Data from: Short-term, low-level nitrogen deposition dampens a trophic cascade between bears and plants

Human activities have substantially increased atmospheric nitrogen (N) deposition in ecosystems worldwide, often leading to higher plant quality for herbivores and greater herbivory. Predators frequently suppress herbivores and indirectly benefit plants via 'trophic cascades', and the strength of these interactions can also depend on N availability. However, the evidence for N deposition effects on cascades primarily comes from studies of high-level N deposition. Most terrestrial ecosystems currently receive elevated, but low-level N deposition, and it is unclear whether this subtle N enrichment has any effect on cascades. Here, I asked whether low-level N deposition alters a trophic cascade from black bears to plants in Colorado. In this ecological network, bears indirectly benefit plants by consuming ants and suppressing positive effects of ants on herbivores. Using a three year N enrichment experiment, I assessed changes in this cascade by measuring plant and arthropod responses to simulated N deposition, bear damage to ant nests, and the presence of mutualist herbivores and ants. I found that low-level N enrichment and bears had interacting effects on plant reproduction. In ambient N conditions, bears indirectly increased plant reproduction by causing ant nests to become inactive and suppressing positive ant effects on herbivores that were detrimental for plants. Yet, bear-induced ant nest inactivity had no effect on plant reproduction in N-enriched conditions. When N was added, ants had greater positive effects on herbivores, but herbivores had weak effects on plants, potentially because plants were more resistant to herbivores. Ultimately, the results indicate that N enrichment strengthened resource control of the community and weakened plant-herbivore interactions and the cascade from bears to plants. This study suggests that common rates of low-level N deposition are changing the strength of trophic cascades and may have already altered resource vs. consumer control of ecological community structure in many ecosystems.人类活动已大幅提升全球陆地生态系统的大气氮（N）沉降水平，通常会提升植食动物的食物植物质量，并加剧植食作用。捕食者通常通过“营养级联（trophic cascade）”抑制植食动物，间接使植物获益，而这类相互作用的强度同样取决于氮的可获得性。然而，现有关于氮沉降对营养级联影响的证据，主要来自高剂量氮沉降的相关研究。当前多数陆地生态系统面临的是升高但仍处于低水平的氮沉降，目前尚不清楚这种轻微的氮富集是否会对营养级联产生影响。本研究旨在探讨低水平氮沉降是否会改变科罗拉多州一条由美洲黑熊到植物的营养级联。在该生态网络中，黑熊通过取食蚂蚁、抑制蚂蚁对植食动物的正向作用，间接使植物获益。本研究通过为期三年的氮富集实验，测定植物和节肢动物（arthropod）对模拟氮沉降、黑熊对蚁巢的破坏以及互利共生的植食动物与蚂蚁存在情况的响应，以此评估该营养级联的变化。研究发现，低水平氮富集与黑熊对植物繁殖存在交互作用。在背景氮沉降条件下，黑熊通过促使蚁巢失活、抑制蚂蚁对植食动物的正向作用（这类植食动物对植物有害），间接提升了植物繁殖成功率。但在氮富集条件下，黑熊诱导的蚁巢失活并未对植物繁殖产生影响。施加氮素后，蚂蚁对植食动物的正向作用更强，但植食动物对植物的影响却较弱，这可能是因为植物对植食动物的抗性有所提升。最终结果表明，氮富集强化了群落的资源调控作用，削弱了植物-植食动物相互作用以及黑熊到植物的营养级联效应。本研究表明，普遍存在的低水平氮沉降正在改变营养级联的强度，且可能已在众多生态系统中改变了资源调控与消费者调控对生态群落结构的相对作用。