Summary
, Liliane Ruess
Naturally occurring stable isotopes of carbon and nitrogen are powerful tools to
investigate food webs, where the ratio of
15
N/
14
N is used to assign trophic levels and
of
13
C/
12
C to determine the food source. A shift in d
15
N 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.
D
15
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N 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 d
15
N value from 2.8% to 4.0%. Different life stages significantly
affected the isotope discrimination by Collembola with juveniles showing a stronger
enrichment (D
15
N ¼ 4.9%) compared to adults (D
15
N ¼ 3.5%). D
13
C varied between
2.1% and 3.3% depending on the food quality, mainly due to compensational
feeding on low quality diet. During starvation d
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C 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.
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