Size-selectivity of predation by brown bears depends on the density of their sockeye salmon prey

Can variation in prey density drive changes in the intensity or direction of selective predation in natural systems? Despite ample evidence of density-dependent selection, the influence of prey density on predatory selection patterns has seldom been investigated empirically. We used 20 years of field data on brown bears (Ursus arctos) foraging on sockeye salmon (Oncorhynchus nerka) in Alaska, to test the hypothesis that salmon density affects the strength of size-selective predation. Measurements from 41,240 individual salmon were used to calculate variance-standardized selection differentials describing the direction and magnitude of selection. Across the time series, the intensity of predatory selection was inversely correlated with salmon density; greater selection for smaller salmon occurred at low salmon densities as bears’ tendency to kill larger-than-average salmon was magnified. This novel connection between density dependence and selective predation runs contrary to some aspect...

猎物密度的变异能否驱动自然系统中选择性捕食的强度或方向发生变化？尽管已有大量关于密度依赖选择的研究证据，但猎物密度对捕食者选择模式的影响却鲜有实证研究予以探究。我们依托阿拉斯加地区棕熊（Ursus arctos）捕食红大马哈鱼（Oncorhynchus nerka）的20年野外调查数据，检验了“大马哈鱼密度可影响体型选择性捕食强度”这一假说。研究共采集了41240条个体大马哈鱼的测量数据，用于计算描述选择方向与强度的方差标准化选择差。在整个时间序列中，捕食选择强度与大马哈鱼密度呈负相关；当大马哈鱼密度较低时，棕熊偏好猎杀体型大于平均个体的倾向会被放大，此时对小型个体的选择强度更高。这一密度依赖与选择性捕食间的全新关联，与部分现有研究结论相悖……