Data from: Relative impacts of environmental variation and evolutionary history on the nestedness and modularity of tree-herbivore networks.

Nestedness and modularity are measures of ecological networks whose causative effects are little understood. We analyzed antagonistic plant–herbivore bipartite networks using common gardens in two contrasting environments comprised of aspen trees with differing evolutionary histories of defence against herbivores. These networks were tightly connected owing to a high level of specialization of arthropod herbivores that spend a large proportion of the life cycle on aspen. The gardens were separated by ten degrees of latitude with resultant differences in abiotic conditions. We evaluated network metrics and reported similar connectance between gardens but greater numbers of links per species in the northern common garden. Interaction matrices revealed clear nestedness, indicating subsetting of the bipartite interactions into specialist divisions, in both the environmental and evolutionary aspen groups, although nestedness values were only significant in the northern garden. Variation in plant vulnerability, measured as the frequency of herbivore specialization in the aspen population, was significantly partitioned by environment (common garden) but not by evolutionary origin of the aspens. Significant values of modularity were observed in all network matrices. Trait-matching indicated that growth traits, leaf morphology, and phenolic metabolites affected modular structure in both the garden and evolutionary groups, whereas extra-floral nectaries had little influence. Further examination of module configuration revealed that plant vulnerability explained considerable variance in web structure. The contrasting conditions between the two gardens resulted in bottom-up effects of the environment, which most strongly influenced the overall network architecture, however, the aspen groups with dissimilar evolutionary history also showed contrasting degrees of nestedness and modularity. Our research therefore shows that, while evolution does affect the structure of aspen–herbivore bipartite networks, the role of environmental variations is a dominant constraint.嵌套性（nestedness）与模块化（modularity）是生态网络的两类度量指标，其背后的作用机制至今仍未被充分阐明。本研究针对两种差异环境下的同质种植园（common garden）展开分析，对象为拮抗型植物-植食性昆虫二元网络，网络中的宿主为具有不同植食性昆虫防御演化历史的颤杨（aspen）。由于大部分生命周期均依赖颤杨的节肢动物植食者具有高度的宿主特化性，这些网络的连接程度十分紧密。两个同质种植园相隔10个纬度，由此导致其非生物条件存在显著差异。我们对网络指标进行了评估，结果显示两个种植园的网络连接度（connectance）相近，但北部同质种植园中每个物种的平均互作链接数更多。相互作用矩阵（interaction matrices）分析显示，无论按环境分组还是按颤杨演化历史分组，所有网络均呈现出清晰的嵌套性，表明二元相互作用可划分为特化类群模块，不过仅在北部种植园中嵌套性数值达到显著水平。以颤杨种群内植食者特化频率衡量的植物脆弱性（plant vulnerability）差异，可由种植园环境显著解释，但无法通过颤杨的演化起源进行区分。所有网络相互作用矩阵均检测到显著的模块化数值。性状匹配分析表明，无论按环境分组还是按演化历史分组，植物的生长性状、叶片形态与酚类代谢物（phenolic metabolites）均会对网络模块化结构产生影响，而花外蜜腺（extra-floral nectaries）的作用则相对微弱。对模块构型的进一步分析显示，植物脆弱性可解释食物网结构中相当一部分的变异。两个种植园的环境差异产生了环境上行效应（bottom-up effects），该效应对整体网络架构的影响最为显著；与此同时，具有不同演化历史的颤杨组群，其嵌套性与模块化程度也存在显著差异。综上，本研究结果表明：尽管演化过程确实会影响颤杨-植食者二元网络的结构，但环境变异的作用才是主导性的限制因素。