Host diversity and trophic status as determinants of species richness and community composition of fungus gnats

Abstract

Making generalisations on trophic interactions is often limited because studies mostly focus on only a few target systems. Despite the important role of fungivores in forest ecosystems, the determinants of their communities are poorly known. This study examined, for the first time, on the basis of quantitative data, the diversity of fungivorous insects in relation to that of their hosts. Variation in range, species richness and community composition of associated partners was compared among fungus gnats (Diptera: Mycetophilidae, Bolitophilidae) and their mushroom hosts (Basidiomycota: Agaricomycetes) using rarefaction procedures and permutational analysis of variance. DNA barcoding aided identification of fungus gnat species. In the studied boreal forests, 80% of the 100 fungal species and 74% of 460 fruitbodies were colonised by fungus gnats. Each infested mushroom species hosted 1 to 12 gnat species, with 37 gnat species reared from 1 to 41 fungal species. Mushroom and fungus gnat species were both associated with three partner species at median. While most of the common gnat species could be considered oligo- or polyphagous, this study showed that earlier works have erroneously categorised some ubiquitous species as polyphagous in the absence of quantitative data. Species richness of fungus gnats and mushrooms correlated strongly at the level of host genera. Regarding partners community composition, analysing frequencies of associations between all pairs of partner species revealed no significant differences among fungus gnats but distinguished between the saprotrophic and ectomycorrhizal mushroom genera. Our results provide empirical evidence to substantiate a recent hypothesis that fungal trophic status affects, via fruitbody characteristics, the community structure of associated organisms. Presented evidence suggests oligophages with host preference to prevail among mushroom-feeding fungus gnats, thereby distinguishing these from many other studied insect guilds, largely composed of broad generalists and strict specialists.