Biased dosimetry and statistics trapped among the spider webs?

We view the article by Moller and Mousseau (2009), “Reduced abundance of insects and spiders linked to radiation at Chernobyl 20 years after the accident”, as being important because it questions established paradigms on declining biodiversity within ecosystems chronically exposed to radiation. The article’s topic of effects from Chernobyl radiation is an area of research that continues to captivate scientists, as well as the general public. The latter was evident by the manuscript’s international press coverage. We also applaud the authors for their sustained field research program at Chernobyl; few others have been able to secure the necessary funding. 

General Causes for Controversy in Chernobyl Ecological Research
•	Lack of proper dosimetry 
•	Not accounting for the extreme spatial heterogeneity of exposure experienced by free-ranging wildlife 
•	Not accounting for confounding variables and indirect effects
•	Lack of analogous controls
•	Questionable analytical methods or statistical analyses and inadequate sample sizes
•	Large uncertainty in extrapolating effects from lower levels of biological organization to populations and communities

However, two aspects of the manuscript are troubling. Foremost is the lack of robust dosimetry data on the organisms in which the dose-response relationships were established. It is the accurate quantification of absorbed dose that is the greatest challenge in Chernobyl field research, and it is the measurement most lacking in many of the controversial papers pertaining to effects from the accident (see text box for general research problems often found in Chernobyl ecological studies).  
	Surveys of external exposure, using hand held instruments at ground level, as measured by Moller and Mousseau (2009), do not adequately represent the absorbed dose received by free-ranging organisms. Such methods are particularly inadequate at Chernobyl, where the horizontal and vertical distributions of exposure are renowned to vary by orders of magnitude over spatial scales that easily encompass the home range of many invertebrate species. A measurement of external exposure taken on the ground will not reflect the external dose received by flying invertebrates or those that spend much of their time dispersed above the ground within the canopy of vegetation. 
	For any terrestrial organism, the absorbed dose (i.e. dose equals the energy released from radioactive decay and absorbed per mass of an organism’s tissue) results from three primary pathways. The measurement technique used by Moller and Mousseau only accounts for one of the three, (i.e. external irradiation), and thus likely resulted in a biased interpretation of data. The external irradiation pathway occurs when radionuclides on the organism’s surface or within its external environment emit radiation with sufficient energy to penetrate the organism’s external surface and be deposited within its tissues. 
	Two other pathways, inhalation and ingestion, are often as important, if not more so; and yet were not captured by their measurement methods. For invertebrates, debatably, the inhalation pathway can likely be ignored for specific life stages. But it is certainly not the case for the third pathway, ingestion, which is highly important to all organisms that obtain some sustenance within contaminated environments. The ingestion of radionuclide contaminated food and water and its subsequent incorporation into bodily tissues is a key component to an organism’s whole-body dose. The ingestion of radioactive contaminants is species-specific and radionuclide-specific, and may vary seasonality according to habitat, mode-of-life and life stage. 	Dose from ingestion should not be ignored. It reflects the intricacies of diverse food webs, as well as radionuclide-specific differences in assimilation and decay-energies. The methods used by Moller and Mousseau fail to account for such intricacies. 
	The second troubling aspect of the manuscript is the statistical analyses. The ordinary ANCOVA model used by the authors to describe the linear relationship between species abundance and radiation may have over estimated effects. In addition, the regression lines shown in their Figure 2 do not reflect the conclusions from the reported ANCOVA models (table S5), which indicate that the linear relationships between abundance and radiation mostly differ between sites. Figure 2, based on a unique linear regression line, is only correct for spider webs (intercept and slope common to all sites), but is not correct for the other groups of invertebrates in their study. Indeed, research “site” has a significant influence on the linear relationship for bumblebees and dragonflies (i.e. the slope is identical for each site but the intercepts are not). The same problem appears for butterflies and grasshoppers where the relationships, both slope and intercept, are different from site to site. A mixed model that assumes random intercepts and slopes among study sites would be more appropriate as it would include all sources of variation and allow generalization beyond the sites actually considered in the article. The statistical approach used in the manuscript does not allow such generalization, and likely underestimated the size of the standard errors. The analyses used by the authors may have resulted in a biased interpretation of data regarding the effects of radiation on terrestrial invertebrates.  
We applaud the authors for their persistent scientific exploration of the Chernobyl accident. The long-term effects of the accident are under-researched, remain controversial, and are in desperate need of being understood. However, we plead with the authors to take a more rigorous approach to dosimetry and statistics; otherwise, most of the radiological scientific community dismisses such data as naïve and inconclusive from the perspective of dose-response phenomena (Smith 2008).  

J. Garnier-Laplace1, C. Della-Vedova2, C. Ritz3, J. Bonzom1, T. Hinton1* 

1 Institut de Radioprotection et de Sûreté Nucléaire, Service d’étude du comportement des radionucléides dans les écosystèmes, 13115 Saint-Paul-Lez-Durance, France 
2 Société Magelis Company, Cadenet, France 
3 Statistics Group; Department of Basic Sciences and Environment Faculty of Life Sciences; University of Copenhagen
* For correspondence (thomas.hinton@irsn.fr)

Moller, A. P. & Mousseau, T. A. 2009 Reduced abundance of insects and spiders linked to radiation at Chernobyl 20 years after the accident. Biol. Lett. 5, 356-359.
Smith, J. T. 2008 Is Chernobyl radiation really causing negative individual and population-level effects on barn swallows? Biol. Lett. 4, 63-64.