Data from: Predatory birds and ants partition caterpillar prey by body size and diet breadth

1.The effects of predator assemblages on herbivores are predicted to depend critically on predator-predator interactions and the extent to which predators partition prey resources. The role of prey heterogeneity in generating such multiple predator effects has received limited attention. 2.Vertebrate and arthropod insectivores constitute two co-dominant predatory taxa in many ecosystems, and the emergent properties of their joint effects on insect herbivores inform theory on multiple predator effects as well as biological control of insect herbivores. 3.Here we use a large-scale factorial manipulation to assess the extent to which birds and ants engage in antagonistic predator-predator interactions and the consequences of heterogeneity in herbivore body size and diet breadth (i.e. the diversity of host plants used) for prey partitioning. We excluded birds and reduced ant density (by 60%) in the canopies of eight northeastern USA deciduous tree species during two consecutive years and measured the community composition and traits of lepidopteran larvae (caterpillars). 4.Birds did not affect ant density, implying limited intraguild predation between these taxa in this system. Birds preyed selectively upon large-bodied caterpillars (reducing mean caterpillar length by 12%) and ants preyed selectively upon small-bodied caterpillars (increasing mean caterpillar length by 6%). Birds and ants also partitioned caterpillar prey by diet breadth. Birds reduced the frequency dietary generalist caterpillars by 24% while ants had no effect. In contrast, ants reduced the frequency of dietary specialists by 20% while birds had no effect, but these effects were non-additive; under bird exclusion, ants had no detectable effect, while in the presence of birds they reduced the frequency of specialists by 40%. As a likely result of prey partitioning by body size and diet breadth, the combined effects of birds and ants on total caterpillar density were additive, with birds and ants reducing caterpillar density by 44% and 20%, respectively. 5.These results show evidence for the role of prey heterogeneity in driving functional complementarity among predators and enhanced top-down control. Heterogeneity in herbivore body size and diet breadth, as well as other prey traits, may represent key predictors of the strength of top-down control from predator communities.1. 捕食者群落（predator assemblages）对植食动物（herbivores）的影响，在很大程度上取决于捕食者间相互作用（predator-predator interactions）以及捕食者对猎物资源的分区程度。目前关于猎物异质性（prey heterogeneity）在催生这类多捕食者效应（multiple predator effects）中的作用，相关研究尚较为有限。 2. 脊椎动物（vertebrate）与食虫节肢动物（arthropod insectivores）是众多生态系统中的两类共优势捕食类群（co-dominant predatory taxa），二者联合作用所产生的涌现属性（emergent properties），既能为多捕食者效应理论提供支撑，也可为昆虫植食动物的生物防治（biological control）提供参考依据。 3. 本研究采用大规模析因操控实验（large-scale factorial manipulation），评估了鸟类与蚂蚁之间的拮抗捕食者间相互作用（antagonistic predator-predator interactions）程度，以及植食动物体型（herbivore body size）和食性宽度（diet breadth，即所利用寄主植物的多样性）的异质性对猎物分区的影响。在连续两年的实验周期内，我们针对美国东北部8种落叶树种的林冠（canopies）开展了鸟类排除与蚂蚁密度降低（降幅达60%）的处理，并对鳞翅目幼虫（lepidopteran larvae，即毛虫）的群落组成与功能性状（traits）进行了测定。 4. 实验结果显示，鸟类并未对蚂蚁密度产生显著影响，表明该系统中两类群间的集团内捕食（intraguild predation）作用有限。鸟类对大体型毛虫具有选择性捕食偏好，可使毛虫平均体长降低12%；而蚂蚁则偏好捕食小体型毛虫，使得毛虫平均体长提升6%。此外，鸟类与蚂蚁还会根据食性宽度对毛虫猎物进行资源分区：鸟类可使广食性毛虫（dietary generalist caterpillars）的出现频率降低24%，而蚂蚁对此类毛虫无显著影响；与之相反，蚂蚁可使专食性毛虫（dietary specialists）的出现频率降低20%，但鸟类对此无显著作用，不过该效应并非加性效应（non-additive）：在排除鸟类的处理组中，蚂蚁未表现出可检测的影响，而在有鸟类存在的处理组中，蚂蚁可使专食性毛虫的出现频率降低40%。由于体型与食性宽度带来的猎物分区效应，鸟类与蚂蚁对毛虫总密度的联合作用呈现加性效应：二者分别使毛虫密度降低44%与20%。 5. 上述研究结果证实，猎物异质性可推动捕食者间的功能互补性（functional complementarity），并增强下行控制（top-down control）效应。植食动物体型与食性宽度的异质性，以及其他猎物功能性状，或可成为预测捕食者群落下行控制强度的关键指标。