Data from: Age and sex-selective predation as moderators of the overall impact of predation

Currently, there is no general agreement about the extent to which predators’ impact prey population dynamics, as it is often poorly predicted by predation rates and species abundances. This could in part be caused by variation in the type of selective predation occurring. Notably, if predation is selective on categories of individuals that contribute little to future generations, it may moderate the impact of predation on prey population dynamics. However, despite its prevalence, selective predation has seldom been studied in this context. Using recoveries of ringed tawny owls (Strix aluco) predated by ‘superpredators’, northern goshawks (Accipiter gentilis) as they colonised the area, we investigated the extent to which predation was sex and age-selective. Predation of juvenile owls was disproportionately high. Amongst adults, predation was strongly biased towards females and predation risk appeared to increase with age. This implies age-selective predation may shape the decline in survival with age, observed in tawny owls. To determine whether selective predation can modulate the overall impact of predation, age-based population matrix models were used to simulate the overall impact of five different patterns of age-selective predation, including the pattern actually observed in the study site. The impact on owl population size varied by up to 50%, depending on the pattern of selective predation. The simulation of the observed pattern of predation had a relatively small impact on population size, close to the least harmful scenario, predation on juveniles only. The actual changes in owl population size and structure, observed during goshawk colonisation were also analysed. Owl population size and immigration were unrelated to goshawk abundance. However, goshawk abundance appeared to interact with owl food availability to have a delayed effect on recruitment into the population. This study provides strong evidence to suggest that predation of other predators is both age and sex-selective and that selective predation of individuals with a low reproductive value may mitigate the overall impact of predators on prey population dynamics. Consequently, our results highlight how accounting for the type of selective predation occurring is likely to improve future predictions of the overall impact of predation.

目前学界对于捕食者对猎物种群动态（population dynamics）的影响程度尚未达成普遍共识，因为这类影响往往难以通过捕食率和物种种群丰度进行准确预测。这一现象的部分成因可能在于所发生的选择性捕食（selective predation）类型存在差异。值得注意的是，若捕食行为对那些对未来种群贡献较低的个体类别具有选择性，那么其对猎物种群动态的整体影响可能会有所缓和。然而，尽管这类选择性捕食普遍存在，但在该研究框架下却鲜有相关探讨。本研究借助“超级捕食者”北部苍鹰（Accipiter gentilis）定居该区域时捕食的环志灰林鸮（Strix aluco）环志回收数据，探究了捕食行为在性别和年龄维度上的选择性程度。结果显示，对幼年灰林鸮的捕食比例显著偏高；在成年个体中，捕食偏好强烈偏向雌性，且捕食风险似乎随年龄增长而升高。这表明年龄选择性捕食可能塑造了灰林鸮种群中随年龄增长而下降的存活模式。为明确选择性捕食能否调节捕食行为的整体影响，本研究使用基于年龄结构的种群矩阵模型（age-based population matrix models），模拟了五种不同的年龄选择性捕食模式（包括研究样地实际观测到的模式）所产生的整体影响。结果表明，根据选择性捕食模式的不同，对鸮类种群规模的影响差异可达50%。其中，模拟实际观测到的捕食模式时，对种群规模的影响相对较小，接近仅捕食幼体这一危害最低的情景。本研究同时分析了苍鹰定居期间观测到的鸮类种群规模与结构的实际变化。结果显示，鸮类种群规模和迁入率均与苍鹰的种群丰度无关，但苍鹰丰度似乎与鸮类的食物可获得性产生交互作用，进而对种群补充（recruitment）产生滞后效应。本研究提供了强有力的证据，表明对其他捕食者的捕食行为同时存在年龄与性别选择性，且对繁殖价值（reproductive value）较低的个体进行选择性捕食，或许能够缓解捕食者对猎物种群动态的整体影响。综上，我们的研究结果凸显了在未来的捕食整体影响预测中，纳入所发生的选择性捕食类型这一因素，将有望提升预测的准确性。