radiation in Fukushima being signiﬁcantly related
to natal dispersal distance (F = 0.31, d.f. = 1, 6, r2 = 0.05, P = 0.60) or
mean population density (F = 1.39, d.f. = 1, 6, r2 = 0.19, P = 0.28).
4. Discussion
The main ﬁndings of this study were that (1) for the regions
surveyed, levels of radioactive contamination were higher and lessvariable at Fukushima-Daiichi than at Chernobyl; (2) the relationship between abundance of animals and radiation was weaker inFukushima than in Chernobyl; and (3) the relationship between
abundance and radiation was more strongly negative in the areawith chronic exposure to radiation for many years (Chernobyl) thanin the more recently contaminated area (Fukushima). There are no
early census data from Chernobyl just after the accident so we cannot directly compare current and past relationships between levelsof background radiation and abundance of different taxa of animals.
If the relationships between abundance and radiation reported here
were caused exclusively by radiotoxicity, we should expect negative relationships in both Chernobyl and Fukushima. That wasA.P. Møller et al. / Ecological Indicators 24 (2013) 75–81

79

Table 1
Species richness of birds and abundance of different animal taxa in Fukushima and Chernobyl in relation to radiation level.
Sum of squares

d.f.

Fukushima
No. bird individuals
No. bird species
No. bumblebees
No. butterﬂies
No. cicadas
No. dragonﬂies
No. grasshoppers
No. spiders

0.775
0.181
0.001
4.553
0.208
0.127
0.004
0.636

1, 2981, 2981, 2981, 2981, 2981, 2981, 2981, 298Chernobyl
No. mammals
No. bird individuals
No. bird species
No. reptiles
No. amphibians
No. bumblebees
No. butterﬂies
No. dragonﬂies
No. grasshoppers
No. spiders

3.669
6.973
4.124
0.093
0.196
1.595
2.153
1.195
0.891
5.738

1, 1591, 896
1, 896
1, 896
1, 896
1, 896
1, 896
1, 402
1, 372
1, 896clearly not what we found. If the relationships were caused byaccumulation of mutations following chronic exposure to radiation across generations, we should expect to see more negative
effects on abundance in the site with more long-lasting exposure.
Consistent with this expectation, we found stronger negative relationships between abundance and radiation in Chernobyl. Theseﬁndings have important implications for monitoring of the biological effects of radioactive contamination because negative effects
are more likely to appear after many generations. Furthermore,
we should expect that the negative effects of radiation on abundance of animal taxa in Fukushima to increase over time. We plan
to repeat the census in Fukushima in 2011 again in 2012, with theprediction that the abundance should decrease further at the mostcontaminated census points.
Radioactive contamination may affect the abundance of animals
through direct radiotoxic effects on physiology or through indirect
effects of mutations that can have deleterious consequences forreproduction and viability. Radioactive contamination may havedirect radiation and chemical toxic effects on animals with consequences for survival and reproduction. Previous studies of animals
at Chernobyl have shown negative effects of radiation on immunity, antioxidant status, reproductive failure by females and sperm
production (see Section 1). Likewise, mutation rates at Chernobyl
have increased by up to a factor 20 relative to the normal background mutation rates (Møller and Mousseau, 2006). Here wehave tested whether differences in time elapsed since exposure
to radioactive contamination may allow discrimination between
decreases in abundance due to direct physiological and indirect
genetic effects due to mutational load. While the abundance of
all animal taxa decreased with increasing level of backgroundF

P

Estimate (SE)

14.89
6.77
0.16
37.18
19.24
0.87
0.22
14.12

0.00010.010
0.69<0.0001<0.0001
0.35
0.64
0.0002−0.105 (0.027)−0.051 (0.020)

57.28
256.89172.85
24.14
14.22
55.71
57.63
34.58
13.58
81.94

<0.0001<0.0001<0.0001<0.0001
0.0002<0.0001<0.0001<0.0001
0.0003<0.0001−0.182 (0.024)−0.078 (0.005)−0.060 (0.005)−0.009 (0.002)−0.005 (0.001)−0.037 (0.005)−0.043 (0.006)−0.049 (0.008)−0.071 (0.019)−0.071 (0.008)−0.254 (0.042)−0.054 (0.012)

0.095 (0.025)

radiation at Chernobyl, that was not the case in Fukushima. We
emphasize that the census methods were the same in the twoareas, and that even the same person conducted all the censuses
thereby reducing variance in effects due to inter-observer variability. Møller and Mousseau (2011) have previously shown a highdegree of repeatability in census results among years and among
countries in studies at Chernobyl, further suggesting that these factors are unlikely to contribute to the results presented here. Several
taxa did not show a reduction in abundance with increasing levelof background radiation at Fukushima despite the fact that levels of
background radiation were higher and less variable at our census
points in Fukushima compared to Chernobyl.
We documented differences in effects of radiation on abundance among taxa, and these differences varied between Chernobyl
and Fukushima. We found a reduced variance in radiation levels at
our census points in Fukushima compared to Chernobyl, and thismay have reduced the power of the statistical analyses. However,
we note that the negative relationships between species richness
and abundance of animals at Chernobyl are not a question of alarge variance in levels of background radiation because these relationships were obvious even when making separate analyses forthe two halves of the range of radiation (see Fig. 2 in Møller andMousseau, 2011). In fact, bumblebees, dragonﬂies and grasshopper
were not signiﬁcantly negatively affected in Fukushima, and theabundance of spiders even increased rather than decreased withlevel of radiation. We suggest that such differences among species
and taxa may be related to ecological characteristics of these taxa
(Møller and Mousseau, 2007b, 2011). Detailed studies of birds haveshown that species with a high rate of use of antioxidants suffer
disproportionately from the effects of radiation on oxidative stress

Table 2
Species richness of birds and abundance of different animal taxa in relation to level of radiation, area (Chernobyl or Fukushima) and their interaction.
Taxon

Radiation (R)Estimate (SE)Area (A)

R×A

Model

No. bird species
No. birds
No. bumblebees
No. butterﬂies
No. dragonﬂies
No. grasshoppers
No. spiders

33.05***
66.49***
4.79*
100.27***
0.03
4.69*
0.61

−0.055 (0.010)−0.092 (0.011)−0.020 (0.009)−0.149 (0.015)−0.003 (0.018)−0.032 (0.015)
0.012 (0.016)

39.50***
0.43
27.76***
532.55***14.83***
15.47***
10.17**

0.23
1.45
3.47
50.39***
6.28*
7.05**
28.57***

113.95***86.55***
47.46***
223.30***13.32***
22.59***
34.84*********

P < 0.05.
P < 0.01.
P < 0.001.

80

A.P. Møller et al. / Ecological Indicators 24 (2013) 75–81

Fig. 3. Abundance of (A, B) spiders, (C, D) butterﬂies and (E, F) birds in relation to background radiation (␮Sv/h) in Fukushima (A, C, E) and Chernobyl (B, D, F).

(Møller and Mousseau, 2007b). The increased abundance of spiders
at higher levels of radiation in Fukushima may arise from negative
effects of radiation on escape ability of insect prey, if prey taxa
are more strongly negatively affected by radiotoxicity. We suggestthat the initial lack of effect of radiation reported here for several
taxa at Fukushima may subsequently change to increasingly suppressed population densities at high levels of background radiation
as mutations accumulate. Møller and Mousseau (2011) showed thatabundance decreased more at high radiation levels in taxa with longdispersal distances and taxa with high mean population densities
in Chernobyl. In Fukushima we found no such evidence of effects ofdispersal or population density. Immigration from uncontaminatedpopulations may result in the maintenance of sink populations incontaminated areas (populations that cannot maintain themselves
without immigration from elsewhere; Møller et al., 2006), but once

the negative effects of radiation appear in populations across large
areas, any rescue effect of immigration from source populations islikely to eventually be diminished or disappear.
In conclusion, we have shown that the relationship between
abundance of animals and radiation differed signiﬁcantly between
Chernobyl and Fukushima. The correlation between radiation andthe abundance of animals were stronger at Chernobyl, wherecontamination with radioactivity has lasted for 25 years, than at
Fukushima, although levels of background radiation generally weremuch higher at our census points in Fukushima. These results are
consistent with the hypothesis that long-term effects of mutation
accumulation are more important determinants of abundance thanshort-term effects of radiotoxicity. The ﬁndings reported here have
implications for biological monitoring of the effects of radioactive
contamination because effects are expected to accumulate over

A.P. Møller et al. / Ecological Indicators 24 (2013) 75–81

time as a consequence of continuous chronic exposure to radiation.
Therefore, any census of long-term effects of radiation on organisms should start from the very beginning of radioactive accidents
to allow documentation of trends in populations of animals overtime.
Acknowledgments
We gratefully acknowledge logistic support and help in Ukraine
by I. Chizhevsky and G. Milinevski. We gratefully acknowledge support from the US National Science Foundation, the University ofSouth Carolina School of the Environment, the NATO CLG program,the CRDF, the Fulbright Program, the National Geographic Society,and the Samuel Freeman Charitable Trust for research in Chernobyl,
and the University of South Carolina and Qiagen GmbH for researchin Japan.
Appendix A. Supplementary data
Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.ecolind.
2012.06.001.
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