Perkins Matthew J, McDonald Robbie A, van Veen F J Frank, Kelly Simon D, Rees Gareth, Bearhop Stuart
Centre for Ecology and Conservation, University of Exeter, Penryn, Cornwall, United Kingdom.
Environment and Sustainability Institute, University of Exeter, Penryn, Cornwall, United Kingdom.
PLoS One. 2014 Mar 27;9(3):e93281. doi: 10.1371/journal.pone.0093281. eCollection 2014.
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.
氮（δ(15)N）和碳（δ(13)C）的稳定同位素比率越来越多地被用于量化营养结构，不过相对较少的研究测试过同位素结构测量方法的准确性。对于跨越四个营养级的实验室饲养和野外采集的植物 - 无脊椎动物食物链，我们使用δ(15)N估计氮范围（NR），使用δ(13)C估计碳范围（CR），它们分别用于量化食物链长度和营养资源的广度。在一系列已知的食物链长度范围内，我们研究了NR和CR在食物链内部和之间是如何变化的。我们对结构的同位素估计是可靠的，因为它们是使用重采样程序计算得出的，该程序将样本均值的方差传播到最终估计值的量化不确定性中。为了确定NR和CR估计值不确定性的来源，我们还研究了营养级之间和食物链之间的分馏（即从源到消费者的δ(15)N或δ(13)C的变化）变化情况。δ(15)N分馏显示出显著的富集，而富集的变化是物种和系统特异性的，范围广泛（1.4‰至3.3‰），重要的是，它将变化传播到了随后的NR估计值中。然而，NR被证明对这种变化具有鲁棒性，并且能够很好地区分食物链长度，尽管较长食物链之间存在一些重叠，这意味着需要意识到这种局限性。δ(13)C分馏并不一致；通常未观察到变化或仅有小的显著富集。因此，随着食物链长度的增加，CR的估计值变化不大，这表明δ(13)C作为能量途径示踪剂的潜在效用。这项研究是对食物链结构同位素量化的有力测试，鉴于全球对水生食物链的估计约为四个营养级，而许多食物链包括无脊椎动物，我们使用四个营养级的植物 - 无脊椎动物食物链使得我们的研究结果与大多数生态系统相关。
