Data from: Stoichiometric imbalances between detritus and detritivores are related to shifts in ecosystem functioning

How are resource consumption and growth rates of litter-consuming detritivores affected by imbalances between consumer and litter C:N:P ratios? To address this question, we offered leaf litter as food to three aquatic detritivore species, which represent a gradient of increasing body N:P ratios: a crustacean, a caddisfly and a stonefly. The detritivores were placed in microcosms and submerged in a natural stream. Four contrasting leaf species were offered, both singly and in two-species mixtures, to obtain different levels of stoichiometric imbalance between the resources and their consumers. The results suggest that detritivore growth was constrained by N rather than C or P, even though 1) the N:P ratios of the consumers’ body tissue was relatively low and 2) microbial leaf conditioning during the experiment reduced the N:P imbalance between detritivores and leaf litter. This surprisingly consistent N limitation may be a consequence of cumulative N-demand arising from the production of N-rich chitin in the exoskeletons of all three consumer species, which is lost during regular moults, in addition to N-demand for silk production by the caddisfly. These N requirements are not commonly quantified in stoichiometric analyses of arthropod consumers. There was no evidence for compensatory feeding, but when offered mixed-species litter varying in C:N:P ratios, detritivores consumed more of the litter species showing the highest N:P and lowest C:N ratio, accelerating the mass loss of the preferred leaf species in the litter mixture. These results show that imbalances in consumer–resource stoichiometry can have contrasting effects on coupled processes, highlighting a challenge in developing a mechanistic understanding of the role of stoichiometry in regulating ecosystem processes such as leaf litter decomposition.

食枯屑腐食者（detritivore）的资源摄取量与生长速率，如何受消费者与枯屑间碳氮磷（C:N:P）化学计量失衡的影响？为解答该问题，本研究选取三种水生腐食者，其体组织的氮磷比（N:P）呈递增梯度，分别为甲壳类、石蛾幼虫与石蝇幼虫。将腐食者置于微宇宙（microcosm）实验装置中，随后将该装置浸没于自然溪流内开展培养。本研究选用四种特性差异显著的植物叶片枯屑作为食物，分别以单种或两种混合的形式投喂，以获取资源与消费者间不同程度的化学计量失衡水平。研究结果显示，腐食者的生长受氮（N）限制而非碳（C）或磷（P），尽管：1）消费者体组织的氮磷比（N:P）相对较低；2）实验过程中的叶片微生物定殖作用，降低了腐食者与叶片枯屑间的氮磷失衡程度。这种出人意料的一致性氮限制现象，可能源于三类消费者物种的累积氮需求：一是其外骨骼中合成富含氮的几丁质（chitin）所需的氮，而该物质会在定期蜕皮过程中流失；二是石蛾幼虫吐丝所需的氮。在节肢动物消费者的化学计量分析中，这类氮需求通常未被量化。本研究未发现补偿性取食（compensatory feeding）的证据，但当投喂碳氮磷比（C:N:P）各异的混合枯屑时，腐食者会更多取食氮磷比（N:P）最高、碳氮比（C:N）最低的枯屑物种，从而加速该偏好叶片物种在混合枯屑中的质量损失。上述结果表明，消费者-资源化学计量失衡可对耦合生态过程产生迥异影响，这凸显了在构建化学计量调控生态过程（如叶片枯屑分解）机制性理解方面所面临的挑战。