title: Application of nitrogen and carbon stable isotopes (δ(15)N and δ(13)C) to quantify food chain length and trophic structure.
abstract: Increasingly, stable isotope ratios of nitrogen (δ(15)N) and carbon (δ(13)C) are used to quantify trophic structure, though relatively few studies have tested accuracy of isotopic structural measures. For laboratory-raised and wild-collected plant-invertebrate food chains spanning four trophic levels we estimated nitrogen range (NR) using δ(15)N, and carbon range (CR) using δ(13)C, which are used to quantify food chain length and breadth of trophic resources respectively. Across a range of known food chain lengths we examined how NR and CR changed within and between food chains. Our isotopic estimates of structure are robust because they were calculated using resampling procedures that propagate variance in sample means through to quantified uncertainty in final estimates. To identify origins of uncertainty in estimates of NR and CR, we additionally examined variation in discrimination (which is change in δ(15)N or δ(13)C from source to consumer) between trophic levels and among food chains. δ(15)N discrimination showed significant enrichment, while variation in enrichment was species and system specific, ranged broadly (1.4‰ to 3.3‰), and importantly, propagated variation to subsequent estimates of NR. However, NR proved robust to such variation and distinguished food chain length well, though some overlap between longer food chains infers a need for awareness of such limitations. δ(13)C discrimination was inconsistent; generally no change or small significant enrichment was observed. Consequently, estimates of CR changed little with increasing food chain length, showing the potential utility of δ(13)C as a tracer of energy pathways. This study serves as a robust test of isotopic quantification of food chain structure, and given global estimates of aquatic food chains approximate four trophic levels while many food chains include invertebrates, our use of four trophic level plant-invertebrate food chains makes our findings relevant for a majority of ecological systems.
authors.affiliation: Centre for Ecology and Conservation, University of Exeter, Penryn, Cornwall, United Kingdom.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Perkins
authors.affiliation: Environment and Sustainability Institute, University of Exeter, Penryn, Cornwall, United Kingdom.
authors.forename: Robbie A
authors.initials: RA
authors.lastname: McDonald
authors.affiliation: Centre for Ecology and Conservation, University of Exeter, Penryn, Cornwall, United Kingdom.
authors.forename: F J Frank
authors.initials: FJ
authors.lastname: van Veen
authors.affiliation: Food and Environment Research Agency, York, Yorkshire, United Kingdom.
authors.forename: Simon D
authors.initials: SD
authors.lastname: Kelly
authors.affiliation: Food and Environment Research Agency, York, Yorkshire, United Kingdom.
authors.forename: Gareth
authors.initials: G
authors.lastname: Rees
authors.affiliation: Centre for Ecology and Conservation, University of Exeter, Penryn, Cornwall, United Kingdom.
authors.forename: Stuart
authors.initials: S
authors.lastname: Bearhop
mesh_terms: D000818:Animals; D001042:Aphids / Q000469:parasitology / Q000502:physiology*; D002247:Carbon Isotopes / Q000378:metabolism; D004175:Diptera / Q000469:parasitology / Q000502:physiology*; D017753:Ecosystem; D020387:Food Chain; D006927:Hymenoptera / Q000502:physiology*; D007814:Larva / Q000469:parasitology / Q000502:physiology; D009587:Nitrogen Isotopes / Q000378:metabolism; D014908:Triticum / Q000254:growth & development / Q000469:parasitology; D003313:Zea mays / Q000254:growth & development / Q000469:parasitology
chemical_list: D002247:Carbon Isotopes; D009587:Nitrogen Isotopes