Pulsed fluxes of organisms across ecosystem boundaries can exert top-down and bottom-up effects in recipient food webs, through both direct effects on the subsidized trophic levels and indirect effects on other components of the system. While previous theoretical and empirical studies demonstrate the influence of allochthonous subsidies on bottom-up and top-down processes, understanding how these forces act in conjunction is still limited, particularly when an allochthonous resource can simultaneously subsidize multiple trophic levels. Using the Lake Mývatn region in Iceland as an example system of allochthony and its potential effects on multiple trophic levels, we analyzed a mathematical model to evaluate how pulsed subsidies of aquatic insects affect the dynamics of a soil-plant-arthropod food web. We found that the relative balance of top-down and bottom-up effects on a given food web compartment was determined by trophic position, subsidy magnitude, and top predators' ability to exploit the subsidy. For intermediate trophic levels (e.g., detritivores and herbivores), we found that the subsidy could either alleviate or intensify top-down pressure from the predator. For some parameter combinations, alleviation and intensification occurred sequentially during and after the resource pulse. The total effect of the subsidy on detritivores and herbivores, including top-down and bottom-up processes, was determined by the rate at which predator consumption saturated with increasing size of the allochthonous subsidy, with greater saturation leading to increased bottom-up effects. Our findings illustrate how resource pulses to multiple trophic levels can influence food web dynamics by changing the relative strength of bottom-up and top-down effects, with bottom-up predominating top-down effects in most scenarios in this subarctic system.

中文翻译：

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脉冲到多个营养级别的资源的瞬态自上而下和自下而上的影响

跨越生态系统边界的生物脉冲通量可以通过对受补贴营养水平的直接影响和对系统其他组成部分的间接影响，在接受者食物网中产生自上而下和自下而上的影响。虽然之前的理论和实证研究证明了外来补贴对自下而上和自上而下过程的影响，但了解这些力量如何协同作用仍然有限，特别是当一种外来资源可以同时补贴多个营养级别时。我们以冰岛米湖地区为例，说明异地生物系统及其对多个营养级的潜在影响，我们分析了一个数学模型，以评估水生昆虫的脉冲补贴如何影响土壤-植物-节肢动物食物网的动态。我们发现自上而下和自下而上对给定食物网隔间的影响的相对平衡取决于营养位置、补贴量级和顶级捕食者利用补贴的能力。对于中等营养水平（例如，食腐动物和食草动物），我们发现补贴可以减轻或加剧来自捕食者的自上而下的压力。对于某些参数组合，在资源脉冲期间和之后依次出现缓解和强化。补贴对食腐动物和食草动物的总影响，包括自上而下和自下而上的过程，取决于捕食者消费随着异地补贴规模的增加而饱和的速度，饱和度越大，自下而上的影响越大。