Structure and dynamics of the population of ground beetles (Coleoptera, Carabidae) in the floodplain of the lower Irtysh River

The carabidofauna of river floodplains in Western Siberia has not been adequately studied. More attention has been paid to the faunistic composition and abundance, while the ecological aspects of forming carabid beetle populations are practically not considered. Therefore, a complex study of ecological and faunistic structure of carabid population at different levels of the floodplain relief determines the relevance of this work. The aim of this research was to reveal the regularities of spatial-temporal distribution of ground beetles in the floodplain profile of the Irtysh river within the southern taiga of Western Siberia. We carried out studies in the basin of the Irtysh river lower reaches in the south of Uvatsky district, Tyumen region. We studied the floodplain of the Irtysh river left bank, including three high layers: site with horsetail and motley grass, 58°43,508' N, 68°41,412' E (low level with height 0.5-1.5 m.), rare willow with different herbs, 58°43,540' N, 68°41,477' E (medium level with height 5.5-6 m), and willow with bushes and high grass, 58°43,565' N, 68°41,453' E (high level with height 6.5-7.5 m). We used Barber's method of soil traps to collect soil entomofauna in 2008-2011. As soil traps, we used 200 ml tin cans, 1/3 filled with 4% formalin solution. At the floodplain sites, we spaced 10 traps linearly with an interval of 8-10 m. The material was sampled every 10 days. Time of exposure of traps was from the first decade of June to the third decade of August in 2008, 2010 and 2011; in 2009 it was from the first decade of May to the third decade of August. During the study period, we worked 8699 trap-nights (excluding water-filled cans) and captured 10766 imago beetles. Ecological groups of carabid beetles were considered in terms of biotopic confinement, relation to humidity, phenology, type of nutrition and the soil-plant layer they occupy (life forms). Indicators of dynamic density (D - value that reflects the number of insect specimens caught in a trapping can in recalculation per 100 trap-nights), numerical abundance, Berger-Parker's index of inverse dominance, Shannon's indeces of species diversity and equalization are shown. The nomenclature of Carabidae is given in accordance with the "Catalogue of Palearctic Coleoptera" (2017). In the floodplain, we revealed 92 species of carabid beetles (See Table 1). As the distance from the riverbed grew, a decrease in species richness was noted (See Table 2). The greatest similarity was found between carabid beetle populations of high and medium floodplain level (Kj = 0.5). The composition of dominants is represented by 6-7 species, only one species of Clivina fossor (Linnaeus, 1758) is distinguished by high abundance in all layers (See Table 1). Closer to the riverbed, the dynamic density of beetles increased from 86.3 to 199.6 specimens per 100 trap-nights; an increase in Shannon's indices of diversity and equalization (See Table. 2) was marked. In the floodplain of high and medium levels, Carabus granulatus Linnaeus, 1758, Oxypselaphus obscurus Herbst, 1784, Pterostichus melanarius (Illiger, 1798), Pterostichus niger (Schaller, 1783), and Trechus secalis (Paykull, 1790) prevailed, in the low floodplain Calosoma investigator (Illiger, 1798), C. fossor, Bembidion litorale (Olivier, 1790) and Platynus assimilis (Paykull, 1790) were dominant. At high and medium level sites, among biotopic groups, forest-meadow species were the most abundant, at low level sites these were forest-swampy and near-water species (See Table 3). Towards the riverbed, there was an increase in the relative abundance of riverside and a decrease in the abundance of forest-meadow species. Hygrophilous species were represented by the largest number of species (56.6%), the number of which decreased up the profile from 65.2 to 58%, the number of mesophiles, on the contrary, increased from 28.8 to 42%. Xerophiles in the communities did not exceed 8%. The basis of the trophic structure was formed by zoophages (96%), mainly stratobionts, of which surface and litter-dwelling forms were the most numerous in the low level, litter-dwelling and litter and crevice-dwelling forms were in the medium and high levels (See Table 4). Mixophytophages were only 4% and mainly represented by geochortobionts harpaloid, the most numerous in the low floodplain. Most carabid species reproduce in the spring-summer period. In the seasonal dynamics of the number of ground beetles there were two (20102011) or three (2009-2010) increases in activity (See Figure). The maximum rates of ground beetles' activity were observed in the population of the low floodplain in the second and third decades of June (686.0 and 575.0 specimens per 100 trap-nights, respectively). In general, the seasonal dynamics of the number of beetles was determined by phenological features of dominant species' reproduction, as well as by the height and duration of the spring flood, affecting the abundance of species, mainly with the autumn type of reproduction (Nebria livida (Linnaeus, 1758), T. secalis). Thus, the structure and dynamics of population of carabid beetles in the high and medium floodplain was the most similar, significantly differing from the low floodplain population, due to changes in humidity and soil and plant conditions of the Irtysh floodplain relief. The paper contains 1 Figure, 4 Tables and 34 References. Acknowledgments: The authors express their deep gratitude to RYu Dudko, Cand. Sci. (Institute of Systematics and Ecology of Animals SB RAS, Novosibirsk, Russia) for help in determining the species of ground beetles.

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