Following the accident at Chernobyl in 1986, the local human population was ... anthropogenic effects characteristic of areas of high human population density.
I. SAMUSENKO, 2004 - Some aspects of White Stork Ciconia ciconia population dynamics in the region of Chernobyl’s accident. In: Anselin, A. (ed.) Bird Numbers 1995, Proceedings of the International Conference and 13th Meeting of the European Bird Census Council, Pärnu, Estonia. Bird Census News 13 (2000):157-160
SOME ASPECTS OF WHITE STORK CICONIA CICONIA POPULATION DYNAMICS IN THE REGION OF CHERNOBYL’S ACCIDENT I. Samusenko1 ABSTRACT. Following the accident at Chernobyl in 1986, the local human population was evacuated over a large area. The White Stork Ciconia Ciconia population in the area began to decline immediately after the accident, and this decline continued into the mid 1990s. However, White Storks did not desert the Chernobyl area entirely, as did some other commensals of man. Breeding success in the Chernobyl area was low – though variable – compared to elsewhere; during the worst years of 1992 and 1994 only 62.5 % of occupied nests were successful. The cause of the storks’ population decline is unknown, but may be a consequence of the marked transformation of its feeding habitat, mainly former agricultural land, which has been abandoned and has become overgrown with tall grasses, shrubs and trees, making it unsuitable for foraging storks. 1
Institute of Zoology, ul. F.Skoriny 27, 220072 Minsk, Belarus
INTRODUCTION Due to its wide range, specific habits and a positive attitude towards it from humans, the White Stork Ciconia ciconia has frequently been used as a model for population studies, particularly those dealing with serious anthropogenic impacts on the environment. After the accident at Chernobyl the local human population was evacuated over a large area so that it was possible to observe White Stork population dynamics under new conditions , virtually free from short-term anthropogenic effects.
MATERIAL AND METHODS Prior to 1986, the extreme south-eastern part of Belarus was seriously affected by the anthropogenic effects characteristic of areas of high human population density. After the accident at Chernobyl, an area of 30 km radius (2 800 km2) around the power station became heavily polluted as a result of the emmission of radioactive substances from the destroyed reactor. The entire local human population was evacuated from this area in 1986 and most of the area was withdrawn from all forms of economic use. Since 1986, the only activities that have been undertaken in the area are those connected with controlling trespass, decontamination (especially immediately after the accident), afforestation and fire control. In 1988, the Polessky Radio-ecological Nature Reserve was created. This included most of the Belarussian part of the evacuated area. Since 1993, its total area has been 2 155 km2. The study reported here was carried out in the Chernobyl area from 1987 onwards. Most of the information was collected during 1990-'95, following the selection of study plots in the late 1980s. Prior to the accident, stork census data were only available from 12 villages (containing a total of 44 nests) in the 30 km radius area. These data were mainly collected by questionnaire censuses undertaken in 1984-'85, though some additional data were available. The same villages, by then unoccupied, were regularly observed during 1990-'95. Only those nests that were visited at least twice during a given season in the period late April-May – late June-July
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have been included in the analyses of breeding success. In total, information about the status and use of 15-40 nests was collected each year (though including some nests from outside the study plots). Data from three study plots outside the Chernobyl area (Zhitkovichi District, Gomel Region; Luninets Dist., Brest Reg. and Novogrudok Dist., Grodno Reg., 10-28 nests in each plot) have been used to compare productivity in polluted and un-polluted areas.
RESULTS AND DISCUSSION Habitat changes, caused by vegetation succession and changes in the hydrological regime in the absence of human interference, led to local fluctuations in White Stork numbers and breeding distribution. The trends observed were complex and their direction cannot yet be determined with any certainty, not at least until the process of ecosystem transformation has stabilised. In the Polessky reserve, where White Storks breed mainly in villages, numbers declined by 33-100 % per village (Table 1). Even in those villages where there were 5-6 occupied nests before the accident, there were never more than two in 1995. Most abandoned villages lost breeding pairs, and this was particularly marked in those located at a distance from populated areas, for example in Borshevka, Krasnoselie, Orevichi, Vepry and Nadtochaevka. In these villages, populations declined by at least 50 %. It appears that the population began to decline in the first post-accident years (Fig. 1) and that the decline was still in progress in the mid 1990s. One single observation supports this conclusion: three occupied nests were censused in Babchin in 1989 while five nests were present prior to 1986. Table 1. Dynamics of White Stork numbers in some abandoned villages of Chernobyl area, 1985-'95 Villages
Krasnoselie Orevichi Babchin Lomachi Borshevka Novosiolki Kozhushki Mokish Vorotec Vepry Dronki Nadtochaevka
1984-85
1990
6 5 5 4 4 4 3 3 3 3 2 2
3/1 2/1 3/3 1/1 4/4 2/2 2/2 1/0 -
Number of nests (occupied / successful) 1991 1992 1993 2/1 3/2 1/1 2/2 4/4 2/2 2/2 1/1 1/0 1/0
2/0 1/0 3/2 1/1 2/0 4/4 2/2 1/1 -
3/3 2/2 2/2 3/3 2/2 1/1 0/0
1994
1995
1/0 2/1 2/1 2/2 1/1 -
1/1 2/1 2/2 2/1 2/1 2/2 2/2 1/1 1/1 1/1 0/0
The proportion of breeding attempts that were unsuccessful seems high in the Chernobyl area compared to elsewhere. During the worst years of 1992 and 1994 only 62.5 % of occupied nests were successful (i.e. raised at least one fledgling), a value typical for endangered West European populations (e.g. Schütz & Szijj 1975; Dallinga & Schoenmakers 1989). In several cases, all young died during the second half of the nestling period, sometimes just before fledgling (two such incidents in 1992). It seems likely that the main reason behind the population decline of the White Stork in the Chernobyl area is the marked transformation of its feeding habitats, mainly former agricultural
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Figure 1. Dynamics of the White Stork population in the Chernobyl area, 1985-'95
120 100 80 occupied
60
succesful
40 20 0 1985 1990 1991 1992 1993 1994 1995
land. These lands are now in the process of becoming overgrown with tall grasses, shrubs and trees. Forests have been planted in large open areas within the Polessky reserve in order to localize radioactive pollution and to prevent the spread of radioactive particles from open sandy areas. This planting is still in progress so the proportion of open habitats suitable for stork foraging is steadily decreasing. Moreover, the peak of grass growth coincides with hatching and the grass would be tall during the period when the food demands of growing nestlings are at their greatest; such tall grass is unsuitable for storks to forage in. An additional factor influencing White Stork numbers in the study area may be changes in nest site availability due to the continuous destruction of old wooden buildings. The number of roof and water-tower nests in 12 censused villages decreased from four in 1990 to two in 1995 for each nest type. All tree nests occupied in 1990 (19 % of the total number in that year) were abandoned by 1993, and the reason for this still remains unclear. Wooden and concrete power poles seem to be the most stable nest supports, and the proportion of occupied nests located on poles has grown from 50 % in 1990 to 75 % in 1995. Table 2 presents the results of a comparison of White Stork breeding success in the area around Chenobyl with that from several study plots elsewhere in Belarus with normal levels of human population density and economic activity. In the Chernobyl area, breeding success fluctuated markedly between years while elsewhere in Belarus it was much more stable. These results may suggest that the White Storks around Chenobyl have been unable to alter their behaviour to adapt to the changes that have occured there. For example, they may have been unable to alter their choice of foraging habitat or the location of their nest site in relation to suitable foraging areas. In addition to those factors that might affect productivity in general (wintering conditions, climatic factors, fluctuations in food avaiability), some local factors, such as the level of aforestation of open areas, the frequency of fire and the extent of recultivation of open areas during the fire control activities, may also have affected productivity in the Chernobyl area. It is important to point out, however, that these measures of variation in breeding success are based on small sample sizes.
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Table 2. Breeding success of White Storks in the Chernobyl area and other parts of Belarus. Years
Number of fledglings (+SD) in: Chernobyl area Regions with intensive anthropogenic pressures Occupied nests Successful nests Occupied nests Successful nests
1991 1992 1993 1995
1.6 + 1.4 (8) 1.3 + 1.3 (16) 3.1 + 1.7 (10) 2.3 + 0.9 (13)
2.6 + 0.6 (5) 2.1 + 1.0 (10) 3.4 + 1.3 (9) 2.5 + 0.5 (12)
-
2.7 + 0.8 (26) 2.7 + 0.9 (71) 2.9 + 0.9 (54) 2.8 + 0.9 (41)
CONCLUSIONS White Storks did not abandon the Chernobyl area following the cessation of all main economic activities and the evacuation of the human population as did true sinanthropic species, such as Feral Pigeon Columba livia domestica, House Sparrow Passer domesticus and some others, (Nikiforov 1994). However, it seems that current conditions in the area are far from optimal for the storks as indicated by the decline in population and fluctuating breeding success. Further monitoring of this population will allow study of this normally human-tolerant species’ reactions and adaptations to the unusual conditions existing in the area after such an unprecendent natural experiment.
ACKNOWLEDGEMENTS I thank Alexey Tishechkin for help in the preparation of this paper. This research was supported by the Soros Fondation, and my participation in the conference was possible because of support from the EBCC. I am deeply indebted to David Gibbons whose comments and criticism have considerably improved the paper.
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