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Ecological Indicators 26 (2013) 112–116

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Ecological Indicators
journal homepage: www.elsevier.com/locate/ecolind

Assessing effects of radiation on abundance of mammals and predator–preyinteractions in Chernobyl using tracks in the snow
Anders Pape Møller a,∗ , Timothy A. Mousseau babLaboratoire d’Ecologie, Systématique et Evolution, CNRS UMR 8079, Université Paris-Sud, Bâtiment 362, F-91405 Orsay Cedex, France
Department of Biological Sciences, University of South Carolina, Columbia, SC 29208, USAa r t i c l e

i n f oArticle history:
Received 23 July 2012
Received in revised form 18 October 2012
Accepted 25 October 2012
Keywords:
Interspecific interactions
Numerical response
Snow tracks

a b s t r a c t
To test whether radioactive contamination reduced the abundance of mammals, and whether species
differed in susceptibility to radiation, we censused mammals by counting tracks in the snow along 161
100-m line transects around Chernobyl during February 2009. The abundance of mammal tracks was
negatively related to level of background radiation, independent of the statistical model, with effects ofradiation accounting for a third of the variance. The effect of radiation differed significantly among species.
There was a positive relationship between abundance of predators and abundance of prey, modified by
the level of background radiation because the number of predators increased disproportionately with
the number of prey at high levels of radiation. These findings suggest that predatory mammals aggregate
in areas with abundant prey, especially when prey are exposed to high levels of radiation. This study
emphasizes the negative effects of level of background radiation on the abundance of mammals andpredator–prey interactions.
© 2012 Elsevier Ltd. All rights reserved.

1. Introduction
Radioactive contamination occurs naturally to varying extent
or due to accidental release of radionuclides into the environment.
Although a large number of accidents has taken place around theworld (Sovacool, 2010), by far the largest release occurred at Chernobyl on 26 April 1986, when radioactive material hundreds oftimes the amount contained in the nuclear bombs in Hiroshima andNagasaki were released into the environment (Shestopalov, 1996).
The effects of this event on free-living organisms and humans still
remain poorly known mainly due to a general lack of research(Møller and Mousseau, 2006; von Wehrden et al., 2012).
Surprisingly, there has been no concerted effort to quantify
the relationship between abundance of animals and plants andbackground level of radiation, and, therefore, the effects of radiation from Chernobyl on fauna and flora remain to be determined.
Recently, extensive censuses have revealed negative relationships between the abundance of a range of animals and thelevel of background radiation around Chernobyl, Ukraine (Møller
and Mousseau, 2007a, 2009a,b), and similar patterns have been
described from Belarus (Møller and Mousseau, 2007a, 2009a,b).
The negative relationships between abundance and radiation were
present in mammals, birds, bumblebees, butterflies, dragonflies,
grasshoppers and spiders (Møller and Mousseau, 2007a, 2009a,b,∗ Corresponding author. Tel.: +33 1 69 15 56 88; fax: +33 1 69 15 56 96.
E-mail address: anders.moller@u-psud.fr (A.P. Møller).

1470-160X/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ecolind.2012.10.025

2011). We have also described negative relationships between
species richness of birds and level of background radiation (Møller
and Mousseau, 2007a). These patterns were present even whencontrolling statistically for differences in habitat, soil quality,
weather conditions that could affect abundance estimates, and timeof day. We are unaware of any other studies covering more than afew sampling sites suggesting that there are no effects of radiation
on abundance or species diversity.
Extensive censuses of animals and plants have the added advantage that they allow for statistical tests of interspecific differencesin susceptibility to radiation. Such tests provide the opportunity toinvestigate ecological factors that predict interspecific differencesin susceptibility and identification of underlying mechanisms.
Møller and Mousseau (2007b) provided the first such study, using
extensive breeding bird surveys to tests for factors associated witha negative impact of radiation on abundance.
The objectives of the present study were to test if the abundanceof snow tracks and by implication the abundance of mammals
in contaminated areas around Chernobyl is a reliable predictorof external level of background radiation (Møller and Mousseau,
2011). Although censuses of mammals based on snow tracks are astandard procedure in ecological studies (e.g. Beauvais and Buskirk,
1999; Hansson, 1994; Rhim and Lee, 2007), this (or any other)method has never been used in Chernobyl. We tested the following four predictions: (1) The abundance of mammals is negatively
related to the level of radiation, even after controlling statistically for potentially confounding variables, as expected if radiation
reduces fecundity, survival rate and the abundance of food. (2) The

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113

Fig. 1. Study sites used for censusing mammals in the area around Chernobyl and level of background radiation (darker colors imply higher levels of radiation). (Forinterpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Modified from European Union (1998).

effect of radiation on abundance of mammals differs among species.
Such an interspecific difference in effect of radiation on abundance
would be expected because radiation is a source of oxidative stressand species differ in their use of antioxidants. Thus, species ofbirds that use more antioxidants for their normal activities show agreater reduction in abundance in areas with high levels of ionizing
radiation (e.g. Møller and Mousseau, 2007b). (3) The abundance of
predators is positively related to the abundance of prey due to afunctional response, with a disproportionate number of predators
occurring at high levels of radiation because the negative effects ofradiation make prey in such areas particularly susceptible to predation. We tested these predictions by analyzing data collected from161 line transects conducted inside or just outside the Chernobyl
exclusion zone in Ukraine in 2009.
2. Materials and methods
2.1. Study sites
The study sites were situated inside or just outside the exclusionzone of Chernobyl (Fig. 1). Sites along roads or paths were chosen
to have a wide range of radiation, varying by more than four ordersof magnitude. The Chernobyl exclusion zone and the surrounding
areas show a patchwork of radiation with a high degree of spatial
heterogeneity. Therefore, it is fully feasible to choose neighboring
study areas that differ in level of background radiation by more than
two orders of magnitude (Fig. 1). We made an exhaustive surveyof 11 study areas that covered this spatial heterogeneity in background radiation to ensure that neighboring sites had both low andhigh levels of radiation to ensure that other confounding variables
would not affect our conclusions. Still, we recorded a number ofenvironmental variables other than level of background radiation(see Section 2.2) to make an effort to statistically control for any

such confounding effects. The 11 study areas are separated by amaximum distance of 43 km, and they cover an area of 800 km2 .
2.2. Transects
Using tacks in snow as an estimate of abundance of animals is an
old method that has been adopted in many studies (e.g. Beauvais
and Buskirk, 1999; Hansson, 1994; Rhim and Lee, 2007). Census
data can be adjusted for time that tracks have accumulated (i.e.
time since last snowfall), although variation in visibility and persistence of tracks among species and sites may also be important(Beauvais and Buskirk, 1999). Here we conducted transects during a short time period of 1.5 days thereby avoiding the possibility
that tracks accumulated over time or they deteriorated in quality and hence information content over time. Our extensive effort
to cover a large number of sites and habitats differing in level ofbackground radiation assured that we covered all habitats relative
to their abundance in the study sites, but also showed that such a
study of general scientific interest is fully feasible within a shortwindow of opportunity.
APM conducted 161 line transects each with a length of 100 m
along roads in a large number of areas in the Chernobyl exclusionzone and just outside the zone during 3–4 February 2009 immediately following heavy snowfall in the early morning (Fig. 1). Eachtransect was separated from others by a minimum distance of 50 m,with most being separated by distances of 100–500 m. We wereunable to make line transects before 3 February because of a deficitof snow, and snow melt on 4 February, combined with little snow
fall in some areas, prevented line transects after 12:00 due to lackof visibility of tracks. We had previously attempted to study mammal tracks in relation to the level of background radiation duringa week of field work in December 2007, but we were unable to
make line transects because the snow melted as we landed in Kiev.

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Transects for the present study were made during intensive fieldwork only lasting 1.5 days. Therefore, we consider that there waslittle opportunity for tracks to accumulate over time. Indeed, when
we entered time since last snowfall as a predictor in our statisticalmodels (see Section 3), this variable only had a marginal importance as a predictor. This justifies our assumption that tracks werelargely made during a single bout of activity with little or no opportunity for the same mammals to make repeated tracks over time.
Even if there were accumulation of tracks over time, the number oftracks would still reflect mammal activity and hence be a functionof actual abundance at a given site. Although the total number of
161 transects was arbitrary due to constraints imposed by changes
in weather, such a large number of transects allows for a very highstatistical power given a significance level of 5%. Thus, for a large
effect size explaining 25% of the variance, as we recorded in ourstudy, the statistical power was 1.00 with 95% confidence limits of0.9998–1.0000.
APM identified for each transect all mammal tracks in the snow
using the field guide by Bang and Dahlstrøm (2001). To avoid counting the same individual repeatedly we only counted the minimum
number of individuals recorded simultaneously in a given transect.
For example, if the tracks of a fox (Vulpes vulpes) crossed the transect to the right, and 30 m later the tracks of a fox crossed to the left,
this was only counted as a single individual, although two individuals may have been involved on some such occasions. In contrast,
if the tracks of two foxes crossed to the left, while none crossed to
the right, this was counted as two foxes.
We recorded for each transect within a distance of 100 m thepercentage cover with grass, bushes and trees to the nearest 5%.
It snowed heavily between 2 and 4 a.m. on 3 February, and werecorded time since this last snowfall because more tracks accumulate with time (Beauvais and Buskirk, 1999; Hansson, 1994;
Rhim and Lee, 2007), starting our first census at 9:00 a.m. on 3
February when light conditions allowed reliable identification of
tracks. Because the snow was fresh, there were no problems of
seeing or identifying tracks.
2.3. Radiation levels
Once having finished a line transect we measured !, " and #radiation 2–3 times at ground level using a hand-held dosimeter(Model: Inspector, SE International, Inc., Summertown, TN, USA).
Cross-validation against data from Shestopalov (1996) revealed astrong positive relationship (linear regression on log–log transformed data: F1,252 = 1546.49, r2 = 0.86, P < 0.0001, slope (SE) = 1.28
(0.10)), suggesting reliability of our estimates of background radiation.
The entire data set is reported in Appendix 1.
Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/
j.ecolind.2012.10.025.
2.4. Statistical analyses
Radiation and body mass were log10 -transformed, while
coverage with grassland, shrub, and forest was square rootarcsine-transformed before analyses in order to obtain normal distributions.
All analyses were made with the statistical software JMP (SAS,2012). For each species we used number of individuals at each of
the 161 transects as the dependent variable and the habitat variables, time since last snowfall and radiation level as independent
variables, using a Poisson error distribution. We tested for an effectof radiation by including radiation level, species and the radiationby species interaction as an estimate of differences in effect among
species.

We tested for a functional response of predators to prey abundance by relating the abundance of predators to the abundance ofprey, the squared abundance of prey, level of radiation, and theinteractions between level of radiation and abundance of prey andsquared abundance of prey, respectively. This model also initially
included time since last snowfall as an additional predictor variable.
We had a variable number of transects per site for logistic
reasons, and this could potentially cause bias in the estimates.
Therefore, we developed random effect models for the models
reported in Tables 1 and 4 by including site as a random factor. We
did not make a random model for the analysis in Table 3 because
random and nested effects cannot be included in the same model.
We estimated an effect size of the relationship between abundance and radiation level from the F-statistics of the partial
regression by transforming these into Pearson product-moment
correlation coefficients, using the equation r2 = (F/(F + denominatord.f.)) (Rosenthal, 1991).

3. Results
We recorded 445 mammals belonging to 12 different species
during the line transects, with the most abundant being fox with147 records followed by wolf with 70 records and hare (Lepus
europaeus) with 49 records (Appendix 1).
The total number of mammals recorded per transect ranged
from 0 to 16, mean (SE) = 2.76 (0.22), median = 2, N = 161 transects. The number of predators per transect was 0–11, mean
(SE) = 1.42 (0.12), median = 1, N = 161 transects. The number of prey
per transect was 0–11, mean (SE) = 1.35 (0.15), median = 1, N = 161transects. The level of radiation ranged from 0.01 to 225.00 $Sv/h,mean (SE) = 5.52 $Sv/h (0.61), median = 0.82 $Sv/h, N = 161 transects. Thus, the level of radiation varied by a factor 22,500 amongtransects.
The abundance of mammals decreased with level of radiation,
depended on species, and the effect of radiation varied among
species as shown by the significant radiation by species interactions(Table 1). The largest negative effect was recorded for the fox (Pearson r = −0.45), while the weakest effect was recorded for the wolf(r = +0.09; see Appendix 1 for a list of effect sizes). A random effectmodel with site as a random factor showed that 44% of the variance occurred among sites. There was still a strong negative effectof radiation (F1,94.52 = 25.46, P < 0.0001, slope (SE) = −0.207 (0.041)),
with a weak additional effect of coverage with shrub (F1,151.2 = 4.53,
P = 0.035).
The total number of mammals per transect decreased significantly with level of radiation (Fig. 2), accounting for 31% of thevariance in abundance. This model showed a good fit to the data
(F138,19 = 1.55, P = 0.14).
The extent to which tracks represented the same individual
should be reflected by time since last snowfall. A longer period
since last snowfall should allow mammals to travel longer distances, and hence they should make more or repeated tracks in thesnow. Indeed, there was a small, non-significant effect of time sincelast snowfall on recorded abundance of mammals despite the shorttime window used for the study (Table 2). The effect size of timesince last snowfall was r2 = (F/(F + denominator d.f.)) (Rosenthal,1991) = 3.03/3.03 + 57.00 = 0.019, or a small effect. Therefore, we
can conclude that this effect is not a major concern.
We made two additional analyses to validate our findings, First,
if we reduced the 161 transects to the mean values for the 11
study areas shown in Fig. 1, we still find a strong negative relationship (F1,9 = 5.81, r2 = 0.39, P = 0.039, estimate (SE) = −0.222 (0.092)).
Indeed, the estimate (SE) in this conservative analysis did not differ
significantly from that found when analyzing the 161 transects asstatistically independent data points (−0.209 (0.025); Table 2).

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Table 1Abundance of mammals in relation to radiation, species and radiation by species interaction in 161 line transects around Chernobyl. The model had the statistics F23,1908 = 22.69,
r2 = 0.21, P < 0.0001.
Variable

Sum of squares

d.f.

F

P

Slope (SE)Radiation (R)Species (S)
R×S
Error

0.833
6.640
1.540
32.953

1
11
11
1908

48.24
34.95
8.10

<0.0001<0.0001<0.0001−0.025 (0.004)

Table 2Abundance of mammals in relation to radiation, habitat (coverage with shrub) and time since snowfall in 161 line transects around Chernobyl. The model had the statistics
F3,157 = 23.70, r2 = 0.31, P < 0.0001.
Variable

Sum of squares

d.f.

F

P

Slope (SE)Radiation
Shrub
Time since snowfall
Error

4.229
0.455
0.184
9.536

1
1
1
157

69.62
7.50
3.03

<0.0001
0.00690.084−0.209 (0.025)−0.176 (0.064)−0.003 (0.002)

increased disproportionately with the number of prey at high levels of radiation, as shown by the significant positive interactionthat had an effect size of 0.18 (Table 4). The interaction between
squared abundance of prey and radiation was not retained in the
final model (F1,154 = 0.04, P = 0.85). Likewise, time since last snowfall was not retained in the model (F1,154 = 0.57, P = 0.45). A random
effect model with site as a random factor showed that 44% of thevariance occurred among sites. There was a strong negative effect ofradiation (F1,70.19 = 10.16, P = 0.0021, slope (SE) = −0.122 (0.038)), a
non-significant effect of number of prey (F1,144 = 0.22, P = 0.64) anda weak additional effect of number of prey squared (F1,155.2 = 4.76,
P = 0.031, slope (SE) = 0.513 (0.235)). The number of prey by radiation interaction did not reach statistical significance (F1,154.4 = 2.19,
P = 0.14).

Fig. 2. Abundance of mammals in relation to background radiation level ($Sv/h)according to line transects in areas around Chernobyl. The line is the linear regression
line.

Second, if we nested background radiation within sites ina nested model we obtained a highly significant model that
accounted for 60% of the variance (Table 3). The model showed agood fit to the data (F105,34 = 1.32, P = 0.18). There was a significant
effect of site implying that the abundance of mammals differedamong the 11 sites (Table 3). In addition, we found a significant
effect of radiation nested within sites (Table 3). This implies thatthe abundance of mammals changed in response to changes inbackground radiation levels (Table 3). This effect of radiation on
mammal abundance was statistically significant at the 0.01 levelfor four of the 11 study sites exceeding the null expectation of
0.01 × 11 = 0.11 significant effects by chance.
The abundance of predators was explained to 24% by abundanceof prey, radiation and their interaction (Table 4). While the linear
term for abundance of prey did not reach statistical significance,
the abundance of predators increased with the squared abundanceof prey, with an effect size of 0.23 (Table 4). In addition, there
were fewer predators in areas with higher levels of radiation, with
an effect size of 0.28 (Table 4). Finally, the number of predators
Table 3Abundance of mammals in relation to radiation nested within study sites around
Chernobyl. The model had the statistics F21,139 = 9.84, r2 = 0.60, P < 0.0001.
Variable

Sum of squares

d.f.

F

P

Radiation [site]
Site
Error

2.703
4.548
5.573

11
10
139

6.13
4.55

<0.0001<0.0001

4. Discussion
The main findings of this study of the abundance of mammal
tracks in relation to level of radiation around Chernobyl were that(1) there were many more mammals in less contaminated areas
independent of the statistical model, (2) species differed significantly in this response to radiation with some showing sharp
decreases in abundance, while others did not, and (3) there wasa functional response of predators to abundance of prey depending
on level of radiation. These effects were demonstrated in generalized linear models, but also in random effect models with site asa random factor thereby controlling statistically for the potential
non-independence of transects within sites.
The abundance of mammals around Chernobyl decreased withincreasing level of background radiation, as revealed by the minimum number of individuals producing snow tracks along 100 mline transects. We also analyzed this relationship by conservatively reducing the entire dataset to mean values for the 11 study
areas being separated by a maximum distance of 43 km althoughthat did not change the conclusions. Likewise, a nested statistical model that considered the effects of radiation within sites
also showed a negative effect of radiation on abundance. In thislatter model the site variable will include variation among sites
due to many different factors that were not considered explicitly
in the study. Such reductions in abundance of animals in morecontaminated areas have already been documented for birds, bumblebees, butterflies, dragonflies, grasshoppers and spiders (Møller
and Mousseau, 2007a, 2009a). There was a significant interspecific difference in the response of mammals to radiation, withsome species showing much stronger declines than others. The
fox showed the strongest negative relationship between radiation
and abundance, while the wolf had the weakest effect. We have

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Table 4Abundance of predators in relation to abundance of prey and time since snowfall in 161 line transects around Chernobyl. Time since snowfall and its interaction with
abundance of prey and radiation were not retained in the model. The model had the statistics F4,156 = 12.35, r2 = 0.24, P < 0.0001.
Variable

Sum of squares

d.f.

F

P

Slope (SE)Abundance of prey (P)Abundance of prey2
Radiation (R)
P×R
Error

0.058
0.398
0.582
0.216
7.660

1
1
1
1
157

1.33
9.07
13.27
4.93

0.25
0.00300.00040.028

0.084 (0.073)
0.719 (0.239)−0.081 (0.022)
0.187 (0.084)

previously demonstrated interspecific differences in relationshipbetween radiation and abundance of birds (Møller and Mousseau,
2007b; Galván et al., 2011).
Line transects may have differed in efficiency among species.
For example, mice and stoats (Mustela erminea) spend most of thetime under the snow, making population density estimates basedon snow tracks unreliable. Therefore, total abundance of mammals
is clearly under-estimating the real abundance. We attempted toassess the reliability of our estimates of mammal abundance basedon snow tracks by measuring the duration of time since last snowfall. We started conducting our line transects a few hours aftersnowfall ceased and we terminated transects during the following 1.5 days. Thus there was very limited time for accumulation ofmultiple tracks by the same individuals. We adopted two different methods for reducing this effect of accumulation. First, we only
made very conservative estimates of the abundance of mammals
by excluding any tracks that could possibly be attributed to morethan the minimum possible number of individuals. Second, we estimated the time since last snowfall and included this variable in theanalyses to account for accumulation of tracks over time. The effectof time since last snowfall was small, suggesting that this was not
an important source of bias. Therefore, there is no reason to believethat the relative abundance of different mammal species in areas
differing in level of radiation would be biased.
Predators are expected to track the populations of prey by showing numerical and functional responses. The Chernobyl situation
is interesting from the perspective of interspecific interactions
because the perturbation caused by radioactive contamination provides a semi-experimental situation where changes in the local
abundance of prey is expected to lead to changes in the local
abundance of predators. The shape of numerical and functional
responses of predators can affect prey community stability andcomposition, with generalist predators either stabilizing (Erlinge
et al., 1988; Henke and Bryant, 1999) or destabilizing the community (Bonsall and Hassell, 1997; Holt, 1977) depending on ecologicalconditions. We found evidence of the functional response of
predators depending on the level of radiation. Predators were disproportionately common at high population densities of prey, andthis effect of high densities of prey was particularly important athigh levels of radiation. We suggest that high levels of radiation
may render prey particularly susceptible to predation, through theeffects of radiation on antioxidant levels, damage to DNA and othermolecules or cyto-toxic effects of radiation. Finally, if the transect
data provided biased estimates of abundance, there is no reason to
expect positive associations between abundance of predators andabundance of prey. Thus, the evidence of a response by predators
to the abundance of prey is consistent with our line transects capturing important biological information about the abundance of
predators and prey.
In conclusion, we have shown that populations of mammals
around Chernobyl have reduced abundance in the most contaminated areas independent of the statistical models, although thiseffect varies markedly among species. The abundance of predators
was related to the level of radiation and prey abundance, with theeffect of abundance of prey being particularly strong at high levels of radiation. Therefore, snow tracks provide a simple method
for assessing the abundance of mammals in radioactively contaminated areas, but also for assessing the effects of backgroundradiation on an interspecific interaction, predator–prey relationships under field conditions.
Acknowledgments
Two anonymous reviewers provided helpful suggestions. We
acknowledge help from G. Milinevski, I. Chizhevsky and O. Bondarenko. We gratefully acknowledge support from US National
Science Foundation, the CNRS (France), the Chernobyl EcoCenter,
the Samuel Freeman Charitable Trust, CRDF, NATO CLG program,
Fulbright Program, National Geographic Society, and the Schoolof the Environment and the College of Arts and Sciences at the
University of South Carolina.
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