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.

消费者与枯落叶的碳氮磷（C:N:P）化学计量失衡，会对食碎屑动物（detritivore）的资源消耗及生长速率产生何种影响？为解答该问题，本研究选取3种水生食碎屑动物作为实验对象，其体组织氮磷比（N:P）呈逐步升高的梯度：分别为甲壳类、石蛾及石蝇。研究人员将食碎屑动物置于微宇宙（microcosms）中，并将其浸没于自然溪流环境。实验选用4种特性迥异的枯落叶，分别以单种及2种混合的形式提供给实验动物，以此构建资源与消费者间不同水平的化学计量失衡（stoichiometric imbalance）状态。研究结果表明，食碎屑动物的生长受到氮（N）限制，而非碳（C）或磷（P）；尽管1）实验动物体组织的氮磷比相对较低，且2）实验过程中枯落叶的微生物定殖降低了食碎屑动物与枯落叶间的氮磷比失衡程度。这种出人意料的一致氮限制现象，可能源于三类消费者物种的共同氮需求：除石蛾需合成丝线消耗氮外，所有物种的外骨骼（exoskeleton）中均含有富氮的几丁质（chitin），且该物质会在定期蜕皮（moult）过程中流失，由此产生累积的氮需求。这类氮需求在节肢动物消费者的化学计量分析中通常未被量化。实验未发现补偿性取食（compensatory feeding）的证据，但当食碎屑动物面对碳氮磷比存在差异的混合枯落叶时，它们会更多取食氮磷比最高、碳氮比最低的枯落叶物种，进而加速该偏好枯落叶物种在混合枯落叶中的质量损失。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差异化影响，这为构建化学计量调控生态过程（如枯落叶分解）的机制性理解带来了挑战。