Stable isotope ratios (SIR) are widely used to estimate food-web trophic levels (TLs). We built systems dynamic N-biomass-based models of different levels of complexity, containing explicit descriptions of isotope fractionation and of trophic level. The values of δ15N and TLs, as independent and emergent properties, were used to test the potential for the SIR of nutrients, primary producers, consumers, and detritus to align with food-web TLs. Our analysis shows that there is no universal relationship between TL and δ15N that permits a robust prognostic tool for configuration of food webs even if all system components can be reliably analysed. The predictive capability is confounded by prior dietary preference, intra-guild predation and recycling of biomass through detritus. These matters affect the dynamics of both the TLs and SIR. While SIR data alone have poor explanatory power, they would be valuable for validating the construction and functioning of dynamic models. This requires construction of coupled system dynamic models that describe bulk elemental distribution with an explicit description of isotope discriminations within and amongst functional groups and nutrient pools, as used here. Only adequately configured models would be able to explain both the bulk elemental distributions and the SIR data. Such an approach would provide a powerful test of the whole model, integrating changing abiotic and biotic events across time and space.

中文翻译：

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对营养结构的机械理解：模拟稳定同位素比率的推论

稳定同位素比 (SIR) 广泛用于估计食物网营养水平 (TL)。我们构建了不同复杂程度的基于动态氮生物质的系统模型，其中包含同位素分馏和营养级的明确描述。δ15N 和 TL 的值作为独立和突现的属性，用于测试营养素、初级生产者、消费者和碎屑的 SIR 与食物网 TL 保持一致的潜力。我们的分析表明，即使可以可靠地分析所有系统组件，TL 和 δ15N 之间也不存在普遍关系，无法为食物网配置提供强大的预测工具。预测能力受到先前饮食偏好、行会内捕食和通过碎屑回收生物质的影响。这些问题都会影响 TL 和 SIR 的动态。虽然 SIR 数据本身解释力较差，但它们对于验证动态模型的构建和功能很有价值。这需要构建耦合系统动态模型，描述块体元素分布，并明确描述功能组和营养池内部和之间的同位素区别，如此处所用。只有充分配置的模型才能解释块体元素分布和 SIR 数据。这种方法将为整个模型提供强有力的测试，整合跨时间和空间变化的非生物和生物事件。