Effects of food quality, starvation and life stage on stable isotope fractionation in Collembola

Naturally occurring stable isotopes of carbon and nitrogen are powerful tools to investigate food webs, where the ratio of 15N/ 14N is used to assign trophic levels and of 13C/ 12C to determine the food source. A shift in δ 15N value of 3‰ is generally suggested as mean difference between two trophic levels, whereas the carbon isotope composition of a consumer is assumed to reflect the signal of its diet. This study investigates the effects of food quality, starvation and life stage on the stable isotope fractionation in fungal feeding Collembola. The fractionation of nitrogen was strongly affected by food quality, i.e. the C/N ratio of the fungal diet. Collembola showed enrichment in the heavier isotope with increasing N concentration of the food source. Δ 15N varied between 2.4‰, which assigns a shift in one trophic level, and 6.3‰, suggesting a shift in two trophic levels. Starvation up to 4 weeks resulted in an increase in the total δ 15N value from 2.8‰ to 4.0‰. Different life stages significantly affected the isotope discrimination by Collembola with juveniles showing a stronger enrichment (Δ 15N=4.9‰) compared to adults (Δ 15N=3.5‰). Δ 13C varied between −2.1‰ and −3.3‰ depending on the food quality, mainly due to compensational feeding on low quality diet. During starvation δ 13C value decreased by 1.1‰, whereas the life stage of Collembola had no significant effect on isotopic ratios. The results indicate that the food resource and the physiological status of the consumer have important impact on stable isotope discrimination. They may cause differences in fractionation rate comparable to trophic level shifts, a fact to consider when analysing food web structure.