Distribution and Abundance of European Earthworm Species in Irtysh Forests

The spread of invasive species of earthworms of predominantly European origin has occurred in the forests of Siberia since the 1980s. The introduction of new lumbricid species, known as ecosystem engineers, can lead to changes in forest communities and their ecosystem functions (climate regulating, water and soil protective, sanitary and hygienic, and recreational). The aim of this research is to study the distribution, abundance, and tolerance of allochthonous species of earthworms of European origin in the forests of the Irtysh region. The main hypothesis is that the occurrence of invasive species is determined by the tolerance degree to edaphic factors (content of organic substances, pH values, electrical conductivity, and soil moisture and temperature). The studies were carried out during the growing seasons of 2018, 2022, and 2023 in Rudnyi Altai region and in the south of the West Siberian plain. The total length of the study area is more than 1500 km; in total, 65 sample sites have been surveyed. Samples have been taken during direct soil excavation. In total, 306 samples have been analyzed, 1133 specimens of earthworms of invasive European species have been identified. Soil conditions have been determined at each sample site layer by layer for each sample. To analyze the correlation between the occurrence of earthworms and their tolerance to edaphic factors, the range of data for each factor and the percentage of the sites with detected presence of the earthworms have been determined for each species. The Irtysh region is inhabited by six allochthonous species (Aporrectodea caliginosa, Aporrectodea rosea, Lumbricus rubellus, Octolasium lacteum, Dendrobaena octaedra, and Eiseniella tetraedra). A. caliginosa is characterized by the greatest range (1450 km) and the highest occurrence (43% of samples); this species is also characterized by a wide range of tolerance (85–100% of the range of the studied factors at the sites) to soil water content, density, pH value, and organic matter content. No factors limiting the distribution of the species in southern Siberia have been detected. The least common (75 km) and rare (found in 2% of the sample) is E. tetraedra, a species with a narrow range of tolerance (5−22%) to all edaphic factors. The occurrence of invasive species correlates significantly with the range of their tolerance to soil organic matter content (r = 0.89) and soil density (r = 0.77). The ranges of tolerance to all invasive species in regard to all studied edaphic factors intersect with those of the native species, Eisenia nordenskioldi.