Data from: Natural selection by pulsed predation: survival of the thickest

Selective predation can lead to natural selection in prey populations and may alleviate competition among surviving individuals. The processes of selection and competition can have substantial effects on prey population dynamics, but are rarely studied simultaneously. Moreover, field studies of predator-induced short-term selection pressures on prey populations are scarce. Here we report measurements of density dependence in body composition in a bivalve prey (edible cockle, Cerastoderma edule) during bouts of intense predation by an avian predator (red knot, Calidris canutus). We measured densities, patchiness, morphology, and body composition (shell and flesh mass) of cockles in a quasi-experimental setting, i.e. before and after predation in three similar plots of 1 ha each, two of which experienced predation, and one of which remained unvisited in the course of the short study period and served as a reference. An individual's shell and flesh mass declined with cockle density (negative density dependence). Before predation, cockles were patchily distributed. After predation, during which densities were reduced by 78% (from 232 m-2 to 50 m-2), the patchiness was substantially reduced, i.e. the spatial distribution was homogenized. Red knots selected juvenile cockles with an average length of 6.9 mm (SD 1.0). Cockles surviving predation had heavier shells than before predation (an increase of 21.5 percentage points), but similar flesh masses. By contrast, in the reference plot shell mass did not differ statistically between initial and final sampling occasions, while flesh mass was larger (an increase of 13.2 percentage points). In this field-study, we show that red knots imposed a strong selection pressure on cockles to grow fast with thick shells and little flesh mass, with selection gradients among the highest reported in the literature.

选择性捕食可引发猎物种群的自然选择，并缓解存活个体间的种内竞争。选择与竞争过程可对猎物种群动态产生显著影响，但二者鲜少被同时研究。此外，针对捕食者引发的猎物种群短期选择压力的野外研究亦较为匮乏。本研究报道了在鸟类捕食者——红腹滨鹬（Calidris canutus）的高强度捕食事件中，双壳类猎物——食用鸟蛤（Cerastoderma edule）身体组成的密度依赖性（density dependence）特征。本研究通过准实验设计，在短期研究周期内对三个面积均为1公顷的相似样地中的鸟蛤开展了密度、空间聚集度（patchiness）、形态及身体组成（壳重与软组织重）的测定：其中两个样地经历了捕食事件，第三个样地未被捕食者造访，作为对照样地。鸟蛤个体的壳重与软组织重随种群密度升高而降低，即表现为负密度依赖效应。捕食前，鸟蛤呈聚集分布；捕食后种群密度下降78%（从232只/平方米降至50只/平方米），其空间聚集度显著降低，即空间分布趋于均质化。红腹滨鹬偏好捕食平均体长为6.9毫米（标准差1.0）的幼年鸟蛤。经捕食存活的鸟蛤，其壳重较捕食前提升21.5个百分点，软组织重则无显著差异。与之形成对比的是，对照样地内鸟蛤的壳重在初始与终末采样间无统计学差异，而软组织重则提升了13.2个百分点。本野外研究表明，红腹滨鹬对鸟蛤施加了强烈的选择压力，促使其快速生长并形成厚壳、软组织占比低的表型，其选择梯度（selection gradients）处于已报道文献中的较高水平。