Assessing trophic position from nitrogen isotope ratios: effective calibration against spatially varying baselines
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Nitrogen isotope signatures (δ15N) provide powerful measures of the trophic positions of individuals, populations and communities. Obtaining reliable consumer δ15N values depends upon controlling for spatial variation in plant δ15N values, which form the trophic ‘baseline’. However, recent studies make differing assumptions about the scale over which plant δ15N values vary, and approaches to baseline control differ markedly. We examined spatial variation in the δ15N values of plants and ants sampled from eight 150-m transects in both unlogged and logged rainforests. We then investigated whether ant δ15N values were related to variation in plant δ15N values following baseline correction of ant values at two spatial scales: (1) using ‘local’ means of plants collected from the same transect and (2) using ‘global’ means of plants collected from all transects within each forest type. Plant δ15N baselines varied by the equivalent of one trophic level within each forest type. Correcting ant δ15N values using global plant means resulted in consumer values that were strongly positively related to the transect baseline, whereas local corrections yielded reliable estimates of consumer trophic positions that were largely independent of transect baselines. These results were consistent at the community level and when three trophically distinct ant subfamilies and eight abundant ant species were considered separately. Our results suggest that assuming baselines do not vary can produce misleading estimates of consumer trophic positions. We therefore emphasise the importance of clearly defining and applying baseline corrections at a scale that accounts for spatial variation in plant δ15N values.
KeywordsStable isotope analysis Trophic structure Biogeochemistry Scale-dependence Nitrogen cycling Selective logging
We thank staff at the Danum Valley Field Centre, especially Bernadus Bala Ola for the identification of plant samples; Adam, Dedy Mustapha and Anthony Karolus for the fieldwork assistance and Glen Reynolds and Apech Karolus for the logistical support and advice. Tom Fayle, Noel Tawatao and Sukarman Sukimin helped with ant identification, and Elly van der Linde assisted with sample preparation and isotope analysis. We thank Yayasan Sabah, the Danum Valley Management Committee, the State Secretary, Sabah Chief Minister’s Department and the Prime Minister’s Department (EPU) for the permission to conduct the research. This study is part of the Royal Society’s Southeast Asia Rainforest Research Programme (Project No. RS266). PW was supported by an Earth and Biosphere Institute studentship from the University of Leeds, and the work was supported by a grant from the Leverhulme Trust.
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