Data from: Hitting the moving target: modelling ontogenetic shifts with stable isotopes reveals the importance of isotopic turnover

Ontogenetic niche shifts are widely prevalent in nature and are important in shaping the structure and dynamics of ecosystems. Stable isotope analysis is a powerful tool to assess these shifts, with δ15N providing a measure of trophic level and δ13C a measure of energy source. Previous applications of stable isotopes to study ontogenetic niche shifts have not considered the appreciable time-lag between diet and consumer tissue associated with isotopic turnover. These time-lags introduce significant complexity into field studies of ontogenetic niche shifts. Juvenile Chinook Salmon (Oncorhynchus tshawytscha) migrate from freshwater to marine ecosystems, and shift their diet from feeding primarily on invertebrates to feeding primarily on fish. This dual ontogenetic habitat and diet shift, in addition to the long time-lag associated with isotopic turnover, suggests that there is potential for a disconnect between the prey sources that juvenile salmon are consuming, and the inferred prey sources from stable isotope analysis. We developed a model that considered ontogenetic niche shifts and time-lags associated with isotopic turnover, and compared this ‘ontogeny’ model to one that considered only isotopic turnover. We used a Bayesian framework to explicitly account for parameter uncertainty. Data showed overwhelming support for the ontogeny model relative to the isotopic turnover model. Estimated variables from best model fits indicate that the ontogeny model predicts a much greater reliance on fish prey than does the stomach content data. Overall, we found that this method of quantifying ontogenetic niche shifts effectively accounted for both isotopic turnover and ontogenetic diet shifts; a finding that could be widely applicable to a variety of systems.

个体发育生态位偏移（ontogenetic niche shift）在自然界中广泛存在，对于塑造生态系统的结构与动态具有重要意义。稳定同位素分析（stable isotope analysis）是评估此类偏移的有力工具，其中δ¹⁵N可用于衡量营养级，δ¹³C则可用于表征能量来源。此前利用稳定同位素开展个体发育生态位偏移研究的相关工作，均未考虑同位素周转（isotopic turnover）相关的、日粮与消费者组织间的显著时间滞后效应。此类时间滞后会为个体发育生态位偏移的野外研究带来显著复杂性。幼年期奇努克鲑（Chinook Salmon，Oncorhynchus tshawytscha）会从淡水生态系统洄游至海洋生态系统，其食性也会从主要以无脊椎动物为食，转变为主要以鱼类为食。这种兼具栖息地与食性的双重个体发育偏移，结合同位素周转相关的较长时间滞后效应，意味着幼鲑当前捕食的猎物来源，与通过稳定同位素分析推断出的猎物来源之间，存在出现脱节的潜在可能。本研究构建了同时涵盖个体发育生态位偏移与同位素周转相关时间滞后效应的模型，并将该"个体发育"模型与仅考虑同位素周转的对照模型进行了对比。本研究采用贝叶斯框架（Bayesian framework），显式地对参数不确定性进行量化。数据分析结果显示，相较于仅考虑同位素周转的对照模型，个体发育模型得到了压倒性的实证支持。最优模型拟合得到的估计变量表明，个体发育模型预测的鱼类猎物依赖程度，远高于胃含物数据所反映的实际依赖水平。总体而言，本研究发现，此种量化个体发育生态位偏移的方法，可以同时有效兼顾同位素周转与个体发育食性偏移的影响；这一发现可广泛应用于各类生态系统的相关研究中。