Singer Michael S, Clark Robert E, Johnson Emily R, Lichter-Marck Isaac H, Mooney Kailen A, Whitney Kenneth D
Department of Biology Wesleyan University Middletown CT USA.
Department of Ecology and Evolutionary Biology University of California at Irvine Irvine CA USA.
Ecol Evol. 2019 Oct 11;9(21):12099-12112. doi: 10.1002/ece3.5662. eCollection 2019 Nov.
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 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.
无天敌空间假说（EFSH）认为，广食性捕食者会促使食草昆虫形成食性特化。在群落层面，EFSH预测食性特化会降低被捕食风险，并且在一些研究单个捕食者类群或捕食者 guilds 的影响的研究中也发现了这种模式。然而，群落层面的捕食也受到多种捕食者类型的组合效应的影响，这就引发了一个问题，即所谓的多种捕食者效应与食草昆虫的食性特化有何关系。在这里，我们通过一项野外实验来检验EFSH，该实验量化了在有和没有鸟类联合捕食效应的情况下，蚂蚁对食草昆虫（毛虫）的捕食风险。在评估一个包含20种毛虫的群落时，我们在系统发育比较框架中使用模型选择来确定最能预测蚂蚁捕食风险的毛虫特征。毛虫物种的丰富度、食性特化和行为防御是其蚂蚁捕食风险的重要预测指标。无论鸟类捕食情况如何，丰富的毛虫物种被捕食的风险都会增加。食性广泛且对攻击有行为反应的毛虫物种，其被捕食风险降低，但这些蚂蚁的影响只在鸟类也能接触到毛虫群落时才会出现。这些发现表明，蚂蚁对毛虫物种的捕食是密度或频率依赖性的，蚂蚁和鸟类可能在同一食草动物群落中对食性特化施加相反的选择，并且多种捕食者的偶然效应可能会产生与食草动物食性广度相关的行为介导的生活史权衡。
