Trophic Structure of the Soil-Dwelling Arthropod Communities at the Border of the Forest and the Steppe in the South of Western Siberia: Isotopic Data

Abstract

Epigeic generalist predators play a crucial role in terrestrial ecosystems, connecting aboveground and belowground food webs. Using stable isotope compositions (δ13C and δ15N values), we assessed the trophic niches of the two main groups of generalist predators (ground beetles (Coleoptera, Carabidae) and spiders (Arachnida: Aranei)), as well as their potential prey (phytophagous and saprophagous insects), plants, and soils in the forest, steppe, and transitional ecosystems located in the forest–steppe of southwestern Siberia. We hypothesized that (1) the trophic niche of carabids is wider than that of spiders, because some ground beetles are omnivorous, and (2) the contribution of invertebrates from the detrital food web (saprophages) to the diet of generalist predators is higher in the “detrital” forest ecosystem than in the steppe, which should be reflected in increased δ13C and δ15N values of the predators in the forest. In total, 16 species of carabid and 17 species of spider were analyzed. The δ15N values of ground beetles suggested a wide range of trophic niches corresponding to two or three trophic levels. Omnivorous carabids of the genera Amara and Harpalus had the minimum δ15N values. The carbon isotope compositions of the ground beetles suggests that most predatory species were predominantly involved in grazing food chains. Spiders had on average increased δ15N values compared to ground beetles, and a relatively narrow range of δ15N values. The isotopic niche occupied by spiders hardly overlapped with the isotopic niche of carabids, which may indicate a significant difference in the range of consumed resources. Thus, our data suggest a significant difference in the trophic niches of the key generalist predators, ground beetles and spiders, which was observed both in the forest and the steppe habitats. Spiders appear to be more closely associated with detrital food webs than ground beetles. Contrary to our expectations, we did not find higher δ13C and δ15N values in ground-dwelling generalist predators in the forest compared to the steppe ecosystem.