Data from: Dietary specialization is conditionally associated with increased ant predation risk in a temperate forest caterpillar community

The enemy-free space hypothesis (EFSH) contends that generalist predators select for dietary specialization in insect herbivores. At a community level, the EFSH predicts that dietary specialization reduces predation risk, and this pattern has been found in several studies addressing the impact of individual predator taxa or guilds. However, predation at a community level is also subject to combinatorial effects of multiple predator types, raising the question of how so-called multiple predator effects relate to dietary specialization in insect herbivores. Here we test the EFSH with a field experiment quantifying ant predation risk to insect herbivores (caterpillars) with and without the combined predation effects of birds. Assessing a community of 20 caterpillar species, we use model selection in a phylogenetic comparative framework to identify the caterpillar traits that best predict the risk of ant predation. A caterpillar species’ abundance, dietary specialization, and behavioral defenses were important predictors of its ant predation risk. Abundant caterpillar species had increased its risk of ant predation irrespective of bird predation. Caterpillar species with broad diet breadth and behavioral responsiveness to attack had reduced ant predation risk, but these ant effects only occurred when birds also had access to the caterpillar community. These findings suggest that ant predation of caterpillar species is density- or frequency-dependent, that ants and birds may impose countervailing selection on dietary specialization within the same herbivore community, and that contingent effects of multiple predators may generate behaviorally mediated life history trade-offs associated with herbivore diet breadth.

天敌自由空间假说（Enemy-Free Space Hypothesis，简称EFSH）提出，广食性捕食者会对昆虫植食者的食性特化产生选择压力。在群落层面，EFSH预测食性特化可降低植食者的捕食风险，这一规律已在多项针对单一捕食者类群或功能群影响的研究中得到证实。然而，群落层面的捕食过程同时受到多种捕食者类型的组合效应影响，由此引出核心问题：所谓的多捕食者效应与昆虫植食者的食性特化之间存在何种关联。
本研究通过野外实验检验EFSH，量化了存在鸟类联合捕食效应与不存在鸟类联合捕食效应两种情境下，昆虫植食者（毛虫）所面临的蚂蚁捕食风险。我们针对包含20种毛虫的群落开展研究，采用系统发育比较框架下的模型选择方法，筛选出能够最优预测蚂蚁捕食风险的毛虫性状。
研究结果显示，毛虫物种的种群丰度、食性特化程度与行为防御能力是其蚂蚁捕食风险的重要预测因子。种群丰度较高的毛虫物种，无论是否存在鸟类捕食，其面临的蚂蚁捕食风险均会升高；而食性广度较广、对攻击具有行为响应性的毛虫物种，其蚂蚁捕食风险则会降低，但这类蚂蚁捕食的效应仅在鸟类也可作用于该毛虫群落时才会显现。
上述研究结果表明：毛虫物种面临的蚂蚁捕食风险具有密度依赖或频率依赖的特征；蚂蚁与鸟类可能会对同一植食者群落内的食性特化施加反向选择压力；多捕食者的条件性效应可能会催生与植食者食性广度相关的、由行为介导的生活史权衡。