Assessing trophic position from nitrogen isotope ratios: effective calibration against spatially varying baselines |
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Authors: | Email author" target="_blank">Paul?WoodcockEmail author David?P?Edwards Rob?J?Newton Felicity?A?Edwards Chey?Vun?Khen Simon?H?Bottrell Keith?C?Hamer |
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Institution: | (1) Institute of Integrative and Comparative Biology, University of Leeds, Leeds, LS2 9JT, UK;(2) School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK;(3) Sepilok Forest Research Centre, Sandakan, Sabah, Malaysia;(4) Present address: School of Marine and Tropical Biology, James Cook University, Cairns, QLD, 4878, Australia |
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Abstract: | 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. |
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