Terrestrial fauna

Ladislav JEDLIČKA, Ivan ORSZÁGH, Tomáš ČEJKA, Alžbeta DAROLOVÁ,   Miroslav KULFAN, Peter MIKULÍČEK, Zbyšek ŠUSTEK, David ŽIAK

CONCLUSIONS  Terrestrial fauna and terrestrial animal communities in the area affected by the Hydroelectric Power Project Gabčíkovo are typical of the Danube inland delta. In the inland delta, an expressive ecological, mostly moisture, gradient exists which is manifested by an ecological catena of communities changing gradually from the aquatic communities, through wetlands, hygrophilous, mesophilous to the xerophilous. Thus, the dominant factor affecting the natural communities is the ground water level influenced by recharge from the Danube. Among the animal taxocoenoses, the edaphic taxocoenoses bound on the soil conditions, predominantly soil moisture, are those which reflect these changes of the hydrological and moisture regime in a larger extent. Taxocoenoses in the overground strata depend, to a certain degree, more on the climatic conditions, inclusively of the microclimate, and on vegetation. Area of the Čunovo reservoir includes monitoring plots with a relatively stabilised development of terrestrial fauna. Changes in individual taxocoenoses in the flood-plain forest were different which resulted from an forest stand opening and thus stimulation of the shrub stratum development. No significant changes were observed and the development of the terrestrial communities was balanced. Observed changes in the taxocoenoses structure and in the population size of some species can be considered to be usual fluctuation. Animal taxocoenoses in the foreststeppe communities Crataegetum danubiale preserved their original character, were stabilised, and expected shift toward the mesohygrophilous type was not significantly manifested. In the by-pass canal area, changes of hydrological regime and changes of micro-climatic factors were reflected by three phases in succession of animal communities. The first phase after putting the Project into operation was characterised by a decline of the strongly hygrophilous species and increase of tolerant species. In the second phase a strong decline even of the moderately hygrophilous species favoured by the decreased ground water level. In the third phase (1996 – 1997), a slow down of the degradation process in the studied zoocoenoses was recorded; it was manifested as a stabilisation of the community structure and increasing of abundance of hygrophilous species. In the area downstream of the tail water canal no significant changes connected with the Gabčíkovo structures were observed in the terrestrial animal communities, the taxocoenoses were balanced and stabilised.

Introduction

Terrestrial fauna and terrestrial animal communities in the area affected by the Hydroelectric Power Project Gabčíkovo are typical of the Danube inland delta. The term inland delta is used for the river flood-plain (inundation area) with a rich network of river arms and standing water bodies having a close relation to the river sediments deposing, river meandering, and frequent floods. The inland delta represents a dynamic system of a specific complex of water, soil and terrestrial ecosystems and of corresponding ecotones. The limiting factor of this system is the occurrence of seasonal floods, the adequate level and fluctuation range of ground waters. The floods, out of the morphological changes, represent an input of organic substances. The entire complex of ecosystems in the inland delta belongs to the cyclical climaxes maintained by the cyclic hydrological regime of the river and adjacent area. The specific conditions in this ecosystem are a crucial limiting factor for the existence of species exhibiting specialised adaptations in their morpho-physiology, life cycles and life strategies for survival in conditions of cyclical climax. All are highly specialised ecologically and their existence in other conditions is not possible. However, their existence is endangered even in such situations in which condition changes do not exceed the ecological tolerance limits of these species, but allow the invasion of widely tolerant species, which the specialised species are unable to compete with. The existence of highly specialised species is manifested in forming the specific zoocoenoses which are maintained and limited just by the conditions of an inland delta characterised by a cyclical incidence of catastrophes, in this case of floods [13, 21,22,26].

In the inland delta, an expressive ecological, mostly moisture, gradient exists which is manifested by an ecological catena of communities changing gradually from the aquatic communities, through wetlands, hygrophilous, mesophilous to the xerophilous (in the terrestrial conditions such a catena is represented by the community sequence of Salici-Populetum. Fraxineto-Populetum. Ulmeto-Fraxinetum, Ulmeto-Quercetum and Crataegetum danubiale) which depends on the ground water level and floods regime. Changes of ground water level and floods and, consequently, of moisture gradient can cause changes of such catena ranging from simple spatial changes to the shift to xeric communities and vanishing of the hygrophilous communities. Thus, the dominant factor affecting the natural communities is the ground water level influenced by, recharge from, the Danube.

Among the animal taxocoenoses, the edaphic taxocoenoses bound on the soil conditions, predominantly soil moisture, are those which reflect these changes of the hydrological and moisture regime to a larger extent. Taxocoenoses in the over-ground strata depend, to a certain degree, more on the climatic conditions, inclusively of the microclimate, and on vegetation.

Animal communities in the studied area are bound on:

a) soft wood flood-plain forests Salici-Populetum (dominated by Salix alba, S. fragilis, to less extent Populus nigra, P. alba, Alnus glutinosa, A. incana) on the plots in the lowest altitudes at water flows and standing water bodies and in depressions with high ground water level (up to depth of 1.5 m) and regular floods, or at least with ground surface transgression.

b) mixed transitional flood-plain forests Fraxineto-Populetum, Ulmeto-Fraxinetum (in E₃ dominated by Populus spp., Fraxinus excelsior, F. oxycarpa, Padus racemosa, Ulmus laevis, U. carpinifolia, to a lesser extent by Alnus incana, A. glutinosa and Salix spp.; in E₂ by Cornus sanguinea) which are sufficiently supplied by ground water during vegetation period and are regularly flooded. At present, the communities of this type are strongly changed by plantation of poplar monocultures. Turning of the original flood-plain forests to poplar monocultures results in a decrease of species diversity and less diversity of the original zoocoenoses, and aninvasion of the expansive xenocoenous species. This process can be intensified by a decrease of ground water level and soil moisture.

c) hard wood flood-plain forests Ulmeto-Quercetum (in E₃ dominated by Quercus robur, Fraxinus excelsior, F. oxacarpa, Ulmus laevis, U. carpinifolia mixed with Populus alba and P. nigra, in E₂ Sambucus nigra, Corylus avellana, Euonymus sp.) with ground water level sinking during vegetation period deeper (2-3 m).

d) Danubian “forest steppe” Crataegetum danubiale (dominated by Quercus robur, Fraxinus sp., Ulmus sp., Crataegus monogyna) with much drier conditions.

In spite of the fact that the selective bibliography of literature about the Podunajská nížina lowland fauna covering works issued up to 1985 [18] includes almost 2000 items (among them only a part deals with the terrestrial fauna), most of these works are oriented faunistically or on the basic ecological problems, e. g. binding on the original, in some cases the already altered or destroyed habitats [34].

In the past, the terrestrial zoocoenoses were studied only sporadically and reliable older source data are missing about most of them. Consistent data were obtained from the beginning of biota monitoring in the area affected by the Gabčíkovo structures in 1990 which, for the present, characterise the zoocoenoses state and changes up to 1997. Thus, the monitoring characterises the state before starting the Gabčíkovo structures operation and situation which followed after it, inclusively of further modifications.

The monitoring of the terrestrial fauna included a wide scale of taxocoenoses of the soil, soils surface and over-ground strata (Mollusca, Oniscidea, Acari, Chilopoda, Collembola, Heteroptera, Coleoptera, Neuroptera, Mecoptera, Lepidoptera, Hymenoptera, Amphibia, Aves, Mammalia) [2, 26, 36, 37]. In this study, the results of study of those taxocoenoses are summarized which reflect more expressively the changes and state of terrestrial animal communities.

Interpretation of changes

When evaluating the changes running in the studied ecosystems, it is to be considered that most of their trends originate in the past, especially in the 1950s (a gradual deepening of the Danube main flow river bed, silviculture etc.). Starting operation of the Gabčíkovo structures caused a mutual overlapping of causes and their negative synergetic effects, while for other trends it seems that the Gabčíkovo structures act against the negative trends from the past. Effects of their antagonisms can be evaluated as late as after a longer lapse of time. The changes can not be evaluated only as changed environmental conditions for individual species or taxocoenoses, but they are also to be evaluated from the point of view of their convergence or divergence with a natural functioning of the studied ecosystems in the inland delta.

The observation series is already sufficiently long to allow making statements about some trends and to separate the changes of a fluctuating character from the general trends of community structure succession conditioned by the site factors and their changes. However, the trend interpretation is not always simple and unambiguous. More changes resulted from the synergetic effect of several factors, which often resulted not only from changes of soil moisture caused by the changed regime of the ground water level and/or floods, but also from antropogenous or other influences (influence of silviculture, immissions, extreme exceptional climatic conditions, hydrological exceptionality of some years, different biotic interactions etc.), concrete modes of the monitoring realization, and situating of the monitoring plots (MP).

For evaluating structural changes in the communities of the model animal groups which can result from construction and operation of the Gabčíkovo structures, the whole area monitoring of the biota monitoring can be divided into three parts with different hydrologic regime:

-         backwater area of the Čunovo reservoir (monitoring plots MP 1, 2, 3, 4),

-         by-passed area representing the area between the original river bed and the by-pass canal of the structures in the section from damming the original river bed down to the outlet of the tail-water canal back into the Danube main river bed (monitoring plots MP 6, 9, 10, 14, 15),

-         area downstream from the by-pass canal, the area downstream from the by-passed section (monitoring plots MP 18, 23).

Contrary to the first and third area, which include monitoring plots with a relatively stabilised development (MP 1, 2, 3, 4, 18, 23), in the monitoring plots of the second area adaptive changes are observed which tend to drier community types and result from floods absence, decrease of ground water level or from both these factors at the same time (MP 6, 9, 10, 14).

In the two years (1993 and 1994) after the Danube main stream damming in autumn 1992, the communities formed themselves under conditions characterised by a continuation of a series of several warm and dry vegetation periods. In 1995, but especially in 1996, an evident cooling began and at the same time (especially in 1996) the precipitation sums increased. In this way, the absence of flood was partly compensated as well as the ground water level decrease (Dunajské kriviny almost by 2 m when compared with the state before 1992, Bodícka brána ca. by 1-1.5 m and Istragov ca. 1 m) and their fluctuations were moderated (they remained preserved only in Istragov).

In the backwater area on the monitoring plots MP 1 - 4, the original hydrological regime was partly restored, however floods in the monitoring plots MP 1 and 3 absented furthermore, similarly as before damming. This state represented, under such conditions, a starting point for the raising of a hard-wood flood-plain forest. Changes in individual taxocoenoses in the flood-plain forests were different and they resulted from an opening of forest stands and stimulation of the shrub stratum development. On one hand, the xerothermophilous species (Lepidoptera) from the adjacent Crataegetum danubiale forest steppe or tolerant forest species (Mollusca) penetrated here, on the other hand some hygrophilous species (Oniscidea, Chilopoda, Mammalia) were recorded here as a consequence of the increased ground water level or, from the same reason, a decline of the species less tolerant to the increased soil humidity was observed. However, there were no striking changes observed and the expected succession towards more hygrophilous communities did not start after the filling of the Čunovo reservoir. In general, the development of the terrestrial communities was balanced. The taxocoenoses were dominated by stenoecious hygrophilous and forest species and the proportion of these species increased. The xenocenous species from the adjacent secondary semisteppe like formations invaded into the taxocoenoses only along forest margins, but this was the usual edge ecotone margin effect. The influence of the structures operation had no significant effect. Observed changes in the taxocoenoses structure and in the population size of some species can be considered to be usual fluctuations.

Animal taxocoenoses in the foreststeppe communities Crataegetum danubiale preserved their original character; they were stabilised and no significant changes were observed in them during the monitoring. A shift toward the mesohygrophilous type was not significantly manifested, termophilous and xerophilous species preserved their position in the dominance structure (Gastropoda), in the typical soil fauna an increase of soil humidity was recorded (Chilopoda). In some taxocoenoses (Lepidoptera) the abundance of xerotermophilous species decreased. This trend, which was recorded already after 1993, could reflect moderation of aridisation and could also result from climatic factors.

The structures operation most influenced the localities situated in the by-passed area in the descendent sequence MP 6, 14, 9, 10.

The most extensive changes caused by the structures operation were observed on the monitoring plot MP 6, Dunajské kriviny. It is due to the fact that this locality is situated out of the arm system supplied by water from the by-pass canal, and under the overflow dam in the original Danube main stream near the Dunakiliti weir. After the diversion of the Danube, the ground water level in this locality sank significantly, the locality dried and the soil moisture decreased. The animal communities in the central part of this locality were entirely dependent on water from precipitation and from floods, the last of which occurred there in 1991. The original wetland communities vanished. In addition, populations of some stenoecious species were affected by the extreme flood in 1991 and later were unable to restore themselves. Contrary to the fact that the marginal parts of this locality were sufficiently supplied by water during the artificially simulated floods from the intake system near the Dobrohošť village, the succession of the terrestrial animal communities was continuously oriented toward the mesic communities of the mixed flood-plain forests, or hard-wood flood-plain forests, or even to communities of the dry shrub formations. A decrease of hygrophilous and typical forests species in the terrestrial communities continued as well as the invasion of xenocoenous species which were, however, moderated in 1997 by more favourable climatic conditions. The moderate increase of the soil moisture caused by the rainy years 1996 and 1997 had a positive influence especially on the euedaphon communities (Acari, Oniscidea, Chilopoda) and, to a certain degree, also on the epigeic taxocoenoses (Coleoptera). The data obtained up to now do not allow establishing whether the trend of aridisation moderation observed in two last years is permanent or whether it represented only a climatic fluctuation. At present the terrestrial communities in monitoring plot MP 6 belong to the most degraded communities in the whole studied area. The facts presented allow stating that the anthropogenic factor had a very strong effect in this monitoring plot and that the revitalisation process started by river arm water supply ran slowly with an uncertain result.

The second locality in which the animal taxocoenoses changes indicate the largest impact of flood absence and a decrease of ground water level was the monitoring plot MP 14 (Istragov). The changes were deepend by the sandy soil type. Until 1992, monitoring plot Istragov belonged among the characteristic hygric sites of soft wood flood-plain forests (Salici-Populetum myosotidetosum sensu Jurko, 1958, [14]).

After the Danube damming, the ground water level sank, the shallow flooded depressions were reduced and the xeroseries started in them [22, 32]. In the drier sites, this secondary succession leads to communities of a drier variety of the soft wood flood-plain forest (Salici-Populetum typicum). In the terrestrial communities, a decline of species indicating the waterlogged habitats of the more hygrophilous species (Mollusca, Coleoptera) was recorded. They were substituted by less hygrophilous forest species. This indicated a process of habitat drying. These changes were also closely connected with extensive timber exploitation, or clear-cutting, and with consecutive changes in lower stands strata. The trend of a strong decline of the strongly hygrophilous species was moderated in 1996 (Mollusca, Lepidoptera) and the adaptive succession changes run slower than those in the vegetation (Mollusca, Chilopoda). Probably the simulated floods contributed to the moderation in the major part of the flood-plain, while in Istragov the backwaters from the confluence with the tail race canal had a moderating effect. In spite of the general decrease of the ground water level by 1-1,5 m, the changes of ground water level in Istragov have the same unsteady character as before the start of the Gabčíkovo structures operation. In other localities the ground water level fluctuations were considerably slower and smoother.

A similar, but a little slower succession trend was observed in 1994-1997 on the monitoring plot MP 10 (Kráľovská lúka). The present structure of the terrestrial taxocoenoses reflected the hydropedological conditions in the preceding years. The changes in communities species composition indicate an instability of conditions. The general trend of the species diversity decrease (Oniscidea, Coleoptera) and an increase of the dominance of mesohygrophilous, tolerant, or xenocenous species continued (Mollusca, Coleoptera) in spite of the fact that this decrease in some taxocoenoses became slower or even stopped (Chilopoda, Coleoptera, Lepidoptera). This signalled the stopping or slow-down of the negative succession trends. However, it is evident that this was a consequence of two considerably moister vegetation periods which were connected, in 1997, with a long term flood. In general the communities´ succession was oriented toward the communities typical of drier types of flood plain forests. In this locality, the spatial distribution of species strongly changed, depending on minute differences in altitude (about 50-60 cm). After the locality began drying, and the strongly hygrophilous species declined, the more tolerant species substituted for them in the depressions, while the higher places were less populated. The changes recorded in the ornithocoenosis can not be considered only as a consequence of the structures operation, but were caused by other strong factors represented by the forest management system. By end of 1996, restoring measures were taken in this locality. Therefore, the positive changes already observed in the herbage stratum can also be expected in the zoocoenoses structure. It is probable than the wetland communities will be restored.

Although the monitoring plot MP 9 (Bodícka brána) is formally in reach of the simulated floods, these floods have not been sufficiently high and they only influence the hydropedological conditions in this permanent study plot (PSP) for a short time. Cutting of the poplars and subsequent changes in the herbage stratum contributed to the drying of the soils surface strata, too. The studied groups of the epigeic animals are bound to the forest stand, which arose on young sandy sediments and on a shallow humus horizon under which the soil dries rapidly. It is known that the arenic fluvisoils belong to those soils which suffer the most from a decrease of ground water level and absence of  floods with sufficient level oscillation. Water level in the arms simulates the natural seasonal pulsing during the artificial floods.

The number of hygrophilous species in the terrestrial communities continued to decrease while the representation of mesohygrophilous and euryecious species increased. Many of the hygrophilous species occur on the observation ability threshold (Mollusca, Coleoptera). This decrease in abundance slowed down or even reversed in some species and the abundance of some hygrophilous species increased in the last year of monitoring (Coleoptera, Lepidoptera, Oniscida, Chilopoda). The euryecious species not needing high humidity (Lepidoptera) preserved their high dominance. In spite of these changes, the state of the zoocoenoses can be considered to be reversible (an increase in the abundance of some hygrophilous species is  possible). It seems that the community succession in the two last years returned to the state which arose immediately after the Danube diversion into the by-pass canal. The stability of this trend is, however, hardly predictable and depends on the interaction of soil moisture changes, timber exploitation and climatic factors fluctuation.

Clear cutting of the stand of poplar cultivars in August, 1997, resulted in a further opening of vegetation. Changes in the herbage and shrub strata composition are expected. They will initiate subsequent changes in the composition of the terrestrial zoocoenoses (invasion of open landscape or of eurytopic species). It is possible that changes in the zoocoenoses structure caused by the above processes could overlap, partly, the changes caused directly by the Gabčíkovo structures operation.

Changes of hydrological regime caused by the structures operation and changes of climatic factors were reflected by three distinctive phases in the succession of communities of the studied animal groups in all localities in the within-dike area.

The first phase followed immediately after the Danube damming. It was characterised by a decline of the strongly hygrophilous species and a sudden increase in abundance of the relatively tolerant, moderately hygrophilous species. It is to be noted that some hygrophilous species survive on the localities in reduced population sizes and the humidity decrease was reflected in a shortening of their activity period to the more humid and colder spring period (Coleoptera, Carabidae). This phase lasted around two years. However, its course in individual localities was not synchronised. In the drier monitoring plot Bodícka brána it represented the natural state. In Istragov, where a part of the monitoring plot has a much drier sandy soil, it came in 1991. In Dunajské kriviny, which is most affected by drying, it came after 1992. A quantitative representation of moderately hygrophilous species before 1992 indicated that these species immigrated in majority of monitoring plots from drier enclaves in flood-plain forests or even from the surrounding fields (Coleoptera, Carabidae).

The second phase was characterised by a strong decline even of the moderately hygrophilous species, which were temporarily favoured by the decreased ground water level and flood absence, approximately on the level before the Danube main stream damming and by a continuing decline of the strongly hygrophilous species.

The third phase came in the two last years of monitoring (1996 and 1997). Its main feature was a slow-down or cessation of the degradation processes in the studied  zoocoenoses. Externally it was manifested as a stabilisation of the communities. In this phase, the abundance of the species tolerant to humidity fluctuations and of the strongly hygrophilous species increased. A slow-down of community degradation was recorded in all studied monitoring plots in 1997, while in Bodícka brána already in 1996.

In the area under the tail race canal in the monitoring plots MP 18 Sporná sihoť and 23 Starý les no changes caused by the Gabčíkovo structures were manifested. No significant changes have been observed in the terrestrial communities corresponding to the flooded soft wood flood-plain forest of the Salici-Populetum association or in the transitional flood-plain forests. Taxocoenoses with a high proportion of the hygrophilous species and with a balanced state exhibited a high degree of originality and adaptation to the typical cyclic climax of the flood-plain forests. The observed changes had the character of usual fluctuations (Mollusca, Lepidoptera), the between-year changes of abundance and dominance were minimal and represented a natural manifestation of population dynamics of individual species. They can be considered to be a normal community dynamics fluctuation of the flood-plain forests in conditions of a normal hydrological regime. The communities were balanced and stabilised. Changes of the moisture conditions indicated by some hygrophilous species could be caused by the stand drying after thinning. The population of the more stenoecious forests species seemed to restore themselves starting in 1996, after the herbage layer regeneration and the impressive simultaneous development of the shrub layer. In some taxocoenoses (Chilopoda, Oniscidea), the largest deviations and decrease in number of species was recorded in 1993-1994. However, in this monitoring plot, it was connected first of all with the extraordinarily dry and warm summer.

After the flood destroyed the litter stratum and, locally, even the humus stratum, populations of some epigeic species were reduced. In contrast, the hygrophilous species, especially the small ones, survived the floods well.

From the view point of nature protection, monitoring plot MP 23 and its zoocoenoses deserve special attention because the species diversity of the hygrobionts had the most original natural character among all studied monitoring plots and several species included in the lists of endangered species occured there. In regard to the original natural character, this locality deserves stricter protection and extension of the protection zone of the Lyon reserve up to this locality.

Proposals

Considerations about protective measures should be based on the basic character of the inland delta ecosystems and on the limiting factors of their existence, among which the main conditioning factors are seasonal floods and an adequate height and oscillation of the ground water level, hence the cyclic hydrological regime maintaining the system of the inland delta cyclic climaxes. Out of the hydrologic regime´s cyclic character, preserving the adequately strong moisture gradient, a condition for the existence of an ecocline of gradually changing communities, is of vital significance. Therefore, the dominant criteria in proposing protective measures are ground water level, soil moisture and floods.

From the view point of the terrestrial animals, it is to be considered that the species are adapted to the conditions of flood-plain forests, with a specific life cycle and adaptation mechanisms enabling them to survive the floods. For this reason, permanent water logging is not required, but what is necessary are adequately high and sufficiently long lasting floods. These must respect the fact that the discharge in the Danube culminates around June to July and, in winter, around December to February and that floods came, as a rule, in summer months.

The measures can not limit the dynamics of the natural processes and fix a state oscillating around average values which would accelerate the ecosystem ageing without initiating succession in some parts, a homogenisation of ecosystems´ mosaics and diversity. The purpose of these measures is not to assure the permanent optimal conditions (in a natural situation such a state does not occur), but to assure such conditions which would not lead to irreversible changes and to secondary succession of nonoriginal communities [22].

The simulated floods, in their present form, are unnatural and, in comparison with the natural ones, too slow. As to the distinguishing of the s. c. inevitable and optimal conditions, it is to be stressed that the present regime of the ground water level is unnatural and the narrow fluctuation of the ground water level (even 10-times narrower than in the past) can influence the pedological processes for a long time and can cause a permanent change of the existing ecosystems on the local edaphic climax (soil gleying) [23].

Because the function of the terrestrial ecosystems depends on the hydrological regime (first of all of the Danube main stream) it is necessary, in the by-passed zone, to:

1. preserve the natural height and seasonal fluctuation of the ground water level;
2. prevent the permanent water logging of higher lying plots;
3. manage the artificial floods according to their natural regime, concentrate their seasonal incidence into the first half of the vegetation period, accelerate the coming of the flood wave, connect it with transgression, but not with water logging, and guarantee their sufficient duration;
4. on the monitoring plot MP 6 the perimeter arm and the depression in the central part should be reconnected. Otherwise a substantial part of the monitoring plot will be supplied only by rain water, which eventually will negatively influence the next development of this community;
5. preserve the dynamics in the main stream, guarantee permanent connection of the arm system with the main stream, preserve permanent discharging of the original main arms and side arms of the parapotamon type [20, 21].

The backwater area of the Čunovo reservoir

MP 1 Ostrov Kopáč

Monitoring plot MP 1 Ostrov Kopáč is surrounded by the Biskupické rameno arm and represents a small area covered by stand of different species composition. In some places the flood-plain forest (Ulmeto-Fraxineto carpinetum) turns into forest steppe like formations with xerothermophilous zoocoenoses. This monitoring plot was not flooded before the Danube damming. The ground water level had sunk deeply, before the filling of the Čunovo reservoir, oscillating in the range of 4-7 m under the surface. Moisture of the soil upper horizons depended only on rain water. Before 1993, the drying process in this locality was especially visible in the tree stratum [30].

The habitat mosaic in this locality was reflected in the malacocoenosis structure. It did not change during the eight-year investigation, irrespective of the irregular penetration of the xenocoenous species from the close surroundings (the open landscape species or even the steppe species like Cecilioides acicula and Granaria frumentum). During the whole investigation, the mesohygrophilous forest species Aegopinella nitens, euryecious Vitrina pellucida and in the depressions, also the forest hygrophilous species Vitrea crystallina predominate in the land snails community. The typical polyhygrophilous species were absent. The tolerant forest species Monachoides incarnatus, Arianta arbustorum and Alinda biplicata also reached a high representation. The number of mollusc species recorded in each year ranged from 17 to 24 (Fig. 1) during the monitoring period and, from 1993, the species number exhibits an increasing trend.

The increase of the ground water level had no influence on the malacocoenosis structure due to the terrain’s higher altitude. The floods were already absent for a long time before the structures operation start. The expected succession of the malacocoenosis toward a more hygrophilous type did not come after filling of the Čunovo reservoir.

The composition of the centipede community also corresponded with the situation in this MP before the Danube damming. The centipedes were discontinuously distributed, they occupied shadowed places, small depressions with higher soil humidity, as well as places with a more continuous litter and leaf cover. In that period, the community consisted of 10 species (Fig. 2), among which the eurytopic species Lithobius forficatus was eudominant and the eurytopic species L. mutabilis and mesohygrophilous L. aeruginosus (Fig. 3) were dominant. Among the species characteristic of the humid Danubian flood-plain forests L. curtipes was recorded. The thermophilous hypogeic species were represented by Clinopodes linearis, which prefers the drier gravel-sandy substrates.

After 1992, the Biskupické rameno arm and depressions in this locality were filled by water. Comparison of the qualitative structure of the centipede taxocoenosis before and after the Danube damming showed a moderate shift from the xerothermophilous species to the mesohygrophilous ones. In spite of the humidity increase, the xerothermophilous species typical of this locality survive. Their number even increased after 1993. Since that time we also record there the species Henia illyrica and thermophilous forest species Crytops anomalans. These thermophilous species were repeatedly recorded in the following years. A certain instability in conditions in this locality resulted in the fluctuation of species number recorded in individual years (Fig. 2). The eudominant species in 1993-1994 were the eurytopic species Lithobius mutabilis, mesohygrophilous L. aeruginosus and the thermophilous species of the Mediterranean origin H. illyrica (Fig. 3).

 

Tab. 1. Between-year species similarity of centiped community in MP 1
1993	41,67
1994	50,00	72,72
1995	33,33	30,77	50,00
1996	30,77	28,57	46,15	88,89
	1991	1993	1994	1995

The terrestrial Isopod community consisted of 6 species (Fig. 4). The eurytopic species Trachelipus rathkii (Fig. 5) able to survive floods preserved its eudominant position during all years of the monitoring. The permanent presence of the species Armadillium vulgare and Trachelipus ratzeburgii, which prefer drier and warmer habitats confirmed the warm forest steppe character of this locality. The high species similarity between individual monitoring years indicate the stability of their environmental conditions.

Tab. 3. Species similarity index of amphibia

The amphibian taxocoenosis was studied in the years 1990, 1993, 1994 and 1996 in the gravel pit and in its surroundings. In individual years, 4-6 amphibian species were recorded (Triturus vulgaris, Bombina bombina, Hyla arborea, Rana arvalis, Rana eculenta and Rana lessonae, Fig. 6). The species composition changed during the study period. The Jaccard index of species similarity between the years 1990 and 1996 was 37,5% (Tab. 3). It indicated a significant change of the community. After the Danube damming the species number decreased and the species B. bufo and B. viridis, typical of steppe and forest steppe habitats were not recorded in 1994 and 1996. On the contrary, the frogs of the Rana esculenta complex (a group of hybrids called sometimes briefly also water frogs) living usually at permanent water bodies began to occur in this locality. After an increase of the ground water level, the water also stayed in the gravel pit, which became a breeding site of the frogs. The recorded species reproduced there, finished their development up to metamorphosis and migrated into surrounding vegetation. The amphibian taxocoenosis seemed to be stabilised. This stabilisation between 1994 and 1996 was indicated by the species similarity index of 83.3%. Changes of taxocoenosis composition and the dominance of individual species result from the increase of ground water level and creating of permanent water surfaces after the Danube damming.

The taxocoenosis of the small mammals (Rodentia, Insectivora) was studied in the flood-plain forest and in the adjacent forest steppe. During the whole monitoring period 10 species were recorded, but all these species were found only in 1990. Among these 10 species, we found the species Mus musculus and Muscardinus avellanarius never more. Besides the obligatory species Sorex araneus, Apodemus flavicollis and Clethrionomys glareolus, also the species Apodemus sylvaticus (Fig. 7) and Microtus arvalis (Fig. 8) penetrating from the adjacent forest steppe were recorded in the flood-plain forest. Their penetration, however, did not represent any special phenomenon, because both habitats – flood-plain forest and forest-steppe have an immediate contact. The increased abundance of the hygrophilous species S. araneus and C. glareolus in the last years was remarkable from our point of view. For example, in C. glareolus the abundance was twice as high in 1996 than in 1995 (25,2 ind. ha^-1 in July 1995 against 54,2 ind. ha^-1 in end of October 1996).

In spite of the expected (and now also proved) increase of ground water level, the forest steppe in the National Nature Reserve (NNR) Kopáč has preserved its typical species composition of small mammals. The steppe character was indicated by the species A. sylvaticus, M. arvalis, but most of all by Crocidura suaveolens. An other stepicolous and xerophilous species, Crocidura leucodon, was not recorded in 1996. Its absence, however, did not signal any significant change, because this species is rare in general and its absence can be purely occasional. The general increase of the small mammals´ abundance, as the average annual (39,2 ind. ha^-1 ) as the maximal value recorded during the year, was remarkable. The most abundant component of the community is the stepicolous species M. arvalis.

In the overground layers, the butterfly taxocoenosis (Lepidoptera - Rhopalocera) in the whole monitoring plot consisted of 63 species (Fig. 9). In the period 1990-1992, hence before filling of the Čunovo reservoir, 59 species were recorded, while in the period 1993-1996 only 47 species. After filling of the Čunovo reservoir the butterfly community changed, qualitatively and quantitatively. The most significant change was the decrease in abundance of the xerothermophilous species Pontia daplidice and Issoria lathonia. The abundance of these species in Ostrov Kopáč in 1996 was considerably lower than in 1995 and preceding years (period before 1993) when the ground water level was lower. Also the abundance of the thermophilous species Colias alfacariensis in 1996 was strikingly lower than in 1995. After 1992, the species Plebejus argyrognomon (an indicator of xerothermophilous sites) was recorded in this locality only once, in 1996. On the contrary the abundance of the mesophilous to hygrophilous species Araschnia levana considerably increased in 1996 in comparison with the preceding years. All these facts indicate the continuing retardation in aridization of this locality, and an improvement in conditions for hygrophilous and mesophilous species.

Evaluation

Animal taxocoenoses of the forest steppe community Crataegetum danubiale preserved their original character. A significant shift to the mesohygrophilous type did not come, thermophilous and xerophilous elements maintained their position in the dominance structure (Land snails), in the typical soil fauna an increase of the substrate humidity was observed (Chilopoda). In some taxocoenoses (Lepidoptera) the abundance of the xerothermophilous species decreased; this continuing trend had been recorded in the preceding years and could be caused by a moderation of aridisation, but it can also result from the climatic factors´ fluctuation.

Changes in individual taxocoenoses in the flood-plain forest had a different character and result from opening of the crown canopy and from the richer development of the shrub stratum. On one hand, the xerothermophilous species from the adjacent forest steppe (Heteroptera, Curculionidae, Lepidoptera) or tolerant forest species penetrate there. On other hand, the hygrophilous species (Oniscida, Chilopoda, Heteroptera, Mammalia) were recorded there probably as a consequence of the increased ground water level and a decline of species less tolerant to increased soil humidity was observed. However, striking changes have not been observed and the expected shift to a mesohygrophilous community type did not occurred after the creation of the Čunovo reservoir.

MP 2 Rusovecké ostrovy

The wetlands habitats, with the corresponding zoocoenoses in the central part of the monitoring plot MP 2b - Dolný Rusovecký ostrov island - were already destroyed as a result of the pre-dam ground water level decrease. In the framework of the technical solution of the so called polder, an artificial arm was excavated along the whole locality margin at the Danube right bank in 1990. Before filling of the Čunovo reservoir, the locality was not influenced by flood during the first two years of the monitoring. After the Danube damming, the Dolný Rusovecký ostrov came to be naturally flooded during high water level in the Danube. In 1997 the local flood level even reached 160 cm above the surface. The ground water level has increased essentially since end of 1992.

A typical land snails community of the transitional flood-plain forest occurred in this locality. The forest species Aegopinella nitens and the forest hygrophilous species Clausilia pumila dominated in it. No xenocenous species were recorded. The number of land snails recorded in individual years is given in Fig. 1. The expected shift of the mallacocoenosis toward the more hygrophilous type did not occur due to the considerable isolation of this locality and a low passive migration of the polygrophilous species.

The small mammal taxocoenoses were monitored in Horný Rusovecký ostrov as in Dolný Rusovecký ostrov. Both islands were regularly flooded in pre-dam conditions, and most of the small mammals inhabiting these island were drowned. At present, the abundance of small mammals is very high in both islands. The autumnal abundance culmination in 1997 was a little lower than in 1996, but this decrease was caused by an occasional weather situation at the time of sampling rather than by a long-term trend in population sizes. The species Apodemus sylvaticus and Microtus arvalis also penetrated there from the nearby dike in the past.

The taxocoenosis of Rhopalocera in this monitoring plot consisted of 64 species (before filling of the Čunovo reservoir in 1990-1992:  43 – 55 species a year, in the period 1993-1997:  32 – 48 species a year, Fig. 10). The increase of the ground water level after filling of the Čunovo reservoir resulted in the gradual extinction of the community Asparago-Crataegetum, in the spreading of hygrophilous vegetation in this locality, and subsequently in changes of species composition and the abundance of butterflies. The mesophilous to xerophilous species Melitea cinxia penetrated here from the Little Carpathians in 1994-1995. The xerothermophilous species Minois dryas, Pontia daplidice, Carcharodus alceae, Thymelicus acteon, Lacaeides argyrognomon, Issoria lathonia and Zerynthia polyxena all gradually disappeared. The species T. acteon and P. daplidice were not recorded in this locality in the two last years (1996-1997) and M. dryas and C. alceae were not recorded in 1997. The abundance of the species L. argyrognomon, I. lathonia and Z. polyxena decreased considerably in this locality. Significant bioindicators of the xerothermic habitats (M. dryas, P. dalidice and I. lathonia) reached a relatively high abundance in this locality before the filling of the Čunovo reservoir and regularly occurred there. This fact as well as the increased abundance of the significant bioindicator of the humid habitats - Lycaena dispar gives evidence of the aridisation slow-down in this locality since 1993 (after filling of the Čunovo reservoir).

Evaluation

The hygrophilous and mesophilous elements (Mollusca, Heteroptera, Lepidoptera, Mammalia) met in the terrestrial communities of the floodplain forest. In general, the development of the terrestrial communities was balanced. The taxocoenoses were dominated by stenoecious hygrophilous and forest species, the xecocoenous species penetrated only into the marginal parts of this monitoring plot (Mollusca: the steppicolous species Cecilioides acicula in 1995) from the surrounding semisteppe formations. Hence their penetration represented a usual edge effect.

In Asparago - Crataegetum, after filling of the Čunovo reservoir, the ground water level increased, the plant community Asparago-Crataegetum gradually vanished and the hygrophilous vegetation spread in this locality. This caused changes in species richness and abundance, and significant bioindicators of xerothermic habitats which reached a relatively high abundance before 1993, gradually disappeared (some Lepidoptera). In contrast to this, the abundance of bioindicators of humid habitas increased. This indicates the continuing extinction of the xerothermic communities after the filling of the Čunovo reservoir and recovery to more humid communities.

MP 3 Ostrovné lúčky

The monitoring plot MP 3 - Čunovo-Ostrovné lúčky was situated on the right Danube bank. Its major part was covered by a soft-wood or hard-wood flood-plain forest. A valuable Crataegetum danubiale covered its central part and represented a transition between the forest and forest steppe. The original marsh habitat (except for the lower parts) vanished. Before the Danube damming, ground water level fluctuated in a depth of 3 – 4 m, after the Danube damming the ground water level increased by approximately 2 m.

Before the damming, the land snails community was dominated, as today, by the forest species Aegopinella nitens, Monachoides incarnatus and Petasina unidentata. In consequence of the decreased ground water level before the Danube damming, in the far past the original wetland habitat became extinct except for the lowest parts of this locality. It is paradoxical that before the structures operation start wetland species (Zonitoides nitidus and Carychium minimum) occurred there in a small number in spite of the fact that humidity was lower than at present. By 1997 their populations became extinct also in the monitoring plot surroundings. The number of recorded mollusc species is presented in Fig. 1.

Despite the favourable humidity, a degraded land snails community of transitional flood-plain forest dominated by the forest species Aegopinella nitens, Petasima unidentata and Urticicola umbrosus occurred in the last year of monitoring (1997). The original populations of hygrophilous species which occurred there to a lesser extent until 1995 have been restored. This phenomenon, which was typical of the flood-plain forest malacocoenoses between Bratislava and Dobrohošť could be explained by the small area and considerable isolation of the localities situated in non flooded places (except for MP 2b), hence without a chance to restore the populations by passive and/or active migration.

The centipede community in this locality consisted of 15 species (Fig. 11), among which the mesohygrophilous species Lithobius aeruginosus and xerothermophilous species Henia illyrica (Fig. 12) predominated. Also two other species Clinopodes linearis, a thermophilous less hygrophilous species typical of the forests steppe areas, and Cryptops parisi, a species characteristic first of all of the submontane zones, were represented there. The mezohygrophilous species were represented by the eudominat Lithobius crassipes. The community’s major part was formed by the tolerant eurytopic species Lithobius erythrocephalus (14%), L. forficatus (25%) and Strigamia acuminata (21%), which penetrated into this locality from the close surroundings neighbouring with agrocoenoses.

After the Danube damming, the dominance of the mesohygrophilous species L. aeruginosus decreased while the dominance of the eurytopic species L. forficatus increased (Fig. 12). This species is considered to indicate progress in ruderalisation. The dominance of the Mediterranean element H. illyrica did not change after the Danube damming. Among the eurytopic species, the species L. mutabilis was dominant in this locality. In 1996, the strongly hygrophilous species Lithobius agilis was recorded there for the first time. In spite of the increase in the ground water level and increase of humidity in this locality, the existence conditions for the thermophilous species H. illyrica, Cryptops anomalans and Clinopodes linearis have been preserved. These species do not occur in the by-passed zone.

The terrestrial Isopod taxocoenosis in this locality consisted of 7 species (Fig. 13), among which the hygrophilous species Hyloniscus riparius, Porcellium collicolum and P. conspersum were recorded as late as 1996-1997. Recording of these species indicates a moderate shift of this locality to more humid habitats. The eurytopic species Trachelipus rathkii (Fig. 14) was dominant in 1993-1996. The species similarity of the one-year samples began to change as late as in 1996-1997 (Tab. 5).

Investigation of amphibians ran in the years 1990, 1993, 1994, 1996, 1997. Only two Rana species of frogs were recorded in this locality in 1990-1993. A stronger increase of species number began in 1994, when the stenotopic species H. arborea and R. arvalis were also recorded. These species indicate a natural character of wetland habitats, good water quality, and its sufficient quantity. In 1996 all three Rana species were recorded which are permanently bound on water. An increase in number of species was also recorded in 1997. It could indicate an improvement in environmental conditions for amphibians due to an increase of the ground water level and filling of natural depressions by water after the Danube damming.

Taxocoenosis of small terrestrial mammals was studied in the forest steppe (Crataegetum danubiale) and in the flood-plain forest. In 1997, as in the preceding years, the xerothermis character of the forest steppe was manifested by occurrence of Crocidura species and by the stepicolous species Microtus arvalis and Apodemus sylvaticus. Their abundance (54 ind. ha^-1 ) was very high for a xerophilous ecosystem. The relatively high abundance was caused by immigration of individuals from the adjacent flood-plain forest showed that the small mammals had a rich food offer in the flood-plain due to sufficient soil moisture.

Altogether 65 species of Rhopalocera were recorded in monitoring plot MP 3 (53 species from 1990 to 1992, 62 species from 1993 to 1997). Considerable qualitative and quantitative changes were observed in the butterfly community after the Čunovo reservoir was filled, Fig. 15. The most significant change was the decline in population of the xerothermophilous species Arethusana arethusa (in Slovakia rare and locally distributed), which disappeared in 1996-1997. Similarly, the abundance of two other significant bioindicators - Pontia daplidice and Issoria lathonia decreased (in 1997 P. daplidice was not recorded at all). After 1992 and 1993, the aridisation slow down was indicated also by other rare xerothermophilous species - Adscita globularie, Zygaena punctum and Z. laeta - which occurred there regularly from 1989 to1993. Z. laeta is included in the Red book [33]. The aridisation slow down in Ostrovné lúčky was indicated also by the abundance increase or first records of some mesohygrophilous or hygrophilous species. During the monitoring period, the mesohygrophilous species Hesperia comma and the hygrophilous Lycaena dispar were recorded in this locality for the first time in 1994. The abundance of the mesohygrophilous to hygrophilous species Araschnia levana increased in comparison with the state before the Čunovo reservoir filling. All these facts indicate the strong reduction of aridisation in the locality Čunovo - Ostrovné lúčky, after the Čunovo reservoir filling.

Evaluation

Communities were stabilised; a good supply to the locality of ground water resulted in an abundance increase of the hygrophilous elements, in some taxocoenoses increased proportion of hygrophilous and eurybiont species (Oniscidea: Hyloniscus riparius), the development conditions for amphibians and small terrestrial mammals have been improved and proportion of xerothermophilous species decreased. In general, the communities corresponded to a drier type of soft-wood flood-plain forest. The long-term increase of the water level in depressions caused a shift of the epigeic and edaphic shore communities into higher places.

The animal taxocoenoses inhabiting the xerothermophilous plant community Crataegetum danubiale were stabilised and no significant changes were recorded in them during the monitoring. Their structure corresponded to the xerothermophilous character of vegetation, the communities exhibited a high species diversity and alpha-diversity, high proportion of xerothermophilous elements, abundant representation of euryecious species, and penetration of mezohygrophilous species from the adjacent flood-plain forest (Coleoptera, Curculionidae, Lepidoptera, small terrestrial mammals). In some taxocoenoses (Lepidoptera) the proportion of xerothermophilous species decreased. After filling of the Čunovo reservoir, the ground water level increased, some deeper depressions were filled by water and process of the locality aridisation was stopped.

MP 4 Topoľové hony

The monitoring plot MP 4 Podunajské Biskupice-Topoľové hony was founded in the hard-wood flood-plain forest (Ulmo-Quercetum covallarietosum). It belonged to the driest habitats in the framework of the monitored flood-plain forests. The floods in this locality had been absent for a long time before the start of the Gabčíkovo structures construction. The ground water level was very deep and soil moisture was not influenced by rising the ground water level after filling of the Čunovo reservoir.

The land snails community had an unambiguously mesohygrophilous character during the whole investigation period and was dominated by the forest species Aegopinella nitens and euryecious species Punctum pagmaeum. The taxocoenosis structure was typical of the hard-wood flood-plain forests. The community consists first of all of the species which do not tolerate floods (A. nitens, Monachoides incarnatus, Cochlodina laminata, Alinda biplicata and the euryecious species P. pygmaeum). After the eight-year monitoring it can be concluded that the malacocoenosis structure was stabilised in spite of the penetration of some xenocoenous species which occurred especially along the plot margins. Changes in 1990-1997 of species number of terrestrial mollusc in four localities in the backwater zone are presented in Fig. 1.

This monitoring plot lay in a larger distance from the Danube. Since the Danube damming and the filling of the Čunovo reservoir, it is exhibited the lowest abundance of small terrestrial mammals. The fact that the sample taken in autumn 1997 exhibited the highest species number among all studied plot was caused by its surrounding by the arable land and by favourable conditions which were indicated by the occurrence of Pitymys subterraneus which occurred there already in 1990 - hence before the structures operation start. P. subterraneus requires a relatively high humidity and does not tolerate the mechanical or chemical disturbance of the soil.

Evaluation

The taxocoenoses structure of land snails (gastropods) and small terrestrial mammals, and its stability, allow concluding that the structures operation had no significant influence on this locality. The observed changes in taxocoenoses structure and in population size of some species lay within the limits of usual fluctuations.

Waterfowls

In a frame of an international winter census of waterfowl organised by the IWRB (International Waterfowl and Wetland Research Bureau) such a census was carried out on the Danube from 1991-1998 and, since 1992, also on the Čunovo reservoir, by-pass canal, tail-water canal, and the seepage canals. Each census was carried out within one day with the participation of many ornithologists. It is to be stressed that the census results were considerably influenced by the winter character, especially by the extent of ice cover on the surface of standing waters, the water flows throughout Europe, and by the conditions at the time of census. Also the whole European population trends influenced, to a certain degree, the observed numbers of individual species.

The course of waterfowl numbers during the January censuses on the Slovak Danube section during 1991-1998 are presented in Fig. 16. In general it can be concluded that the number of waterfowl on the Danube gradually increased from 1991 up to 1993 when it culminated. Then it dropped suddenly (the drop in 1994 probably resulted from a moderate winter). In the next period, the maximal number of water fowl was recorded in 1995. After 1995 its number decreased until 1998, when it moderately increased. The general trend of the waterfowl number in the January censuses is very moderately and insignificantly decreasing. Except for the winter of 1998, when 61,6% of the waterfowl were recorded on the by-pass canal and Čunovo reservoir, the waterfowl in all years preferred the original Danube canal (55,9-75,6% of all recorded birds). This can be explained by a warming of the water in the original Danube canal by seeping warm ground water, which prevents freezing of the water surface even during long frosty periods. Besides this, except for 1998 and 1996, the trends in waterfowl number observed on the Čunovo reservoir conformed with those on the original Danube canal. This means that waterfowls use both, the reservoir and the original Danube river bed, to an equal extent. This is, however true only generally and some species clearly preferred only one of these habitats. According to the data [1, 17] Ardea cinerea, Anas platyrhynchos, Bucephala clangula and Cygnus olor (except for 1997-1998) prefer the Danube original canal; on the contrary Aythya ferina prefers all artificial water bodies, while Tachybaptus ruficollis only the seepage canals. In this connection it is to be stressed that concentration of waterfowl on the Čunovo reservoir and by-pass canal was considerably influenced by hunting and poaching. Just after the start of the shooting season, the waterfowl behaviour changes suddenly. The birds become very shy and escape away from humans to larger distances. From this point of view, the Čunovo reservoir and by pass canal offer better conditions to the waterfowl. They allow the birds to concentrate at a larger distance from the banks, and to better escape the range of shooting or to better register the hunters or poachers on boats earlier than when the shooters can hide themselves in the river bank vegetation along the original Danube canal. In this regard, and only in this regard, the vegetation free banks of the by-pass canal are even more secure for the waterfowl then the Čunovo reservoir shores, with irregularly dispersed reed tufts.

In individuals years, the number of water fowl species recorded during the winter census (Fig. 17) ranged from 23 to 32. The recorded number of waterfowl species generally had a moderately increasing trend, in the Gabčíkovo structures this trend was steeper (Fig. 18). In general, the number of species recorded on the original Danube canal was a little higher than on the structures and represented 52,2-62,5% of the total number. The only species in which the individual numbers increased during the censuses was Tachybaptus rufucollis. This species is not an object of hunting. In addition, its concentration on seepage canals where the hunters did not focus increased its security.

The most abundant wintering species on the Danube was Anas platyrhynchos whose abundance ranged from 5209 to 25701 individuals. The next most abundant species were Aythya ferina, A. fuligula, Phalacrocorax carbo, Anser fabalis, Larus ridibundus and Larus canus. In 1992 – 1997, the majority of water fowl (55,8 – 75,6% of individuals) was concentrated on the original Danube canal, only in 1998, during the very moderate winter, were 61.1% of birds in the Gabčíkovo structures. This confirmed the opinion that birds prefer the warmer water in the original canal. Except for 1994, the number of species was higher (or equal in 1995) on the old Danube river bed than on other water tables in the area of the Gabčíkovo barrage step (52,2 – 62,5%).

The by-pass canal zone

Five monitoring plots with different ground water regimes were situated in the by-pass zone ranging from Čunovo to Sap. The drainage influence of the original Danube main river bed worsened the existence conditions for the soft wood flood-plain forest - the willow stands along the river banks began to dry up as well as the depressions filled previously by water, and the arms which have no contact with the arm system supplied by water from the intake structure at Dobrohošť. At present, the ground water flows from the supplied river arms toward the original Danube river bed. In a strip along the original main river bed whose width ranges, according to the course of the main arms from 80-250 m, the ground water level had no contact with the capillary zone of the trees´ rhizosphere. As a consequence of this, the leaves fell before their time and the production stands in this strip withered away [31].

MP 6 Dunajské kriviny

After the Danube damming, the largest changes were observed just on the monitoring plot MP 6. As a consequence of the draining effect of the original Danube river bed the old side arm was not filled by water. The ground water level stabilised more than 2 m lower than before the Danube damming. The position of this locality, above the intake structure at Dobrohošť supplying the river arm system with water from the by-pass canal, also complicated the situation. Changes in the previous ground water level regime and an absence of floods (the last one reaching a majority of this monitoring plot was in 1991) cause serious changes in the ecosystem. While in 1991 the ground water level moved at the depth of 2,5-3 m, since November 1992 it stabilised with minimal fluctuations at the depth of 4,2 m. Thus the preconditions for of the driest types of flood-plain forests arose. The soil moisture ranged from 15-25%. Originally. this plot was grown by an elm-ash stand with admixed poplars growing in thick layers of sandy sediments.

During the three years before the Danube damming the land snails community had a structure typical of moderately moist varieties of the flood-plain forests. Moisture sufficiency was indicated by the wetland land snails Vertigo antivertigo, Carychium minimum and Zonitoides nitidus. After the Danube damming their abundance and dominance  decreased suddenly. Changes in the number of recorded species is presented in Fig. 19,

After the Danube damming the original malacocoenoses, typical of soft-wood flood plain forests with a predominance of hygrophilous and polyhygrophilous species, changed to a mesohygrophilous community characteristic of the transitional flood-plain forest (Fig. 20). Only the deeper depressions working as refuges for the moderately hygrophilous species represent an exception [3]. Polyhygrophilous species V. antivertigo and Pseudotrichia rubiginosa disappeared in 1993. Z. nitidus did not occur in the samples from 1995. Changes of the between year species similarity are presented in Tab. 6. In 1991 the flood considerably reduced populations of some stenoecious species of land snails. Populations of some of them have not recovered (Vertigo antivertigo, Oxyloma elegans).

Restoration of the hygrophilous community was possible in the closest surrounding of the arm on the locality perimeter which, since 1995, was filled by the water from the intake structure during the simulated floods. The simulated flood in 1997 had great efficiency. Also, at that time the depression in the central part of the locality was filled by seeping water.

Altogether 12 chilopod species were recorded in the community until 1993 (Fig. 21). Among the regularly occurring species which indicated a higher degree of soil moisture (from 20-30%) the mesohygrophilous species Lithobius curtipes, L. crassipes, Necrophloeophagus flavus and L. microps were found. The last species occurred for the last time in 1991. The eurytopic species L. forficatus (Fig. 22) belongs to the eudominant species. L. mutabilis penetrated regularly from the monitoring plot surroundings. In 1992 the species Lithobius lucifugus and L. cyrtopus, untypical for this area, were found. They were probably brought by water from the mountain forested areas.

A clear shift toward a drier habitat type began in this locality. This was connected with rearrangement of ground surface of immediate vicinity of the canal supplying the branch system with water from the by-pass canal. The moisture in the upper layers of the surface humus moves in the interval of 0-20%. This fact was reflected in species composition of the centipede taxocoenosis. In the first year after the Danube damming (1993), the species number dropped considerably on the lowest level observed so far (Fig. 21) and, at the same time, the dominance of the eurytopic species L. forficatus not requiring a high moisture (Fig. 22), increased suddenly. The high dominance of this species was maintained in the next years. The dominance increase in L. forficatus was probably caused by individuals penetrating from the surrounding localities after the recession of conditions typical of flood-plain forests. At the same time, dominance of the mesohygrophilous species L. curtipes, which is characteristic for the Danubian flood-plain forests, was observed. In contrast, dominance of L. crassipes which is more tolerant of soil moisture changes, did not change in this locality. In the years after the Danube damming, the species Clinopodes flavidus, L. microps, L. lucifugus and L. cyrtopus were not recorded in monitoring plot MP 6. The former two species were untypical of this territory. In contrast, after the Danube damming the species Lithobius lapidicola, L. pussilus and L. calcaratus were found. All of these do not need a high soil moisture. In 1995, the number of the recorded species increased to 12 (Fig. 21) and dominance of the eurytopic L. forficatus decreased considerably (Fig. 22). These changes could be caused by the artificial summer flood. Species similarity of the one year catches between the year 1993 and period 1994-1997 ranges from 50-60% (Tab. 7).

During the investigation period in monitoring plot 6, five species of terrestrial isopods were recorded (Fig. 23). The eurytopic species Trachelipus rathkii and Hyloniscus riparius, preferring very moist habitats (Fig. 24), were eudominant. The taxocoenosis species compositions changed as late as in 1997 when Haplophthalmus mengii indicating that the soil moisture increase was recorded for the first time. The species similarity of the one year catches was very high after the Danube damming.

From the beginning of monitoring this locality has been characterised by a relatively low cumulative abundance of ground beetles, which did not exceed 500 individuals per 10 pitfall traps and vegetation period. In the first years, the bottom of an arm network between small islands on which the traps were exposed was regularly flooded. This resulted in a relatively high dominance of the hygrophilous species Agonum moestum (13,35%) (Fig. 25), Oxypselaphus obscurus (22,39%)(Fig. 26), Patrobus atrorufus (11,87) (Fig. 27) and Platynus assimilis (3,56%)(Fig. 28). Other hygrophilous or ripicolous species (e. g. Bembidion dentellum, Bembidion biguttatum, Oodes helopioides) occurred individually.

The abundance of the hygrophilous species dropped considerably in 1993 or some species ceased occurring after 1993. In contrast, for 1992 and 1993, the abundance of the species needing less moisture, especially Carabus granulatus (1993: 21,59%)(Fig. 29) and Pterostichus niger (1992: 25,52%)(Fig. 30), increased strongly. This state lasted two to three years. In 1994-1995, the abundance of these species decreased again and in 1997 they were not recorded. In 1994 an invasion of the expressively xenocoenous species Pseudoophonus rufipes started, and it culminated in 1995 (17,12% of all carbids). The course of its invasion coincided in time with the individual occurrence of several xerophilous species represented especially by Licinus depressus and with some Amara species. The invasion intensity of Pseudoophonus rufipes, however, did not depend only the habitat changes in monitoring plot 6, but to a considerable degree on the intensity of its migration during warm nights in the second half of July and in August. The most stable abundance, during the whole investigation period, (about 10%) was found in Stomis pumicatus which does need a high moisture.

The succession form the state in 1989 to that of 1997 was characterised by a continuous decrease of three similarity indices (Fig. 31) between the one-year catches of 1990-1997 and the catch of 1989. Their values ranging between 43-62% decreased to the values ranging from 16 to 30%. The smallest decrease was observed in the species similarity index, while the largest one was in the proportional similarity index (dominance identity).

When compared with other monitoring plots, the carabid community in Dunajské kriviny (MP 6) was most affected by deep structural changes resulting from an about two-meter decrease of the ground water level, the absence of natural flood and insufficient efficiency of the simulated flood, as well as by the large changes in the vegetation cover. Extraordinarily strong colonies of ants (Lasius spp.) contributed to the community degradation.

In the mite (Acari) taxocoenosis, the abundance of all trophic groups increased [26], as well as the proportion of mycetophags and detritophags (cumulative dominance 47,3%). The increase of their abundance was especially observed in two last years. In contrast, the species number decreased from 46 species in 1994, to only 29 in 1997. The decrease in abundance of the medium sized and large predators could be caused by an increase of the soil humidity due to a supply of water from the near intake structure.

The amphibian taxocoenosis was monitored in the years 1991, 1994 and 1997. A high species number was recorded in the first two years (Fig. 32). Still in 1994, the drop of the ground water level after the Danube damming did not influence this taxocoenosis significantly, but in 1997 the species number decreased to 2. This decrease was obviously caused by the drying of this locality. In 1991 the species B. bombina and R. esculenta were eudominant, while in 1994 B. bombina and R. lessonae. An increase of dominanced R. lessonae, which is not as closely bound to water as R. esculenta, was also caused by the decrease of the ground water level and drying of this habitat. Changes of the species composition of this taxocoenosis are demonstrated by Jaccard´s index of species similarity (Tab. 8), indicating a significant change when compared with the original state.

Tab. 8. Index of species similarity of the amphibioan taxocoenosis in the monitoring plot Dunajské kriviny.

The taxocoenosis of small terrestrial mammals consisted of 8 species: S. araneus, S. minutus, C. leucodon, M. minutus, A. flavicollis, A. sylvaticus, C. glareolus and M. arvalis. The abundance of S. araneus, Fig. 34, and A. sylvaticus was decreased after 1991, and stabilised at a low level. A moderate decrease in abundance was also recorded in C. glareolus. The abundance of A. sylvaticus culminated in 1995-1996, Fig. 33, and was higher than before the Danube damming. The abundance changes, especially within one year, were not as strong as in the monitoring plots in the back water zone.

Altogether 34 species of Rhopalocera were recorded in monitoring plot MP 6 during the seven-year investigation (23 species in the period 1991-1992, 33 species in the period 1993-1997), Fig. 35.. Significant changes in the species composition of butterflies were observed after putting the structures into operation. Several xerotherophilous species and some species typical of the ruderal habitats began to occur (Carcharodus alceae, Aricia agestis, Erynnis tages, Pyrgus malvae and Leptidea sinapis) and indicated a strong aridisation of this locality.

The bird (Aves) taxocoenosis was monitored in an older willow-poplar stand with scattered Padus racemosa. A considerable portion of this monitoring plot was represented by open areas with some old osiers and well developed E₂ stratum. Depressions were filled by water before the Danube damming (a reed stand (Phragmites australis) of about 5 x 15 m).

The main parameters of the bird taxocoenosis did not change during the monitoring period. The average number of species in one observation in the breeding period was stabile in individual years, while the average number of individuals was moderately decreasing (Fig. 36). Out of the breeding period (Fig. 37), the average number of species in one observation was stabilised, too, but the species number slightly increased. In the breeding period, the average number of species in an individual year ranged from 15 to 21,5, and the average number of individuals from 43 to 59. Out of the breeding period the average number of species ranged form 13,2 to 15,8 and the average number of individuals from 33,6 to 82,6.

Before construction of the intake structure supplying the river arm system with water from the by-pass canal, shrubs grew on the margins of this locality. The endangered species Sylvia nisoria nested in them. After their cutting during the construction of the intake structure, this species stopped nesting in this locality and ceased to occur here even as a non-nesting species.

Evaluation

In spite of the fact that the arm on the perimeter of this locality was sufficiently supplied by water from the intake structure near Dobrohošť and that the year 1997 was hydrologically exceptional, the succession of the terrestrial taxocoenoses to a mesic type of community of the transitional and hard wood flood-plain forests, or even to dry shrub formations, continues. The decrease of abundance and species number of the hygrophilous and typical forest species continues, as well as the penetration of xenocoenous species. Their penetration was moderated in 1997 by more humid and colder weather. The moderate increase of the soil moisture in the last years positively influenced communities of the euedaphon (Acari, Oniscidea, Chilopoda), to a limited extent also the communities of epigeic groups (Coleoptera, Heteroptera) as well as those of the small terrestrial mammals.

The obtained data do not allow saying whether the trend of aridisation moderation observed in the last two years was permanent or whether it represented a climatically conditioned fluctuation. At present, the terrestrial communities in this locality belong to the most degraded in the whole monitored area. On the base of the above facts it is possible to conclude that the antropogenic factors had a very strong impact on this monitoring plot and the simulated flood did not reached it.

MP 07 Žofín

The monitoring plot MP 07 Žofín was not included into the regular monitoring of biota until as late as 1995. Therefore, an observation series which would document a long-term succession trend was not available. This monitoring plot was overgrown by a soft-wood flood-plain forest (Salici-Populetum). The substrate was formed by thick layers of sand and gravel, its humidity ranged from 25% to 35%.

In the years 1995-1997, a total of 10 species of centipedes were recorded here (Fig. 38). This locality was valuable due to the presence of a strong population of the mesohygrophilous species Lithobius austriacus, whose dominance ranged in the years 1995-1997 from 57% to 64%. The high moisture in this locality was confirmed by finding the strongly hygrophilous Lithobius agilis in 1996. The originality of this flood-plain forest is also confirmed by the presence of characteristic species like L. crassipes, L. curtipes, Pachymerium ferrugineum and by a low dominance of the eurytopic species L. forficatus and L. mutabilis. Therefore, the Centipede taxocoenosis in monitoring plot 7 can be considered, for the present, to be characteristic and little disturbed.

Five species of the terrestrial Isopods were recorded in this locality (Fig. 39). The eurytopic species Trachelipus rathkii and the hygrophilous species Porcelium collicolum (Fig. 40) were eudominant.

The taxocoenosis of the terrestrial Isopods did not exhibit significant changes during the three years of the monitoring. The predominance of hygrophilous species indicated a stable and high soil moisture.

Altogether 18 species of Rhopalocera were recorded in monitoring plot 7 during 1993 – 1996. This number is relatively low. Some of the recorded species are typical of the ruderal habitats (Pieris brassicae, P. rapae, P. napi, Inachis io, Vanesa atalanta, Cynthia cardui, Maniola jurtina, Coenonympha pamphilus and Polyommatus icarus). Because the monitoring started in this locality after the start of the Gabčíkovo structures operation, it is not possible to compare the present state with that before 1993.

Evaluation

The terrestrial animal communities had a similar structure as in other monitoring plots in the soft-wood flood-plain forests with adequate soil moisture in the by-pass zone. According to some indices, the communities were not stabilised, some ruderal elements indicate a decrease in humidity. The studied edaphic and epigeic communities have the character of communities of soft-wood flood-plain forests with high soil moisture (Oniscidea, Chilopoda, Heteroptera). An abundant occurrence of xerophilous and thermophilous species and of the widely tolerant species was observed. The proportion of the hygrophilous species also increased in these taxocoenoses in 1996 – 1997, in comparison with the preceding years. From the point of view of the terrestrial communities it would be desirable to enlarge the flooding of the wider surroundings of the river arms. The locality is less endangered by the Gabčíkovo structures operation than by the increasing frequency of tourists (bathing, angling in the near gravel pits). There is not a sufficiently long series of observations for a more detailed evaluation of the changes trend. The monitoring results characterise the present state, which can be only generally compared with the state before the Gabčíkovo structures constructing. At that time, the water communities were characterised by a high species richness, diversity and productivity.

MP 9 Bodícka brána

The monitoring plot MP 9 Bodícka brána is an Salici – Populetum stand. The soil moisture was high and ranged from 25 to 35%. The ground water level oscillated at a depth of 2,0-3,0 m After the Danube damming, floods occurred irregularly or the locality was flooded by flood simulation for a short time. In August 1997, some part of a poplar cultivar stand was clear-cut and in some other parts thinning was carried out. As a consequence of this, the surface was more exposed and further changes in the structure of herbage and shrub strata can be expected, which will result in changes in the taxocoenoses of the terrestrial animals (penetration of eurytopic species and of the open landscape species). The cutting of the poplars contributed to a decrease in soil moisture of 30% to 20%, which creates changes in the vegetation cover.

In this monitoring plot, a land snails community dominated by the species Carychium minimum, Zonitoides nitidus and by the forest hygrophilous species Vitrea crystallina existed even before the Gabčíkovo structures operation started. In the past, the xenocenous species Pupilla muscorum, Monacha cartusiana and Vallonia pulchella occurred in this community. They penetrated from the grassy banks of the original Danube main stream. Species number of the land snails recorded in this monitoring plot from 1990 to 1997 is presented in Fig. 19.

After the Danube damming, this monitoring plot was sporadically flooded and the proportion of hygrophilous land snails decreased considerably, stabilising on a low level. The short-term simulated flood could have a negligible influence on the moisture condition due to the drainage effect of the original river bed and the sandy substrate. It can be concluded that the simulated flood has not yet positively influenced the gastropod taxocoenosis structure. The original hygrophilous community existing in this locality before the Danube damming suffered a considerable decline in the proportion of hygrophilous species and an increase in the proportion of mesohygrophillus forest species and euryecious species (Fig. 41).

In 1994, the dominance of the euryecious species Cochlicopa lubrica increased suddenly. From that time up to 1997, this species belonged together with the forests species Aegopinella nitens, Monachoides incarnatus and with another euryecious species Punctum pygmaeum, to the eudominant species. Changes in community species composition were also reflected by changes in similarity indices of individual one-year samples (Tab. 11).

The so called edge effect caused by the penetration of species preferring sparsely stocked stands or open areas especially Vallonia costata, and to a lesser extent Pupilla muscorum, was observed in this locality. In August 1997, the adjacent stand of poplar cultivars was clear-cut, opening the stand on this monitoring plot. As a consequence of this, changes in the structure of herbage and shrub strata in the stand and in its surrounding were expected as well as subsequent changes in the taxocoenosis of the land snails (penetration of the eurytopic and open landscape species). It is obvious that the structural changes of the mollusc taxocoenosis caused by the clear-cutting of the adjacent stand will partly overlap the changes caused by the operation of the Gabčíkovo structures.

Before the Danube damming, the centipede community consisted of 10 species (Fig. 42). The hygrophilous species Lithobius crassipes and L. curtipes occurred among them and reached a higher dominance than in monitoring plot 6 (Fig. 42). Besides these two species, the hygrophilous species L. agilis also contributed to the community characteristics. It requires a higher degree of soil moisture. The community also included the eurytopic species L. forficatus, L. erythrocephalus and L. mutabilis which occurred and occur in the whole monitored area.

After the Danube damming, the species number decreased on 7 (Fig. 42). Iin the next fewyears it returned to the original level, before the damming. The eurytopic species L. forficatus was dominant during whole monitored period (Fig. 43). The dominance of the characteristic species L. crassipes and L. curtipes oscillated and, in some years, L. crassipes was not recorded at all. Dominance of the hypogeic species Pachymerium ferrugineum increased by 20-30% after the damming (Fig. 43). These changes in the centipede taxocoenosis structure show a shift of this monitoring plot toward the habitats characterised by a drier soil surface. This was confirmed by the soil moisture measurements. The simulated flood in this locality had only a short-term effect.

 

Tab. 12. Between-year species similarity of the centipoed community in MP 9.
1992	46,15
1993	54,55	45,45
1994	54,55	60,00	75,00
1995	38,46	35,71	50,00	50,00
1996	42,86	46,15	54,55	54,55	80,00
1997	46,15	38,46	45,45	60,00	54,55	58,3
	1991	1992	1993	1994	1995	1996

Monitoring plot MP 9 is characterised by the occurrence of seven species of terrestrial isopods. Changes in their number in individual years are presented in Fig. 44. During all years, the eurytopic species Trachelipus rathkii (Fig. 45) belonged to the eudominant species. In the last monitoring year (1997), a sudden drop in dominance of the hygrophilous species Porcellio scaber was recorded. This indicated a shift toward a drier habitat. The changes of species similarity in individual years are given in the Tab. 13.

In 1991, the hygrophilous carabids Platynus assimilis (12,96%, Fig. 46), Patrobus atrorufus (12,41%, Fig. 47), Oxypselaphus obscurus (3,31%, Fig. 48) accompanied by the species Asaphidion flavipes (30,42%, Fig. 49), Carabus granulatus (8,09%, Fig. 50), Pterostichus strenuus (9,65%, Fig. 51) and Clivina fossor (4,32%) which do not need such high moisture or are more tolerant, strongly predominated in the carbids taxocoenosis. Presence of the  hygrophilous species Bembidion dentellum and Badister sodalis significantly contributed to the community characteristics.

In 1993, representation of the more tolerant species Carabus granulatus (11,11%, Fig. 50), Pterostichus melanarius (25,90%, Fig. 52), Pterostichus niger (24,26%, Fig. 53) and Stomis pumicatus (3,89%) increased suddenly, both absolutely as well as relatively. At the same time, representation of all species needing a high moisture decreased absolutely and relatively as well.

In 1994-1996, the abundance of a majority of all species strongly decreased. This decrease resulted in the decrease of the carbid cumulative abundance to ca. 1/3-1/2 of the level reached in 1993 (Fig. 54). The most hygrophilous species, first of all Platynus assimilis (a drop from 106-141 individuals in 1991-1993 to 5-67 individuals in 1994-1997), Clivina fossor (a drop from 21-47 individuals in 1991-1992 to 2-9 individuals in 1993-1997), Bembidion dentellum and Badister sodalis were affected at most. However, also some less hygrophilous or tolerant species, Pterostichus strenuus, Asaphidion flavipes and Oxypselaphus obscurus , were affected in spite of the fact that their abundance in some years even temporarily increased (P. strenuus 1996, A. flavipes 1995, O. obscurus 1997). The fact that the last three species are small and have more chance to find prey indicates that the abundance changes reflected not only the changes in the hydrological regime, but subsequently also a limiting of the food offer. The proportion of representation of individual species in 1997 showed that the moisture conditions improved in this locality in the last year of monitoring. The worst state was observed in 1995.

Since 1993, an invasion of the euryecious xenocoenous species Trechus quadristriatus run in this monitoring plot (1993 - 69 individuals, 1994 - 75 ind., 1995 - 190 ind., 1996 - 165 ind., 1997 - 140   ind.). Its invasion represents a manifestation of the strong degrading changes in the community structure. The trend of community structure changes indicated by the three similarity indices (Fig. 55) is relatively moderate. The values of the index of species similarity were most stable (oscillation range of 41-60%), the index of proportional similarity decreased moderately, the index of abundance similarity decreased the most (from 44,7% to 25,7%).

Tab. 14. Species similarity of the amphibian taxocoenosis in MP 9 in the years 1993-1997

During the monitoring, altogether 8 species (Triturus vulgaris, Bombina bombina, Hyla arborea, Pelobates fuscus, Rana arvalis, R. esculenta, R. lessonae, R. ridibunda) of amphibian were recorded in this plot, while in individual years 2-5 species were observed. The species composition of the taxocoenosis changed considerably during the monitored period (Fig. 56). The largest changes were observed in 1996, when only two species - B. bombina and R. ridibunda were recorded. However, from a long-term point of view (1991-1997), the observed changes are not strong. The species of the R. esculenta complex concentrated close to the discharged Bodícke rameno arm. The taxocoenosis did not change, but in their reproduction period the frogs search for places with a stable water table, or they concentrate in the vicinity of the discharged arm, searching for hatching sites.

Altogether 7 species of small terrestrial mammals were recorded in this plot during the monitoring period: S. araneus, C. leucodon, M. minutus, A. flavicollis, A. sylvaticus, C. glareolus and M. arvalis. The quantitative representation of individual species did not change essentially during the monitored period (1991-1997) and no striking trends in abundance were observed, inclusive of the usual cyclical fluctuations, Fig. 57, 58. It is possible that the Gabčíkovo structures operation inhibited fluctuations which would have appeared under natural conditions.

During seven years of monitoring, altogether 25 species of butterflies were recorded in this locality (18 species in 1991-1992, 21 species in 1993-1997), Fig. 59. After the start of the Gabčíkovo structures operation, new mesohygrophilous and hygrophilous species were recorded in this monitoring plot: Thymelicus lineolus, Erynnis tages, Apatura ilia, Araschnia levana, Lasiommata megera and the strongly hygrophilous species Lycaena dispar). This indicated an improvement in the existence conditions for hygrophilous and some mesohygrophilous species of Rhopalocera in this locality.

Evaluation

After the Danube damming, the moisture conditions did not change significantly (ground water level in the depth of 2-3 m, moisture of the soil surface decreased ca. by 10%), the drying of the soil surface was caused by cutting of the poplars and by subsequent changes in herbage stratum. In the taxocoenoses of the terrestrial animals, the number of hygrophilous species continued to decrease and, at the same time, the proportion of mesohygrophilous  and/or euryecious species increased. Many of the hygrophilous species (Mollusca, Coleoptera) already occur on the observability threshold. This trend slowed down, or stopped, in a part of the hygrophilous species. The abundance of some hygrophilous species even increased again in the monitoring last year (Coleoptera, Carabidae, Lepidoptera, Oniscidea, Chilopoda). The euryvalent or less hygrophilous species (Lepidoptera) preserved their high dominance. In spite of the observed changes, the state of the communities can still be considered reversible (an increase in abundance of some hygrophilous species). It seems that during the two last years a succession of the communities returned to the state which arose immediately after the Danube diversion into the by-pass canal. Stability of this trend is, however, hardly predictable and depends on the interactions of changes of soil moisture, timber exploitation and climatic fluctuations.

Clear cutting of the poplar cultivars stand in August, 1997, caused a further opening of the existing stands. Changes in the structure of the herbage and shrub stratum are expected as well as subsequent changes in the communities of terrestrial animals (penetration of eurytopic or open landscape species). It is possible that the changes caused by these processes can partly overlap the possible changes caused by the Gabčíkovo structures operation. It would be desirable to co-ordinate all major interventions in the monitoring plots (in this case the intensive timber exploitation) with the monitoring of the biota.

MP 10 Kráľovská lúka

The monitoring plot MP 10 - Bodíky - Kráľovská lúka had a gravel substrate. the soil surface layer reached 25-40% moisture, ground water level oscillated between 2-3 m under the surface. The monitoring plot was regularly flooded each year, in some years even twice. In the by-passed zone, MP 10 was the most remote locality from the discharging arms. Thus it was most dependent, until 1997, on the coming of flood. Until 1997, a moderate decline of the ground water level was observed several times (especially in the dry years) and drying of the shore swampy zone. The dead arm, which was a part of this locality, clearly separated a drier habitat, situated closer to the dike and covered by a poplar monoculture, from a humid soft-wood flood-plain forest (Salici-Populetum). Since 1991 no stronger extensive flood occurred. After the damming the average ground water level increased by approximately 1 m.

Before the Danube damming the monitoring plot´s predominantly grown was the Leucojum aestivum (in the direction from the dike behind the dead arm). The predominant land snails were the polyhygrophilous species Carychium minimum and Zonitoides nitidus. other hygrophilous species (Oxyloma elegans, Succinea putris, Pseudotrichia rubiginosa) also reached a strong quantitative representation. In spite of the fact that the willow stand neighbours with the poplar monoculture, invasion of the eurytopic species in this monitoring plot was not observed. The number of species occurring in this plot is given in Fig. 19.

The absence of large floods since 1991 resulted, especially in 1996, in a strong increase in abundance of the mesohygrophilous forest snail Aegopinella nitens (intolerant to floods). In 1997 its abundance decreased moderately, but in spite of this it maintained its eudominant position. In the last years the dominance of the eurytopic forest species Alinda biplicata also increased. This species does not tolerate floods. Among  the strongly hygrophilous species, only Carychium minimum appears in the samples from 1997. Other hygrophilous species (Zonitoides nitidus, Pseudotrichia rubiginosa, Succinea putris and Oxyloma elegans) did not already occur (Fig. 60). By end of 1996 restoration measures were undertaken in this locality which could result in positive changes in the community of land snails, similar to what has already been manifested in the herbage stratum.

The centipede taxocoenosis in this locality consisted of 12 species (Fig 61). These included typical ripicolous and hygrophilous Lamyctes fulvicornis  and the hygrophilous species Lithobius agilis and L. microps. Lamyctes fulvicornis was eudominant and L. aeruginosus and L. curtipes (Fig. 62) were dominant. Before the Danube damming, the centipede community in Kráľovská Lúka was among the richest communities. In the first year after the damming (1993), the species diversity decreased considerably (Fig. 61). Altogether 17 species were recorded here during the period 1991-1997.

After 1993 the stenotopic hygrophilous species L. agilis and the submontane forest species L. cyrtopus were not recorded. L. cyrtopus is a xenocenous element in the flood-plain forest fauna and it entered into this territory with the floods. Construction of the Čunovo dam probably interrupted that way. L. microps was also absent in the centipede community after 1993. Hypogeic species Necrophloeophagus flavus, Pachymerium ferrugineum and Strigamia acuminata recorded before the Danube damming reappeared after a temporary absence as late as in 1997 (an effect of the artificial flooding?). Changes of the centipede community did not finish in 1997; when for the first time Lithobius pelidnus and Pachymerium tristanicum were recorded here. Changes in the species composition of the centipede community indicate unstable conditions in this locality. For the present, the eurytopic Lithobius forficatus and mesohygrophilous species L. aeruginosus, L. crassipes and L. curtipes (Fig. 62) maintained their dominant to eudominant position in this locality. Dominance of the ripicolopus species Lamyctes fulvicornis decreased in 1993-1995, in 1996 it increased again, but it was absent in 1997 (Fig. 62). This fluctuation reflected a gradual drying of the dead arm shores situated closer to the dam. Unstable conditions in this monitoring plot were also reflected in the similarity index, which reached its lowest value between two consecutive years in the period 1992 and 1993 (values on diagonal in Tab. 16).

chapter 23 - part 2