The Isotopic Signature of the “Arthropod Rain” in a Temperate Forest: Wingless Are From Crowns, Winged Are From Soil

Invertebrate phyto-, sapro-, and microbophages, as well as predators and parasitoids, densely populate tree crowns. Eventually, all crown inhabitants fall from the trees and become a food source for litter-dwelling predators, scavengers, and saprophages. However, the functional significance of the arthropod rain, i.e., the flux of invertebrates falling from the crowns, remains unexplored. We collected arthropod rain in a temperate mixed forest throughout the growing season. The δ13С and δ15N values of the arthropods (730 samples in total) were compared to a large reference dataset of the isotopic composition of soil animals from temperate forests. The most numerous taxa in the arthropod rain were collembolans and mites. The most diverse orders were Diptera (18 families) and Coleoptera (29 families), which formed the major portion of the winged specimens. The total ranges of δ13С and δ15N values of individual animals forming arthropod rain reached 14‰ and 26‰, respectively. Nevertheless, invertebrates forming arthropod rain were on average depleted in 13C and 15N by 1.6‰ and 2.7‰, respectively, compared to the soil-dwelling animals. This difference can be used to detect the contribution of the arthropod rain to detrital food webs. Low average δ13С and δ15N values of the arthropod rain were primarily driven by the presence of microphytophages, represented mainly by Collembola and Psocoptera, and macrophytophages, mainly aphids, caterpillars, and heteropterans. Furthermore, wingless arthropods were depleted in heavy isotopes relative to winged specimens. Among wingless invertebrates, predators and parasitoids differed significantly in δ15N values from phytophages and microbi/saprophages. In contrast, there was no consistent difference in δ values between saprophages and predators among winged insects, all of them being similar in the isotopic composition to soil-dwelling invertebrates. This result suggests that winged insects in the arthropod rain represented a random assemblage of specimens originating in different biotopes, but most were tightly linked to soil food webs. Overall, our data suggest that invertebrates falling from the crown space and flying arthropods originating from the soil are an important channel connecting food webs in tree crowns and in the soil.