Stable Isotope Trophic Fractionation (13C/12C and 15N/14N) in Mycophagous Diptera Larvae

Abstract

The use of isotopic analysis for reconstructing the structure of food webs requires determination of the trophic fractionation of carbon and nitrogen stable isotopes (Δ¹³C and Δ¹⁵N). Fungi and mycophagous animals play a key role in soil communities, but there are very limited field data on the degree of isotope fractionation in animals that feed on fungi. We studied the bulk isotopic composition of mycetophagous Diptera larvae inhabiting fruit bodies of saprotrophic and mycorrhizal macromycetes, as well as larva feeding on parasitic rust fungi. Trophic enrichment in ¹³С and ¹⁵N was at the minimum (0.0 and 0.9‰, respectively) in the larvae of gall midges Mycodiplosis sp. feeding on rust fungi (Pucciniales). In the larvae of dipterans inhabiting fruiting bodies of saprotrophic and mycorrhizal macromycetes, the Δ¹³C and Δ¹⁵N values averaged 0.9 and 3.4‰, respectively. This corresponds to the values usually observed in grazing food chains. The accumulation of ¹⁵N was more pronounced in the larvae that fed on saprotrophic fungi, but no clear relationship was found between the degree of trophic fractionation and the taxonomic affiliation of animals or fungi. As suggested by our data and the analysis of published studies, the variations in the trophic fractionation in mycophages are strong, but they are not likely to impede the identification of the “mycorrhizal” and “saprotrophic” energy channels in the soil food webs.