Abstract Food-chain length, or the trophic level of an apex predator, is among the most important properties of food-webs with implications for community structure, ecosystem processes and pollutant accumulation in forests. Three main hypotheses (ecosystem-size, productivity, and disturbance) have been erected to explain variation in food-chain length in freshwater ecosystems, yet the support for these hypotheses in less spatially restricted terrestrial ecosystems has not been extensively studied. Here, we used nitrogen (δ15N) and carbon (δ13C) stable isotopes to explore variation in the realized trophic positions (δ15N) of a beetle Carabus lineatus lateralis and a wasp Vespula vulgaris in 32 chestnut woodland patches in northwestern Spain, while accounting for the insects’ relative mobility by using inferences based on δ13C. We used five potential predictors of ecosystem-size productivity, seven of disturbance and six covariates in anticipation that biological assemblages in woodlands might be influenced by ecosystem-size productivity, and from a change from the human management to a much-increased activity of ecosystem engineers, especially wild boar. Our results provided support for the disturbance hypothesis and suggested that the beetle FCL seems to be more affected by wild boar disturbance than by human-forest management, possibly due to increased forest cover and to rural abandonment in recent decades. Moreover, we found a negative association between the wasp FCL and the ecosystem-size productivity hypothesis, as indicated by the plant Ellengberg’s indicator value for nitrogen, which contrasts to the hypothesis that ecosystem-size productivity should increase FCL. Our findings are discussed in relation to: (1) differences in intensity and frequency between human- and wild boar-induced disturbances; (2) the diets and mobilities of the two predators; and (3) the near lack of hard ecological boundaries in terrestrial ecosystems, such as chestnut woodlands.

中文翻译：

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野猪生根和农村遗弃可能会改变欧洲森林地区节肢动物组合的食物链长度

摘要 食物链长度或顶端捕食者的营养水平是食物网最重要的特性之一，对森林中的群落结构、生态系统过程和污染物积累有影响。已经建立了三个主要假设（生态系统规模、生产力和干扰）来解释淡水生态系统中食物链长度的变化，但在空间受限的陆地生态系统中对这些假设的支持尚未得到广泛研究。在这里，我们使用氮 (δ15N) 和碳 (δ13C) 稳定同位素来探索西班牙西北部 32 个板栗林地斑块中甲虫 Carabus lineatuslateralis 和黄蜂 Vespula vulgaris 已实现的营养位置 (δ15N) 的变化，同时考虑到通过使用基于 δ13C 的推论来计算昆虫的相对移动性。我们使用了五个生态系统规模生产力的潜在预测因子、七个干扰因素和六个协变量，以预测林地中的生物组合可能受到生态系统规模生产力的影响，以及从人类管理转变为生态系统工程师活动的大幅增加，尤其是野猪。我们的结果为干扰假设提供了支持，并表明甲虫 FCL 似乎更受野猪干扰而不是人类森林管理，这可能是由于近几十年来森林覆盖率增加和农村遗弃。此外，我们发现黄蜂 FCL 与生态系统规模生产力假设之间存在负相关，如植物 Ellengberg 的氮指标值所示，这与生态系统规模生产力应该增加 FCL 的假设形成对比。我们的研究结果与以下方面相关：（1）人类和野猪引起的干扰强度和频率的差异；(2) 两种掠食者的饮食和活动；(3) 陆地生态系统中几乎没有硬生态边界，例如栗树林。