Data from: The evolution of foraging rate across local and geographic gradients in predation risk and competition

Multiple theories predict the evolution of foraging rates in response to environmental variation in predation risk, intraspecific competition, time constraints, and temperature. We tested six hypotheses for the evolution of foraging rate in 24 spotted salamander (Ambystoma maculatum) populations from three latitudinally divergent sites using structural equation models derived from theory and applied to our system. We raised salamander larvae in a common-garden experiment and then assayed foraging rate under controlled conditions. Gape-limited predation risk from marbled salamanders solely explained foraging rate variation among populations at the southern site, which was dominated by this form of selection. However, at the middle and northern sites, populations evolved different foraging rates depending on their unique responses to local intraspecific density. The coupling of gape-limited predation risk from marbled salamanders and high intraspecific density at the middle site jointly contributed to selection for rapid foraging rate. At the northernmost site, intraspecific density alone explained 97% of the interpopulation variation in foraging rate. These results suggest that foraging rate has evolved multiple times in response to varying contributions from predation risk and intraspecific competition. Predation risk often varies along environmental gradients, and, thus, organisms might often shift evolutionary responses from minimizing predation risk to maximizing intraspecific competitive performance.

多项理论均预测，觅食率（foraging rate）会因捕食风险（predation risk）、种内竞争（intraspecific competition）、时间限制（time constraints）以及温度等环境因子的变化而发生演化。
本研究针对源自3个纬度梯度各异样点的24个斑点钝口螈（Ambystoma maculatum）种群，采用基于理论推导且适配本研究体系的结构方程模型（structural equation models），检验了6个与觅食率演化相关的假说。我们通过同质园实验（common-garden experiment）培育钝口螈幼体，并在可控条件下测定其觅食率。
南部样点的种群间觅食率差异仅可由大理石钝口螈（marbled salamanders）带来的口器限制型捕食风险（gape-limited predation risk）解释，该样点的选择压力以该类型为主。但在中部与北部样点，种群的觅食率演化差异则取决于其对本地种内密度的独特响应。中部样点中，大理石钝口螈带来的口器限制型捕食风险与高种内密度的协同作用，共同催生了对高觅食率的选择压力。在最北端的样点，种内密度单独解释了97%的觅食率种群间变异。
上述结果表明，觅食率会因捕食风险与种内竞争的相对贡献差异，多次独立发生演化。捕食风险通常随环境梯度发生变化，因此生物的演化响应往往会从以降低捕食风险为目标，转向以提升种内竞争能力为目标。