Trophic niche differentiation and utilisation of food resources in collembolans based on complementary analyses of fatty acids and stable isotopes

Abstract

Feeding on a variety of resources, collembolans have been assumed to be food generalists. However, recent stable isotope analyses documented that collembolan species occupy different trophic levels, but detailed studies on species-specific food resources are sparse. Further, it still is little understood whether food resources of collembolan species are constant or shift, e.g., with habitats. Fatty acid (FA) and stable isotope analysis allow insight into utilisation of basal resources and trophic levels of consumers, respectively. We combined these methods to investigate variations in trophic niches of six collembolan species (Lepidocyrtus lanuginosus, Folsomia quadrioculata, Parisotoma notabilis, Ceratophysella denticulata, Isotomiella minor and Protaphorura armata) in beech and spruce forests. We correlated collembolan FAs with that of litter microorganisms at the respective study sites to identify links to resources. FA composition and stable isotope signatures in collembolan species did not differ significantly between forest types but between species suggesting pronounced trophic niche differentiation between species. The trophic niche of C. denticulata and P. armata was similar and significantly differed from the other species with 13C and 15N signatures being least depleted. We suggest that C. denticulata has a predatory life style feeding on nematodes. Due to intermediate to low 13C and 15N levels and specific FA compositions the other collembolan species could be ascribed to secondary decomposers with I. minor being more closely associated with bacterial resources and F. quadrioculata, L. lanuginosus and P. notabilis with fungal resources. However, dietary niches presumably comprise a mixture of resources rather than single resources. FAs in collembolan species correlated poorly with those in litter suggesting that the diet of collembolan species is rather independent of the structure of microbial communities in bulk litter. The results suggest that collembolans consume resources of microsites rather than bulk material thereby benefitting from the small-scale heterogeneity in resource distribution in soil. Low variation in trophic niches with forest type suggests that these microsites are similar across different forests and provide similar food resources despite marked differences in e.g., litter materials and humus structure.