RADIATION-INDUCED EFFECTS ON PLANTS AND … · RADIATION-INDUCED EFFECTS ON PLANTS AND ANIMALS: FINDINGS OF THE UNITED NATIONS CHERNOBYL FORUM ... The response of biota to Chernobyl

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RADIATION-INDUCED EFFECTS ON PLANTS AND ANIMALS:FINDINGS OF THE UNITED NATIONS CHERNOBYL FORUM

Thomas G. Hinton,* Rudolph Alexakhin,† Mikhail Balonov,‡ Norman Gentner,§

Jolyn Hendry,‡ Boris Prister,** Per Strand,†† and Dennis Woodhead‡‡

Abstract—Several United Nations organizations sought to dis-pel the uncertainties and controversy that still exist concerningthe effects of the Chernobyl accident. A Chernobyl Forum ofinternational expertise was established to reach consensus onthe environmental consequences and health effects attribut-able to radiation exposure arising from the accident. Thisreview is a synopsis of the subgroup that examined theradiological effects to nonhuman biota within the 30-kmExclusion Zone. The response of biota to Chernobyl irradia-tion was a complex interaction among radiation dose, doserate, temporal and spatial variation, varying radiation sensi-tivities of the different taxons, and indirect effects from otherevents. The radiation-induced effects to plants and animalswithin the 30-km Exclusion Zone around Chernobyl can beframed in three broad time periods relative to the accident: anintense exposure period during the first 30 d following theaccident of 26 April 1986; a second phase that extendedthrough the first year of exposure during which time theshort-lived radionuclides decayed and longer-lived radionu-clides were transported to different components of the envi-ronment by physical, chemical and biological processes; andthe third and continuing long-term phase of chronic exposurewith dose rates <1% of the initial values. The doses accumu-lated, and the observed effects on plants, soil invertebrates,terrestrial vertebrates and fish are summarized for each timeperiod. Physiological and genetic effects on biota, as well as theindirect effects on wildlife of removing humans from theChernobyl area, are placed in context of what was knownabout radioecological effects prior to the accident.Health Phys. 93(5):427–440; 2007

Key words: National Council on Radiation Protection andMeasurements; Chernobyl; health effects; accidents, nuclear

INTRODUCTION

ALTHOUGH IT has been 20 y since the Chernobyl nuclearaccident of 26 April 1986, controversy and uncertaintystill exist relative to the extent of death, damage, andlong-term effects. To counter such confusion, the Inter-national Atomic Energy Agency (IAEA), in cooperationwith seven other United Nations’ organizations, estab-lished the Chernobyl Forum. The Forum’s mission wasto reach consensus on the environmental consequencesand health effects attributable to radiation exposurearising from the accident, as well as to provide advice onenvironmental remediation, special health care programs,and areas where further research is required. The otherUnited Nations’ organizations involved were the Foodand Agricultural Organization, the United Nations De-velopment Program, United Nations Environment Pro-gram, the United Nations Office for the Coordination ofHumanitarian Affairs, the United Nations ScientificCommittee on the Effects of Atomic Radiation, theWorld Health Organization, and the World Bank.

In 2003–2004, two groups of experts from 12countries, including those most affected by the accident(Belarus, Russia, and Ukraine), assembled as the Cher-nobyl Forum to assess the accident’s environmental andhuman health consequences. A subgroup of the environ-mental section examined the radiological effects to non-human biota. This review presents their synopsis on theradiological effects to wildlife within the 30-km Exclu-sion Zone. It starts with a brief overview of what wasknown about the radiological effects on biota prior to theChernobyl accident. Then the Chernobyl data are high-lighted from three perspectives. First, general findingsare presented as a function of three distinct time periodsin which very different radiological conditions domi-nated the post-accident environment. Second, generalfindings of effects are presented from research on majorclasses of biota for which the most data exist (i.e., plants,soil invertebrates, terrestrial animals, fish). Third, themost recent and somewhat controversial cytogenetic data

* Savannah River Ecology Laboratory, University of Georgia,Aiken, SC; † Russian Institute of Agricultural Radiology and Agroecol-ogy, Obnisk, Russia; ‡ International Atomic Energy Agency, Vienna,Austria; § United Nations Scientific Committee on Effects of AtomicRadiation, Vienna, Austria; ** Ukrainian Institute of Agricultural Radi-ology, Kiev, Ukraine; †† International Union of Radioecology, Oslo,Norway; ‡‡ Centre for Environment, Fishery and Aquaculture, UnitedKingdom.

For correspondence contact: Thomas G. Hinton, Savannah RiverEcology Laboratory, University of Georgia, Aiken, SC, or email [email protected].

(Manuscript accepted 29 June 2007)0017-9078/07/0Copyright © 2007 Health Physics Society

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are summarized. An emphasis is placed on the impor-tance of secondary effects, such as adaptation, and theconfounding indirect effects caused by human abandon-ment of the area. This comprehensive review is importantbecause it encompasses the research of scientists fromBelarus, Russia, and Ukraine, as well as many Westerncountries. The review forms a consensus of internationalexpertise on the environmental radiological effects fol-lowing the world’s worst nuclear accident.

PRE-CHERNOBYL KNOWLEDGE OFRADIATION EFFECTS ON BIOTA

The biological effects of radiation on plants andanimals have long been of interest to scientists; in fact,much of the information on human effects evolved fromstudies on plants and animals. Effects research paralleledthe development of nuclear energy and concerns aboutthe possible impacts of increased discharges of radionu-clides into the terrestrial and aquatic environments. Afterthe mid-1970’s, sufficient information had accrued thatseveral reviews summarized the effects of ionizing radi-ation on plants and animals (Whicker and Fraley 1974;IAEA 1976, 1988, 1992; UNSCEAR 1996; Whicker1997; Whicker and Hinton 1997).

Some broad generalizations about effects from radi-ation exposure can be gleaned from the research that hasbeen conducted over the last 100 y. Foremost are therelatively large differences in doses required to causelethality among various taxonomic groups (Fig. 1). Con-siderable variation in response occurs within a taxon dueto enhanced radiosensitivity of some species or lifestages. Differences in radiosensitivities were also appar-ent when the doses required to transition from minor tointermediate effects were documented for relatively short-term exposures. Data are summarized for various plantcommunities, soil invertebrates and rodents (Fig. 2).

Within the plant kingdom, trees are generally moresensitive than shrubs, which in turn are more sensitivethan herbaceous species. Primitive forms such as lichens,mosses, and liverworts are more resistant than vascularplants. Radiation-resistant plants frequently have molec-ular and cellular characteristics that enhance their abilityto tolerate radiation stress, and differences in plant-community response can be explained, in part, by theearly work of Sparrow (1961). He showed that specificnuclear characteristics are associated with high radiosen-sitivity in plants, but that sensitivity can be modified intime due to seasonal processes [e.g., dormancy, or theonset of growth in spring (Table 1)].

Scientific reviews (e.g., IAEA 1992) indicated thatmammals are the most sensitive organisms and thatreproduction is a more sensitive endpoint than mortality.For acute exposures of mammals, mortality generallyoccurs at doses �3 Gy while reproduction is affected atdoses �0.3 Gy. Chronic exposures alter the responses,with mortality occurring at �0.1 Gy d�1 and reproduc-tion affected at �0.01 Gy d�1. Among aquatic organ-isms, fish are the most sensitive, with gametogenesis and

Fig. 1. Acute dose ranges that result in 100% mortality in varioustaxonomic groups. Humans are among the most sensitive mam-mals and, therefore, among the most sensitive organisms (Whickerand Schultz 1982).

Fig. 2. Range of short-term radiation doses (delivered over 5 to60 d) that produced effects in various plant communities, rodentsand soil invertebrates. Minor effects include chromosomaldamage, and changes in productivity, reproduction and physiol-ogy. Intermediate effects include changes in species compositionand diversity through selective mortality. Severe effects (massivemortality) begin at the upper range of intermediate effects(Whicker and Fraley 1974; Whicker 1997).

Table 1. Nuclear characteristics and factors influencing the sen-sitivity of plants to radiation (Sparrow 1961; adapted fromWhicker and Schultz 1982).

Factors increasing sensitivity Factors decreasing sensitivity

Large nucleus (high DNA content) Small nucleus (low DNA content)Much heterochromatin Little heterochromatinLarge chromosomes Small chromosomesAcrocentric chromosomes Metacentric chromosomesNormal centromere Polycentric or diffuse centromereUni-nucleated cells Multi-nucleated cellsLow chromosome number High chromosome numberDiploid or haploid High polypolidSexual reproduction Asexual reproductionLong intermitotic time Short intermitotic timeLong dormant period Short or no dormant periodSlow meiosis Fast meiosis

428 Health Physics November 2007, Volume 93, Number 5

embryo development being the more sensitive stages.Effects on animal populations can be reduced by theirmobility (in terms of moving from areas of high exposureto areas of low exposure). Comparatively stationary soilinvertebrates do not have such abilities and can receivesubstantial doses relative to the rest of the animalkingdom, particularly because the soil is a sink for mostradioactive contamination.

The response of a plant or animal to radiationdepends on the dose received, as well as its radiosensi-tivity. The former is largely determined by its habitatpreference in relation to the evolving distribution ofradioactive contaminants as a function of time, as well asthe organism’s propensity to accumulate radionuclidesinto its organs and tissues. Because of their particular useof the habitat, plants and animals within a contaminatedarea may receive radiation doses that can be substantiallygreater than those of humans occupying the same area(e.g., humans gain some shielding from housing and mayobtain food and water from less contaminated sources;IAEA 1992).

Although all exposures to ionizing radiations havethe potential to damage biological tissue, protraction of agiven total absorbed dose in time can, depending on thedose rate, result in a reduction in response due to theintervention of cellular and tissue-repair processes. Thishas led to the conventional, but somewhat artificial,distinction between the so-called acute and chronicradiation-exposure regimes. In general, an acute radia-tion exposure is one that usually occurs at a high-doserate and in a short period of time relative to that withinwhich obvious effects occur. Chronic exposures aretaken to be continuous in time, often over a significantportion of an organism’s lifespan, or throughout someparticular life-stage (e.g., embryonic development), andusually at a sufficiently low-dose rate that the cumulativedose does not produce acute effects.

The earlier reviews noted above were consistent inconcluding that it is unlikely that there will be significantdetrimental effects:

● to terrestrial and aquatic plant populations, and aquaticanimal populations, if the maximally exposed individ-uals receive chronic dose rates �10 mGy d�1; or,

● to terrestrial animal populations if the maximallyexposed individuals receive dose rates �1 mGy d�1.

It should be emphasized, however, that these doserates were not intended for use as limits to provideprotection of the environment; they were simply the doserates below which the available evidence, admittedlylimited in the range of organisms and the biologicalresponses investigated, indicated little likelihood of anysignificant response. The above dose rates are with

reference to population-level effects, not to impacts onindividual organisms. Alternative approaches to the“maximally exposed individuals” have been suggested(Wilson and Hinton 2003).

The more recent reviews of the effects of irradiationon individual organisms, carried out in the framework oftwo European Community projects, FASSET (Frame-work for the Assessment of Environmental Impact) andEPIC (Environmental Protection from Ionising Contam-inants in the Arctic), have produced broadly consistentconclusions (Woodhead and Zinger 2003; Sazykina et al.2003; Real et al. 2004). Although minor effects may beseen at lower dose rates in sensitive cell systems orindividuals of sensitive species (e.g., hematological cellcounts in mammals, immune response in fish, growth inpines and chromosome aberrations in many organisms),the threshold dose rate for significant effects in moststudies is �0.1 mGy h�1 (2.4 mGy d�1). Detrimentalresponses then increase progressively with increasingdose rate and usually become clear at �1 mGy h�1 (24mGy d�1) given over a large fraction of the lifespan. Thesignificance of the minor morbidity and cytogeneticeffects to the individual, or to populations more gener-ally, seen at dose rates �2.4 mGy d�1, has yet to bedetermined (Real et al. 2004).

The recently compiled EPIC database covers a verywide range of radiation dose rates (from below 10�5 Gyd�1 up to more than 1 Gy d�1) to wild flora and faunaobserved in northern parts of Russia, including wildlifein the Chernobyl contaminated areas (Sazykina et al.2003). The general conclusion from the EPIC database isthat the threshold for deterministic radiation effects inwildlife lies somewhere in the range of 0.5–1 mGy d�1

for chronic low-linear energy transfer (LET) radiation.These broad conclusions concerning the impact of

radiation on plants and animals provide an appropriatecontext within which to consider the available informa-tion on the effects that have been observed from theincreased radiation exposures following the accident atChernobyl.

CHERNOBYL DATA

Temporal dynamics of radiation exposureIt is critical to frame any discussion of Chernobyl

environmental effects within the specific time period ofinterest. Effects observed now, nearly 20 y after theaccident, are drastically different from those that oc-curred during the first 20 d. Three distinct phases ofradiation exposure have been identified in the area localto the accident (UNSCEAR 1996). In the first 20 d,radiation exposures were essentially acute because of thelarge quantities of short-lived radionuclides present in

429Radiation-induced effects on plants and animals ● T. G. HINTON ET AL.

the passing cloud of contamination (99Mo, 132Te/I, 133Xe,131I, and 140Ba/La). Most of these short-lived, highlyradioactive nuclides deposited onto plant and groundsurfaces resulting in the accumulation of large doses thatmeasurably impacted biota. High exposures to the thy-roids of vertebrate animals also occurred during the firstdays/weeks following the accident from the inhalationand ingestion of radioactive iodine isotopes or theirradioactive precursors.

On the day of the accident, measured exposure ratesin the immediate vicinity of the reactor reached 20 Gyd�1. Extreme heterogeneity of dose rates was common,with dose rates declining rapidly in short distances. Forexample, dose rates decreased by three orders of magni-tude from the reactor to the town of Pripyat, just 3 kmaway (Fig. 3). These exposure rates were mainly due togamma irradiation from deposited radionuclides. How-ever, for surface tissues and small biological targets (e.g.,mature needles and growing buds of pine trees) there wasa considerable additional dose from the beta radiation ofthe deposited radionuclides. Taking into account thehigh-dose rates during the relatively short exposureperiod from the short-lived radionuclides, this first phaseof 20 to 30 d can be generally characterized as an acuteexposure regime that had pronounced effects on biota,including mortality to the more sensitive species.

The second phase of radiation exposure extendedthrough the summer and autumn of 1986, during whichtime the short-lived radionuclides decayed and longer-lived radionuclides were transported to different compo-nents of the environment by physical, chemical and

biological processes. Dominant transportation processesincluded rain-induced transfer of radionuclides fromplant surfaces onto soil, and bioaccumulation throughplant tissues. Although the dose rates at the soil surfacedeclined to much �10% of the initial values due toradioactive decay of the short-lived isotopes, damagingtotal doses were still accumulated.

Approximately 80% of the total radiation doseaccumulated by plants and animals was received within 3mo of the accident, and over 90% of this was due to betaradiation (UNSCEAR 1996). This finding agrees withearlier studies on the importance of beta radiation,relative to the gamma component, to the total dose fromradioactive fallout (Prister et al. 1982). Measurementsmade with thermoluminescent dosimeters on the soilsurface at sites within the 30-km Exclusion Zone indi-cated that the ratio of beta to gamma dose was about 26:1[i.e., 96% of the total dose was from beta radiation(Krivolutsky et al. 1999)].

In the third and continuing phase of radiation expo-sure (commencing about 1 y post-accident) dose rateshave been chronic, �1% of the initial values, and derivedmainly from 137Cs contamination. With time, the decay ofthe short-lived radionuclides and the migration of muchof the remaining 137Cs into the soil have meant that thecontributions to the total radiation exposure from the betaand gamma radiations have tended to become morecomparable. The balance does depend, however, on thedegree of bioaccumulation of 137Cs in organisms and thebehavior of the organism in relation to the main source ofexternal exposure from the soil. Aside from the spatial

Fig. 3. Measured exposure rates (R h�1) on 26 April 1986 in the area close to the Chernobyl reactor (UNSCEAR 2000;1 R h�1 is �0.2 Gy d�1).


heterogeneity in dose rate arising from the initial depo-sition, large variations in the radiation exposure ofdifferent organisms occurred at different times due totheir habitat niche (e.g., birds in the canopy vs. rodentson the ground). Immigration of animals into the 30-kmExclusion Zone and recruitment of plants and animalsfrom those that are present means that new animals areconstantly being introduced into the radioactively con-taminated conditions that exist around Chernobyl today.Because exposures are now chronic with greatly reduceddose rates, the effects to biota are reduced by efficientrepair processes and adaptive responses. Indirect effects,such as human abandonment of the area, now appear tohave a more striking impact on biota within the 30-kmExclusion Zone than do radiological effects.

Radiation-induced effects on major classes of biota

Effects on plants. Doses received by plants fromthe Chernobyl fallout were influenced by the physicalproperties of the various radionuclides (i.e., their half-lives, radiation emissions, etc.), the physiological stageof the plant species at the time of the accident, and thedifferent species-dependent propensities to take up radio-nuclides into critical plant tissues. The occurrence of theaccident in late April heightened the damaging effects ofthe fallout because it coincided with the period ofaccelerated growth and reproduction in plants. The dep-osition of beta-emitting contamination onto critical planttissues resulted in their receiving a significantly largerdose than animals living in the same environment (Pristeret al. 1982, 1991). Of the absorbed dose to critical partsof trees, 90% was due to beta irradiation from thedeposited radionuclides and 10% to gamma irradiation(Arkhipov et al. 1994). Beta irradiation also dominatedthe dose to the herbaceous species Arabidopsis, contrib-uting 82–96% (Abramov et al. 2005). Large, apparentinconsistencies in dose-response observations occurredwhen the beta-irradiation component was not appropri-ately accounted for (Grodzinsky et al. 1991).

Within the 30-km Exclusion Zone of Chernobyl,deposition of total beta activity and associated doses toplants were sufficient (0.7–3.9 GBq m�2) to causemortality, short-term sterility, and reduction in produc-tivity of some species (Prister et al. 1991). By August1986, crops that had been sown prior to the accidentbegan to emerge. Growth and development problemswere observed in plants growing in fields with contam-ination densities of 0.1–2.6 GBq m�2, and with estimateddose rates initially received by plants reaching 300 mGyd�1. Spot necroses on leaves, withered leaf tips, inhibi-tion of photosynthesis, transpiration and metabolite syn-thesis, as well as an increased incidence of chromosome

aberrations in meristem cells were detected (Shevchenkoet al. 1996). The frequency of various anomalies inwinter wheat exceeded 40% in 1986–1987, with someabnormalities apparent for several years afterwards(Grodzinsky and Gudkov 2001).

Coniferous trees were already known to be amongthe more radiosensitive plants, and pine forests 1.5–2 kmwest of the Chernobyl Nuclear Power Plant (CNPP)received sufficient dose to cause mortality (Tikhomirovand Shcheglov 1994) at dose rates that exceeded 20 Gyd�1 (UNSCEAR 2000). The first signs of radiation injuryin pine trees in close proximity to the reactor wereyellowing and needle death, which appeared within 2–3wk. During the summer of 1986, the area of radiationdamage expanded in the northwest direction up to 5 km;serious damage was observed at a distance of 7 km. Inthe Exclusion Zone that received 10–20 Gy, 90% of thetrees died by 1997, with the remaining eventually re-stored. At doses that exceeded 20 Gy the pine plantationperished. This area became known as the “red forest”because of the orange-colored needles. Since 1988, thered forest has undergone a community shift and has beenreplaced by grasses, shrubs, and young deciduous speciesof trees more tolerant of radiation than pines.

Mortality rate, reproduction anomalies, stand viabil-ity, and re-establishment of pine-tree canopies weredependent on absorbed dose (Kozubov and Taskaev1990; Kal�chenko and Fedotov 2001; Arkhipov et al.1994; Tikhomirov and Shcheglov 1994). In the earlyperiod 26 April–1 June 1986, external irradiation ac-counted for 80% of the dose to trees (Kal�chenko andFedotov 2001). Two months after the accident mostradionuclides moved to the leaf-litter and then remainedin the upper 3–5 cm soil layer for a long period [7 y(Kal�chenko and Fedotov 2001)]. Acute irradiation ofPinus silvestris at doses of 0.5 Gy caused detectablecytogenetic damage; doses of �0.1 Gy did not cause anyvisible damage to the trees. From 0.5–1.0 Gy the treesexhibited stimulatory growth of secondary shoots; from1–5 Gy, slight damage was observed, characterized by adecrease in annual growth, and morphological alterationof vegetative organ (variable needle length, intensebudding at the tips of annual shoots). Such visible effectswere manifested for the first 2 y after the accident andthen disappeared. Moderate damage was observed atdoses of 5–15 Gy and included disturbed growth ofmeristem tissue, decreased trunk diameter, shortenedshoots with intense needle growth (broomlike shots),distorted spatial orientation of needles and shoots, as wellas gigantic and dwarfed needles. Seeds from these treeshad low germinating capacity. Normal growth was re-stored within 3 y, and no subsequent change in morpho-genesis was observed.


As early as 1987, recovery processes were evident inthe surviving tree canopies and young forests werere-established in the same place as the perished trees byreplanting via reclamation efforts (Arkhipov et al. 1994).In 2000, Kal�chenko and Fedotov (2001) examined thehealth of the forest along the western track at theperiphery of the red forest (1.5–2 km from the Chernobylreactor). They found no mortality, and young pines to bereappearing (dose rates were 0.4–1 mGy d�1). Theyconsidered the genetic load in the damaged pine popu-lations exposed to 15 Gy still high and hindering theirnatural restoration.

In a herbaceous species, Arabidopsis, the effect perunit dose was lower for high-dose irradiation (i.e., themore contaminated the site, the lower the embryoniclethal frequency per unit dose). The data suggest thatlow-dose chronic irradiation exerts a greater effect perunit dose as compared with high-dose irradiation; that is,the effectiveness of irradiation decreases with increasingdose. This pattern persisted for each of the 6 y studied,starting in 1987 (Abramov et al. 2005).

Effects on soil invertebrates. Although between 60and 90% of the initial fallout was captured by thevegetation (Tikhomirov and Shcheglov 1994), withinweeks to a few months the processes of wash-off by rainand leaf-fall moved the majority of the contamination tothe litter and soil layers, where soil and litter inverte-brates were exposed to high radiation levels for pro-tracted time periods. The potential for impact on soilinvertebrates was particularly large because the timing ofthe accident coincided with their most radiosensitive lifestages: reproduction and molting following their winterdormancy.

Within 2 mo after the accident, the numbers ofinvertebrates in the litter layer of forests 3 to 7 km fromthe nuclear reactor were reduced by a factor of 30(Krivolutsky et al. 1999), and reproduction was stronglyimpacted (larvae and nymphs were absent). Doses of�30 Gy (estimated from thermoluminescent dosimetersplaced in the soil) had catastrophic effects on the inver-tebrate community, causing mortality of eggs and earlylife stages, as well as reproductive failure in adults.Within a year, reproduction of invertebrates in the forestlitter resumed, due in part to the migration of inverte-brates from less contaminated sites. After 2.5 y, the ratioof young to adult invertebrates in the litter layer, as wellas the total mass of invertebrates per unit area, were nodifferent from control sites. However, species diversityremained markedly lower (Krivolutsky et al. 1999).

The diversity of invertebrate species within the soilfacilitates an analysis of community-level effects (i.e.,changes in species composition and abundance). For

example, only five species of invertebrates were found in10 soil cores taken from pine stands in July 1986, 3 kmfrom the CNPP, compared to 23 species at a control site70 km away. The mean density of litter fauna wasreduced from 104 individuals per 225 cm2 core at thecontrol location to 2.2 at the 3-km site. Six species werefound in all 10 cores taken from the control site, whereasno one species was found in all 10 cores from the 3-kmlocation (Krivolutsky and Pokarzhevsky 1992). Thenumber of invertebrate species found in the heavilycontaminated sites was only half that of controls in 1993,and complete species diversity did not recover until1995, almost 10 y after the accident (Krivolutsky et al.1999).

The change in species diversity observed within thesoil invertebrate communities, presented above, is per-haps the most obvious published example of community-level change and subsequent recovery following theChernobyl accident. At 30 Gy, this effect would not havebeen predicted from pre-Chernobyl data (Fig. 2). Thedeath of pine stands close to the Chernobyl reactor andthe subsequent establishment of grasslands and decidu-ous trees are striking visual examples.

Compared with invertebrates within the forest litterlayer, those residing in arable soil were not as impacted.A four-fold reduction in earthworm number was found inarable soils, but no catastrophic mortality in any group ofsoil invertebrates was observed. There was no reductionin soil invertebrates below a 5 cm depth in the soil.Radionuclides had not yet migrated into deeper soillayers, and the overlying soil shielded the invertebratesfrom beta irradiation, the main contributor (94%) to totaldose. The dose to invertebrates in forest litter was three-to ten-fold higher than to those residing in arable soil(Krivolutsky et al. 1999).

Developmental instability of morphological charac-ters has been documented in several taxons at Chernobylby measuring the degree of fluctuating asymmetry.Asymmetry has been observed in morphological charac-teristics of four species of plants, four species of insects,two species of fish, one species of amphibian, onespecies of bird and three species of mammals from theChernobyl area (Moller 1992, 1998; Medvedev 1996;Zakharov and Krysanov 1996). For example, the asym-metry of male stag beetles from Chernobyl was signifi-cantly greater than that of beetles from control areas, orfrom museum collections of beetles taken from theChernobyl area prior to the accident. The asymmetry ofmated males was significantly smaller than that ofunmated male beetles, suggesting potential impacts onmating success (Moller 2002).


Effects on terrestrial animals. Four months afterthe accident, surveys and autopsies of wildlife and ofabandoned domestic animals that remained within the10-km Exclusion Zone of Chernobyl were conducted(Krivolutksy et al. 1999). Fifty species of birds wereidentified, including some rare ones; all appeared normalin appearance and behavior. No dead birds were found.Swallows and house sparrows were found to be produc-ing progeny that also appeared normal. Forty-five speciesof mammals from six orders were observed and nounusual appearances or behaviors were noted.

During the fall of 1986, the number of small rodentson highly contaminated research plots decreased by afactor of 2–10. Estimates of absorbed dose during thefirst 5 mo after the accident ranged from 12–110 Gy forgamma and 580–4,500 Gy for beta irradiation. Numbersof animals were recovering by the spring of 1987, mainlydue to immigration from less affected areas. In 1986 and1987, the percentage of pre-implantation deaths in ro-dents from the highly contaminated areas increased two-to three-fold compared with controls. Resorbtion ofembryos also increased markedly in rodents from theimpacted areas. However, the number of progeny perfemale did not differ from controls (Taskaev and Testov1999).

Barn swallows at Chernobyl were found to haveelevated germline mutation rates of 13–15% expressedphenotypically as partial albinism and in noncodingmicrosatellite loci (Ellegren et al. 1997). Moller andMousseau (2001) used partial albinism as a phenotypicmarker of mutation in barn swallows from Chernobyl.Partial albinism was elevated by a factor of 10, and wasrelated to poor mating success. Camplani et al. (1999)hypothesized that male barn swallows from Chernobylmay use large amounts of carotenoids for free-radicalscavenging in response to irradiation, leaving little leftfor the production of plumage coloration. A loss offitness in Chernobyl barn swallows has also been attrib-uted to the radiation exposures. The fraction of nonre-producing adults was on average 23% in Chernobyl,compared to close to zero at the control site; clutch size,brood size and hatching success were all reduced in birdsfrom Chernobyl (by 7, 14, and 5%, respectively), andadult survival was 12% lower (Moller et al. 2005a).Moller et al. (2005a) calculated that the recorded survivaland reproductive rates of barn swallows are insufficientto maintain a stable population. Most recently, Moller etal. (2005b) observed a significant increase in abnormalsperm from barn swallows at Chernobyl and a reducedlevel of antioxidants in blood and liver.

Effects on aquatic organisms. Cooling water forthe CNPP was obtained from a 21 km2 man-made

reservoir located southeast of the plant site. The coolingreservoir became heavily contaminated following theaccident with over 7 � 3 � 1015 Bq of a mixture ofradionuclides in the water and sediments (Kryshev1995). Aquatic organisms were exposed to externalirradiation from radionuclides in water, contaminatedbottom sediments, and irradiation from contaminatedaquatic plants. Internal irradiation occurred as organismstook up radioactively contaminated food and water orinadvertently consumed contaminated sediments. Theresultant doses to aquatic biota over the first 60 dfollowing the accident are depicted in Fig. 4.

The maximum dose rates for aquatic organisms(excluding fish) were reported in the first 2 wk after theaccident, when short-lived radionuclides (primarily 131I)contributed 60 to 80% of the dose. During the secondweek, the contribution of short-lived radionuclides todoses of aquatic organisms decreased by a factor of two.Maximum dose rates to fish were delayed due to the timerequired for their food webs to become contaminatedwith longer-lived radionuclides (largely 134/137Cs, 144Ce/Pr, 106Ru/Rh, and 90Sr/Y). Differences in dose ratesamong fish species occurred due to their trophic posi-tions. Nonpredatory fish (carp, goldfish, bleak) reachedestimated peak dose rates of 3 mGy d�1 from internalcontamination in 1986, followed by significant reduc-tions in 1987. Dose rates in predatory fish (perch),however, increased in 1987 and did not start to declineuntil 1988 (Kryshev et al. 1992). Accumulated doseswere greatest for the first generation of fish born in 1986and 1987. Bottom-dwelling fish (goldfish, silver bream,

Fig. 4. The dynamics of absorbed dose rate (cGy d�1) to organismswithin the Chernobyl reactor cooling pond during the first 60 dfollowing the accident. Data are model results based on concen-trations of radionuclides in the water and in lake sediments(adapted from Kryshev et al. 1992).


bream, carp) that received significant irradiation from thebottom sediments attained accumulated total doses of�10 Gy.

In 1990, the reproductive capacity of young silvercarp was analyzed (Ryabov 1992). The fish were in liveboxes within the cooling pond at the time of the accident.By 1988, the fish reached sexual maturity. Over theentire post-accident period they received a dose of 7–8Gy. Biochemical analyses of muscles, liver and gonadsindicated no difference from controls. The amount offertilized spawn was 94%; 11% of the developing spawnwere abnormal. Female fertility was 40% higher than thecontrols, but 8% of the irradiated sires were sterile. Thelevel of fluctuating asymmetry in offspring did not differfrom the controls, although the level of cytogeneticdamage (22.7%) significantly exceeded controls (5–7%).In contrast, Pechkurenkov (1991) reported that in 1986–1987 the number of cells with chromosome aberrationsin carp, bream flat, and silver carp was within the norm.It is worth noting that the cooling pond was subject notonly to radioactive contamination but also to chemicalpollution. Recent reviews of chronic effects of ionizingradiation on reproduction in fish, with the Chernobyl dataincluded (Table 2), have been summarized.

Genetic effects

Early studies. Cytogenetic studies at Chernobyl cancrudely be separated into those conducted during the firstfew years post-accident and those collected during thelast decade. The former were characterized by higherdose rates and perhaps fewer confounding variables,whereas the latter can be characterized by chronic expo-sure to a declining dose rate. Most results from the earlystudies clearly showed increased mutation burdens inplants and animals. Results from the last decade havebeen more controversial.

For example, an increased mutation level was ap-parent in 1987 in the form of various morphologicalabnormalities observed in plants of Canada flea-bane,common yarrow, and mouse millet. Abnormalities in-cluded unusual branching of stems; doubling the numberof racemes; abnormal color and size of leaves andflowers; and development of “witch’s brooms” in pinetrees. Similar effects in the 5-km Exclusion Zone near thereactor also appeared in deciduous trees (leaf gigantism,changes in leaf shapes). Morphological changes wereobserved at an initial gamma exposure rate of 0.17–0.26Gy h�1. At 0.65–1.30 Gy h�1 enhancement of vegetativereproduction (heather) and gigantism of some plantspecies were observed (Tikhomirov et al. 1993; Arkhipov etal. 1994; Kozubov and Taskaev 1994; Tikhomirov andShcheglov 1994).

Cytogenetic analysis of cells from the root meristemof winter rye and wheat germ of the 1986 harvestdemonstrated a dose dependency in the number ofaberrant cells. A significant excess over the control levelof aberrations was observed at an absorbed dose of 3.1Gy, inhibition of mitotic activity occurred at 1.3 Gy, andgermination was reduced at 12 Gy (Geraskin et al. 2003).The analysis of three successive generations of winter ryeand wheat on the most contaminated plots revealed thatthe rate of aberrant cells in the intercalary meristem in thesecond and third generations were higher than in the first.

From 1986–1992, mutation dynamics were studiedin populations of Arabidopsis within the 30-km Exclu-sion Zone (Abramov et al. 1992). On all study plots in thefirst 2–3 y after the accident, Arabidopsis populationsexhibited an increased mutation burden. In later years,the level of lethal mutations declined; nevertheless themutation rate in 1992 was still four to eight times higherthan the spontaneous level. The dose dependence of the

Table 2. Chronic effects of ionizing radiation on reproduction infish, data from the FASSET database (Copplestone et al. 2003).

Dose rate(�Gy h�1)

Dose rate(mGy d�1) Reproductive effects

0−99 0−2.4 Background dose group, normalcell types, normal damageand normal mortalityobserved

100−199 2.4−4.8 No data available200−499 4.8−12 Reduced spermatogonia and

sperm in tissues500−999 12−24 Delayed spawning, reduction in

testes mass1,000−1,999 24−48 Mean lifetime fecundity

decreased, early onset ofinfertility

2,000−4,999 48−120 − Reduced number of viableoffspring

− Increased number of embryoswith abnormalities

− Increased number of smoltswith undifferentiated sex

− Increased brood size reported− Increased mortality of

embryos5,000−9,999 120−240 − Reduction in number of fish

surviving to 1 mo of age− Increased vertebral

abnormalities�10,000 �240 − Interbrood time tends to

decrease with increasing doserate

− Significant reduction inneonatal survival

− Sterility in adult fish− Destruction of germ cells

within 50 d in medaka fish− High mortality of fry, germ

cells not evident− Decrease in number of male

salmon returning to spawn


mutation rate was best approximated by a power functionwith a power index of less than one.

Zainullin et al. (1992) observed elevated levels ofsex-linked recessive lethal mutations in natural Drosoph-ila melanogaster populations living under conditions ofincreased background radiation due to the Chernobylaccident. The mutation levels were increased in 1986–1987 in flies inhabiting the more contaminated areas withinitial exposure rates �200 mR h�1. In the subsequent 2years, mutation frequencies gradually returned to normal.

Studies of adverse genetic effects in wild mice werereported by Shevchenko et al. (1992) and Pomerantsevaet al. (1997). These involved mice caught during 1986–1991 within a 30-km radius of the Chernobyl reactorwith different levels of gamma radiation and in 1992–1993 on a site in Bryansk Oblast, Russia. The estimatedtotal doses of gamma and beta radiation varied widelyand reached 3–4 Gy mo�1 in 1986–1987. One endpointwas dominant lethality, measured by embryo mortality inthe offspring of wild male mice mated to unexposedfemale laboratory mice. The dominant lethality rate waselevated for a period of a few weeks following capture inmice sampled at the most contaminated site. At doserates of �2 mGy h�1, 2 of 122 captured males producedno offspring and were assumed to be sterile. The remain-der showed a period of temporary infertility and reducedtestes mass, which, however, recovered with time aftercapture.

The frequencies of reciprocal translocations inmouse spermatocytes were consistent with previous stud-ies. For all collected mice, a dose-rate-dependent inci-dence of increased reciprocal translocations (scored inspermatocytes at meiotic metaphase I) was observed. Thefrequency of mice harboring recessive lethal mutationsdecreased with time post-accident (Pomerantseva et al.1997). Shevchenko et al. (1992) found that the frequencyof reciprocal translocations in mice was relatively lowand increased linearly with increasing dose rate; anincrease in embryonic mortality and frequency of abnor-mal sperm heads was also observed.

More recent studies. Several studies have beenconducted of the genetic impacts on rodents at Chernobylduring the last decade and results range from virtually noeffect (Baker et al. 1999, 2001; Matson et al. 2000;Wickliffe et al. 2003a, 2003b) to significantly elevatedmutation rates and the formation of micronucleated cells(Goncharova and Ryabokon 1995). Some results aredifficult to interpret. For example, bank voles (Clethri-onomys glareolus) have been among the most heavilycontaminated rodents at Chernobyl (Baker et al. 2001),with average internal radiocesium levels of 28 kBq g�1

and dose rate of 20 mGy d�1 (Baker et al. 2001).

Examination of mitochondrial DNA (mtDNA) sequencesindicated genetic diversity was elevated in voles fromChernobyl compared to controls. Although this might beattributed to an increased mutation rate in the mtDNA, itis just as likely due to immigration of individuals fromsurrounding areas into the contaminated environment(Baker et al. 2001). The researchers were unable toattribute any significant detrimental effects to bank volepopulations inhabiting the Chernobyl area, even thoughcontaminant levels are the highest published for anymammal. Age and sex distributions, diversity, abun-dance, and gross physiological conditions of small mam-mal populations in the 30-km Exclusion Zone currentlyappear to be similar to background locations in otherparts of Ukraine (Baker et al. 1996; Baker and Chesser2000; Jackson et al. 2004).

Analysis of mutations in the natural rodent popula-tion exposed to different anthropogenic impacts oftenyields ambiguous results (Bol�shakov et al. 2003). Forexample, mole voles chronically irradiated in Byelorus-sia at dose rates of up to 0.7 mGy d�1 had an increasedfrequency of chromosome aberrations (Goncharova andRyabokon 1995), whereas neither cytogenetic nor mo-lecular methods revealed any increase in mutagenesis inthe same species living in the 10-km Zone of Chernobyland receiving 97 mGy d�1 (Wickliffe et al. 2002).Variation in the levels of genomic instability in rodentslargely depends on specific features of their population-demographic structure and population cycles, with viralinfections and fluctuations of population size havinglarge impacts (Bol�shakov et al. 2003).

Considerable controversy exists relative to mutationfrequencies and positive vs. negative results observed inChernobyl rodents during recent years (Dubrova 2003;Wickliffe et al. 2003b). Microtus living within the 30-kmExclusion Zone were estimated to receive 19–53 mGyd�1 for over 6 mo, a total dose from chronic exposure of3.5 to 10 Gy (Chesser et al. 2000). These doses approachthe LD50/30 for voles (10 Gy; Chesser et al. 2000). Nodifference in frequency of chromosomal breaks betweenexposed and control animals were observed (Baker et al.1996; Wiggens et al. 2002). The authors attributed thelack of difference to several possibilities: that the rodentswere exposed to chronic irradiation rather than acute; thatthe rodents had developed a radioresistance; or a lack ofsensitivity in their methods (Wiggins et al. 2002). Thelow-dose rates currently at Chernobyl did not inducepoint mutations in mice and the cumulative, chronicallyabsorbed doses did not induce the same genetic effects asacute doses of the same magnitude (Wickliffe et al.2003a). DeWoody (1999) did not see a difference ingenetic diversity among mice living at Chernobyl com-pared to controls. Nor did he see evidence of mutations in


the p53 gene within rodents from Chernobyl. Wiggens etal. (2002) suggest that the long-term chromosomal ef-fects of the Chernobyl disaster are not as great as hasgenerally been predicted from previous laboratory stud-ies. The magnitude of chromosomal rearrangements involes exposed to the Chernobyl environment is less thanexpected based on traditional paradigms.

Kovalchuk et al. (2000), however, did demonstratean increased frequency of homologous recombination inplants exposed to chronic irradiation at Chernobyl. Like-wise, the temporal dynamics of cytogenetic damage isevident in the damaged pine stands. By 1993, in thezones that received up to 5 Gy the level of cytogeneticeffect in pines was close to control values (Kal�chenkoand Fedotov 2001). In 1993, cytogentic damage waseight times greater in trees that received 5–15 Gy than incontrols. In 1997 and 1998, the number of cells bearingchromosome aberrations decreased in all zones, but wasstill 2–3 times higher than controls in the 5–15 Gy zone.In the sublethal damage zone, pine trees did not bearseeds for 5–7 y.

Advances in the sophistication and associated tech-nologies of detecting molecular and chromosomal dam-age have occurred since the early genetic studies atChernobyl. Such advances have allowed researchers toexamine endpoints not previously considered. Mostprominent, and controversial, is the mutation frequenciesin repeat DNA sequences termed “minisatellite loci,”“microsatellite loci” or “expanded simple tandem re-peats” (ESTR). These are noncoding, repeat DNA se-quences that are distributed throughout the germline andthat have a high background (spontaneous) mutation rate.Presently, the function of ESTRs is not known, althoughthis is a matter of much interest and discussion (ICRP2003; CERRIE 2004). ESTR mutations have only rarelybeen associated with recognizable genetic disease(Bridges 2001).

Laboratory examination of mutations in mouseESTR loci shows clear evidence of a mutational doseresponse (Fan et al. 1995; Dubrova et al. 1998); whereas,data on elevated levels of ESTR mutations in plants oranimals residing in the Chernobyl-affected have beenmore problematic. In general, quantitative interpretationof the ESTR data is difficult because of conflictingfindings, their weak association with genetic disease,dosimetric uncertainties and methodological problems(CERRIE 2004).

Dubrova et al. (1996) concluded that ESTR muta-tions are initiated by DNA double-strand breaks; how-ever, they calculated that the high frequency of mutationscannot be explained solely by this mechanism. Theobserved increase in mutation rate would require some6,000 extra double-strand breaks per gamete. This is such

an elevated and unlikely number of breaks that Dubrovaet al. (1996) suggested that the increased mutation rate isnot caused by ESTR-specific events (i.e., double-strandbreaks), but by radiation damage elsewhere in the ge-nome, possible in protein-coding genes that are involvedin DNA replication. Data supporting such epigenetic,nontargeted effects are provided by Kovalchuk et al.(2000). They examined mutation frequencies in wheatgrown within contaminated fields of Chernobyl. A six-fold increase in mutation rate over control samples,within a single generation of exposure, was observed inplants that obtained approximately 0.3 Gy of chronicirradiation. The increase in ESTR mutation rates wasthought to be due to nontargeted effects of ionizingradiation and attributed to chronic exposures from inter-nal and external irradiation. The low-level exposureshould not have caused such an increase in mutations andit is too high to be due to direct targeting of the ESTRloci. The significance of epigenetic effects on genomicinstability and ultimately on populations is an area ofscience that needs much additional research.

AdaptationPrior to the accident, much of the area around

Chernobyl was covered in 30–40 y old pine stands that,from a successional standpoint, represented mature, sta-ble ecosystems. The high-dose rates from ionizing radi-ation during the first few weeks following the accidentaltered the balanced community by killing sensitiveindividuals, altering reproduction rates, destroying someresources (e.g., pine stands), making other resourcesmore available (e.g., soil water), and opening niches forimmigration of new individuals. All these components,and many more, were interwoven in a complex web ofaction and reaction that altered populations and commu-nities of organisms.

Irradiation is an environmental stress, in many wayssimilar to other environmental stresses, such as pollutionby metals or the destruction caused from forest fires. Ifsuch stressors are sufficient, the community organizationis changed and generally reverts to an earlier succes-sional state. However, when the stress is subsequentlyreduced and sufficient time passes, recovery occurs, andthe ecosystem again regains stability advancing toward amore mature state.

One of the more interesting questions relative to theChernobyl accident is: “How do organisms (populations)adapt to chronic radiation exposures?” An increase invariation is one of the main adaptive responses to stresscaused from exposure to contaminants, such as radiation(Wurgler and Kramers 1992). Increased variation inmorphogenetic changes has been documented in 96 plantspecies belonging to 28 families widespread around the


Chernobyl reactor site (Kordium and Sidorenko 1997). Ahigher cytogenetic variation compared to control valuesindicates active adaptation. Adaptation is a complexprocess by which populations of organisms respond tolong-term stresses by permanent genetic change(Kovalchuk et al. 2003).

DNA methylation (the addition of a methyl group tothe carbon 5 position of the cytosine ring) is the mostcommon epigenetic DNA modification in plants and is astrategy used to stabilize their genomes (Mittelsten-Scheid et al. 1998). Kovalchuk et al. (2003) found thatgenomic DNA of exposed pine tress was considerablyhypermethylated at Chernobyl. Such hypermethylationmay be viewed as a defense strategy of plants thatprevents genome instability and reshuffling of the hered-itary material, allowing survival in an extreme environ-ment (Kovalchuk et al. 2003). According to Kovalchuket al. (2003):

“Adaptation could occur based on changes at theDNA level and may include increased mutation rates inessential genes, such that favorable mutations could leadto higher tolerance to radiation. However, such an eventwould be rare, assuming a germline mutation rate inplants of about 10�5 to 10�6 per gamete, one expectsabout 1 mutation in 500,000 plants. Even if all mutationswere favorable, it is difficult to imagine that this wouldlead to sufficient and rapid adaptation of all plants in apopulation within several years, even though radiationwas shown to increase the mutation rate. Mechanismsable to explain such rapid adaptation to chronic radiationexposure are epigenetic changes that control expressionof genes and genome stability. Changing of methylationpatterns may lead to changes in the expression of variousstress-related and housekeeping genes, thus resulting in‘stronger’ plants.”

Human abandonmentLayered on top of the impacts from the irradiation

was the abrupt and drastic change that occurred whenhumans were removed from the 30-km Exclusion Zone.Pripyat, the town adjacent to the Chernobyl reactor, wasabandoned when �50,000 people were evacuated. Agri-cultural activity, forestry, hunting, and fishing within the30-km Exclusion Zone were stopped because of theradioactive contamination of the products. Only activitiesdesigned to mitigate the consequences of the accidentwere carried out, as well as those supporting the livingconditions of the cleanup workers.

For some years after the accident, the agriculturalfields still yielded domesticated produce, and manyanimal species, especially rodents and wild boars, con-sumed the abandoned cereal crops, potatoes and grassesas an additional food source. This advantage, along with

the special reserve regulations established in the Exclu-sion Zone (e.g., a ban on hunting), tended to compensatefor the adverse biological effects of radiation, and pro-moted an increase in the populations of wild animals.Significant population increases of game mammals (wildboar, roe deer, red deer, elk, wolf, fox, hare, beaver, etc.)and bird species (black grouse, duck, etc.) were observedsoon after the Chernobyl accident (Gaichenko et al.1990; Suschenya 1995).

More than 400 species of vertebrate animals, includ-ing 67 ichthyoids, 11 amphibians, 7 reptilians, 251 birds,and 73 mammals, inhabit the territory of the evacuatedtown of Pripyat and its vicinity; more than fifty of thembelong to a list of those protected according to nationalUkrainian and European Red Books on Threatened andEndangered Species. The Chernobyl Exclusion Zone hasbecome a breeding area of rare species such as thewhite-tailed eagle, spotted eagle, eagle owl, crane, andblack stork (Gaschak et al. 2002). In both the Belarusianand Ukrainian parts of the Exclusion Zone, State radio-ecological reserves have been created with a regime ofnature protection (Mycio 2005).

Since human abandonment of the area, the PripyatRiver floodplain, a developed system of artificial drain-age channels, now supports about a hundred families ofbeavers. Recognizing the value of the abandoned landaround Chernobyl, 28 endangered Przewalski’s wildhorses were introduced in 1998. After 6 years, theirnumber doubled (Gaschak et al. 2002). The wild horse ofAsia, Equus przewalski, is the only extant wild horsethat, in the purebred state, is not descended from thedomestic horse. In the 1960’s, the horse became extinctin the wild; but in the early 1990’s was reintroduced intoseveral locations in Mongolia and Kazakhstan, as well asChernobyl, using animals bred in European zoos.

As has been shown many times before, when hu-mans are removed, nature flourishes. This phenomenonexists in U.S. National Parks such as Yellowstone andthe Grand Tetons, and at large U.S. Department ofEnergy sites where the general public has been excludedfor over 50 y. Human presence in any environment is adisturbance to the natural biota. Normal activities offarming, hunting, logging, and road building, to name buta few, fragment, pollute, and generally stress the pro-cesses and mechanisms of natural environments. Theremoval of humans alleviates one of the more persistentand ever-growing stresses experienced by natural eco-systems.

The removal of human residents complicates anyexamination of population-level impacts to wildlife fromradiological exposures at Chernobyl. The absence ofpermanent human residency for 20 y has resulted in aflourishing natural ecosystem around Chernobyl. The


30-km Exclusion Zone has become a wildlife sanctuary;it is one of the more profound ironies of Chernobyl(Mycio 2005).

CONCLUSION

The environmental response to the Chernobyl acci-dent was a complex interaction of dose, dose rate, time,and the radiosensitivities of the different taxons. Duringthe first year post-accident, radiation levels were suffi-ciently intense within the 30-km Exclusion Zone to causemortality among the most radiosensitive species. Thoseeffects propagated up biological organizations such thatimpacts to some communities were observed. The acuteradiobiological effects documented in the Chernobyl areawere consistent with radiobiological data obtained inexperimental studies prior to the accident.

Twenty years later, radiation levels have dropped to1% of the original levels due to radionuclide decay anddeeper migration of the radionuclides into the soil.Various cytogenetic anomalies attributable to radiationcontinue to be reported from experimental studies per-formed on plants and animals within the 30-km Exclu-sion Zone. However, it is also true that many recentstudies have been unable to document any negativeeffects. Whether the observed cytogenetic anomalies insomatic cells have any detrimental biological signifi-cance to populations is still not known. Mousseau et al.(2005) cautioned that it is too early to assess the overallimpact of radionuclide exposures on plant and animalpopulations at Chernobyl because the possible conse-quences of multigenerational accumulation of geneticdefects are not known.

Long-term effects to biota in the Exclusion Zonehave been confounded by nature’s overriding positiveresponse to human abandonment of the area. Without apermanent human residency for 20 y, the ecosystemsaround the Chernobyl site are now flourishing in re-sponse to the wildlife sanctuary-type environment.

Radiological effects to biota are an important topicof research because some results may be appropriatemodels for chronic, low-level exposures that humansmight experience within the nuclear workforce, or fol-lowing widespread release of contamination. Addition-ally, the paradigms for regulating permissible exposurelevels to biota are currently being re-evaluated at aninternational level (ICRP 2003; Brechignac 2003; Hintonet al. 2004). One endpoint of concern being considered isdamage to DNA; and, yet, insufficient data exist toformulate guidance on acceptable levels of moleculardamage relative to effects on the exposed population(Garnier-Laplace et al. 2004). The Chernobyl ExclusionZone provides a unique area for conducting much needed

radiological research; in particular, studies on the multi-generational effects of genomic instability, adaptive re-sponses, and epigenetic mechanisms.

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RADIATION-INDUCED EFFECTS ON PLANTS AND … · RADIATION-INDUCED EFFECTS ON PLANTS AND ANIMALS: FINDINGS OF THE UNITED NATIONS CHERNOBYL FORUM ... The response of biota to Chernobyl