Data from: Important impacts of tissue selection and lipid extraction on ecological parameters derived from stable isotope ratios

1. The nitrogen (δ15N) and carbon (δ13C) isotope ratios of animal tissues can help identify the composition of diets and open up a myriad of ecological applications. However, consumers do not ingest or assimilate all components of food items, and it is not well understood how sampling different tissues of sources and consumers may affect isotopic values ascribed, and thereby how such variation affects derived ecological measures. 2. Utilising a simple prey–predator feeding relationship in insects, we examined isotopic differences in soft, exoskeleton and whole tissues using samples with and without lipid extraction. As a derived ecological measure, we calculated trophic discrimination factors, changes in δ15N or δ13C between source and consumer, for the different prey - predator tissue combinations. 3. Lipid extraction did not affect δ15N values and we found significant tissue differences in δ15N that varied between prey and predator. Lipid extraction enriched δ13C values in most instances, and it was only after extraction of lipids that we observed consistent depletion of δ13C in exoskeleton relative to soft tissues in prey and predator. 4. Isotopic differences between tissue types propagated marked variation in derived ecological parameters. Common sampling practice using whole tissue for prey and predator (whole : whole) resulted in a trophic discrimination factor of 0.48‰ for δ15N, compared with correct factors of 0.97‰ (soft : whole) and 2.18‰ (soft : soft) using prey soft tissue actually ingested by the predator. For δ13C, variation across discrimination factors was less, with whole : whole tissue of -0.14‰, whilst correct factors were -0.55‰ (soft : whole) and -0.04‰ (soft : soft). 5. Our results indicate tissue selection and preparation are important considerations for isotopic studies using arthropods. Lipid extraction is necessary to derive accurate δ13C values based on proteins, whilst consequences of tissue selection are likely context-dependent: In poorly defined systems where sources are isotopically similar or have larger variance our results indicate tissue selection within sources is important to avoid significant error, whether estimating trophic positions or dietary proportions using mixing models. In such cases we strongly recommend exclusion of source materials not assimilated in consumers.1. 动物组织的氮(δ¹⁵N)与碳(δ¹³C)同位素比值，可用于解析动物的日粮组成，为生态学研究开辟了众多应用方向。然而，消费者并不会摄入或同化食物的所有组分，目前学界尚未明确：对食物源与消费者的不同组织进行采样，会如何影响所测得的同位素数值，进而此类差异又会如何改变推导得到的生态学指标。2. 本研究以昆虫间简单的猎物-捕食者取食关系为研究模型，针对是否经过脂质提取的样本，分别检测了软组织、外骨骼及全组织的同位素差异。作为一项核心推导生态学指标，我们针对不同猎物-捕食者组织组合，计算了营养级判别因子(trophic discrimination factors)——即食物源与消费者之间δ¹⁵N或δ¹³C的数值变化量。3. 脂质提取并未对δ¹⁵N数值产生影响，且我们发现，猎物与捕食者的δ¹⁵N在不同组织间均存在显著差异，且该差异在两类生物间并不一致。在多数情况下，脂质提取会使δ¹³C数值富集；仅在完成脂质提取后，我们才观察到：在猎物与捕食者体内，外骨骼的δ¹³C相较于软组织均呈现一致的贫化现象。4. 不同组织间的同位素差异，会导致推导得到的生态学参数出现显著差异。当前通用的采样惯例以全组织分别作为猎物与消费者的采样样本（全组织:全组织），此时δ¹⁵N的营养级判别因子为0.48‰；而若采用捕食者实际摄入的猎物软组织作为样本，则准确的判别因子分别为0.97‰（软组织:全组织）与2.18‰（软组织:软组织）。针对δ¹³C，不同组织组合下的判别因子差异相对更小：全组织:全组织的判别因子为-0.14‰，而准确的判别因子则为-0.55‰（软组织:全组织）与-0.04‰（软组织:软组织）。5. 本研究结果表明，在节肢动物(arthropods)的同位素研究中，组织选择与样本前处理是至关重要的考量因素。基于蛋白质组分推导准确的δ¹³C数值时，脂质提取步骤必不可少；而组织选择所带来的影响则大概率取决于研究场景：在食物源同位素特征相似或方差较大的复杂难界定系统中，无论通过混合模型(mixing models)估算营养级位置还是日粮占比，我们的结果均显示，对食物源内部的组织进行合理选择，可有效避免显著的估算误差。针对此类场景，我们强烈建议剔除消费者无法同化的食物源组分。