Data from: Understanding spatial distributions: negative density-dependence in prey causes predators to trade-off prey quantity with quality

Negative density-dependence is generally studied within a single trophic level, thereby neglecting its effect on higher trophic levels. The ‘functional response’ couples a predator's intake rate to prey density. Most widespread is a type II functional response, where intake rate increases asymptotically with prey density; this predicts the highest predator densities at the highest prey densities. In one of the most stringent tests of this generality to date, we measured density and quality of bivalve prey (edible cockles Cerastoderma edule) across 50 km² of mudflat, and simultaneously, with a novel time-of-arrival methodology, tracked their avian predators (red knots Calidris canutus). Because of negative density-dependence in the individual quality of cockles, the predicted energy intake rates of red knots declined at high prey densities (a type IV, rather than a type II functional response). Resource-selection modelling revealed that red knots indeed selected areas of intermediate cockle densities where energy intake rates were maximized given their phenotype-specific digestive constraints (as indicated by gizzard mass). Because negative density-dependence is common, we question the current consensus and suggest that predators commonly maximize their energy intake rates at intermediate prey densities. Prey density alone may thus poorly predict intake rates, carrying capacity and spatial distributions of predators.

负密度依赖（negative density-dependence）的研究通常局限于单一营养级（trophic level），因而忽略了其对更高营养级的影响。“功能响应（functional response）”将捕食者的摄食率与猎物密度相关联。其中最为普遍的是II型功能响应（type II functional response），即摄食率随猎物密度呈渐近式增长；该响应预测，捕食者种群密度最高的区域对应猎物密度最高的区域。在迄今为止对这一普遍性结论最严格的检验之一中，我们在50平方千米的潮间带泥滩范围内，测定了双壳类猎物（bivalve prey）——可食用鸟蛤（Cerastoderma edule）的密度与个体质量（individual quality），同时采用一种新型到达时间法（time-of-arrival methodology）追踪了它们的鸟类捕食者（avian predators）：红腹滨鹬（Calidris canutus）。由于鸟蛤的个体质量存在负密度依赖效应，红腹滨鹬的预测能量摄入率（energy intake rates）在高猎物密度下出现下降，此时对应的是IV型功能响应（type IV functional response）而非II型功能响应。资源选择模型（resource-selection modelling）分析显示，红腹滨鹬确实会选择鸟蛤密度处于中等水平的区域——在该区域中，结合其表型特异性消化限制（phenotype-specific digestive constraints，以肌胃质量（gizzard mass）为指标），能量摄入率可达到最大化。鉴于负密度依赖效应普遍存在，我们对当前的主流共识提出质疑，并提出：捕食者通常会在中等猎物密度下实现能量摄入率的最大化。因此，仅依靠猎物密度，可能无法准确预测捕食者的摄食率、承载量（carrying capacity）与空间分布（spatial distributions）。