Data from: 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 aspects of optimal foraging theory and differs from many observations of density-dependent selection because (1) the direction of selection remains constant while its magnitude changes as a function of density and (2) stronger selection is observed at low abundance. These findings indicate that sockeye salmon may be subject to fishery-induced size selection from both direct mechanisms and latent effects of altered predatory selection patterns on the spawning grounds, resulting from reduced salmon abundance.猎物密度的变化能否驱动自然系统中选择性捕食（selective predation）的强度或方向发生改变？尽管已有大量关于密度依赖选择（density-dependent selection）的实证证据，但猎物密度对捕食选择模式的影响却鲜有针对性的实证研究。本研究利用阿拉斯加地区20年间棕熊（Ursus arctos）捕食红大麻哈鱼（Oncorhynchus nerka）的野外监测数据，检验“红大麻哈鱼密度会影响体型选择性捕食（size-selective predation）强度”这一假说。研究共纳入41240条红大麻哈鱼个体的测量数据，用于计算方差标准化选择差（variance-standardized selection differentials），以量化描述捕食选择的方向与强度。在整个监测时间序列中，捕食选择的强度与红大麻哈鱼密度呈负相关；当红大麻哈鱼种群密度较低时，棕熊偏好捕杀体型大于平均个体的倾向会被显著放大，因此对小型红大麻哈鱼的选择强度更高。这一密度依赖效应与选择性捕食间的全新关联，与最优觅食理论（optimal foraging theory）的部分核心观点相悖，且与诸多已报道的密度依赖选择观测结果存在差异，具体体现在两点：（1）捕食选择的方向保持恒定，但其强度随种群密度的变化而改变；（2）在种群丰度较低时，捕食选择的强度更强。上述研究结果表明，红大麻哈鱼可能同时面临两类渔业诱导体型选择（fishery-induced size selection）：一是直接捕捞带来的选择效应，二是由于鲑鱼丰度下降导致产卵场捕食选择模式改变所引发的潜在间接效应。