Data from: Linking songbird nest predation to seedling density: sugar maple masting as a resource pulse in a forest food web

The ecological literature presents considerable evidence for top-down forcing on the maintenance of species diversity. Yet, in temperate forests, bottom-up forces often exert a strong influence on ecosystem functioning. Here, we report on the indirect influence of a pulsed resource, sugar maple (Acer saccharum) seed production, on nest survival in a migratory songbird. We hypothesized that seed production in year t would determine daily nest survival rate in year t + 1 through its effects on seed-eating rodents. We used the density of sugar maple seedlings (with cotyledons) in year t + 1 as a proxy for seed production in year t and predicted that it would be inversely related to songbird nest survival the same year. We estimated the density of sugar maple seedlings, eastern chipmunk (Tamias striatus) activity, and daily nest survival rate in the ovenbird (Seiurus aurocapilla) over four successive years in a northern hardwood forest of New Brunswick, Canada. Seedling density varied by two orders of magnitude between years, whereas an index of chipmunk activity changed by an order of magnitude. Both variables were positively correlated and negatively correlated to daily nest survival rate. A logistic-exposure model including only seedling density received the greatest level of support (lowest AICc). Previous studies have reported the effect of sugar maple masting on seed-eating rodent populations, but the strong link we report between seedling density and songbird nest survival is novel. A nocturnal seed-eating nest predator, deer mouse (Peromyscus maniculatus), was not considered in our models, which may explain why chipmunk was not the best predictor of daily nest survival rate. The trophic linkages we observed are remarkably strong for a temperate forest ecosystem and might become more prevalent in northeastern North America, at least on calcium-rich soils, with the loss of large-diameter beech trees as a result of beech bark disease.生态学领域已有大量文献证实，下行调控（top-down forcing）对物种多样性的维持具有关键驱动作用。然而在温带森林生态系统中，上行调控（bottom-up forcing）往往对生态系统功能产生更强的调控效应。本研究聚焦于波动型资源——糖枫（Acer saccharum）结实——对迁徙鸣禽巢存活率的间接影响。我们提出研究假说：t年的糖枫结实量将通过作用于食籽啮齿类，决定t+1年的鸣禽日巢存活率。我们以t+1年的糖枫幼苗（具子叶（cotyledons））密度作为t年糖枫结实量的替代指标（proxy），并预测该密度与同年鸣禽的巢存活率呈负相关。我们在加拿大新不伦瑞克省的一处北方硬叶林中，连续四年开展野外调查，测定了糖枫幼苗密度、东部花栗鼠（Tamias striatus）活动强度，以及橙顶灶莺（Seiurus aurocapilla）的日巢存活率。结果显示，不同年份间的糖枫幼苗密度差异可达两个数量级，而花栗鼠活动指数的变化幅度亦达一个数量级。上述两个变量彼此呈正相关，且均与鸣禽日巢存活率呈负相关。仅包含幼苗密度的逻辑暴露模型（logistic-exposure model）获得了最高的模型支持度（修正赤池信息准则（AICc）值最低）。此前已有研究报道了糖枫结实大年（masting）对食籽啮齿类种群的影响，但本研究揭示的幼苗密度与鸣禽巢存活率之间的强关联仍属首次发现。我们的模型未纳入夜行性食籽巢捕食者——鹿鼠（Peromyscus maniculatus）——这或许可以解释为何花栗鼠并非日巢存活率的最佳预测因子。我们观测到的营养联系（trophic linkages）在温带森林生态系统中表现得尤为显著；随着山毛榉树皮病（beech bark disease）导致北美东北部地区（至少在富钙土壤生境中）大径级山毛榉林木的消亡，这类营养联系在该区域可能会愈发普遍。