Data from: Effects of plant diversity on the concentration of secondary plant metabolites and the density of arthropods on focal plants in the field

1. The diversity of the surrounding plant community can directly affect the abundance of insects on a focal plant as well as the size and quality of that focal plant. However, to what extent the effects of plant diversity on the arthropod community on a focal plant are mediated by host plant quality or by the diversity of the surrounding plants remains unresolved. 2. In the field, we sampled arthropod communities on focal Jacobaea vulgaris plants growing in experimental plant communities that were maintained at different levels of diversity (1, 2, 4 or 9 species) for three years. Focal plants were also planted in plots without surrounding vegetation. We recorded the structural characteristics of each of the surrounding plant communities as well as the growth, and primary and secondary chemistry (pyrrolizidine alkaloids, PAs) of the focal plants to disentangle the potential mechanisms causing the diversity effects. 3. Two years after planting, the abundance of arthropods on focal plants that were still in the vegetative stage decreased with increasing plant diversity, while the abundance of arthropods on reproductive focal plants was not significantly affected by the diversity of the neighbouring community. The size of both vegetative and reproductive focal plants was not significantly affected by the diversity of the neighbouring community, but the levels of PAs and the foliar N concentration of vegetative focal plants decreased with increasing plant diversity. Structural equation modelling revealed that the effects of plant diversity on the arthropod communities on focal plants were not mediated by changes in plant quality. 4. Synthesis. Plant quality can greatly influence insect preference and performance. However, under natural conditions the effects of the neighbouring plant community can overrule the plant quality effects of individual plants growing in those communities on the abundance of insects associated to this plant.

1. 周边植物群落的多样性可直接影响目标植物（focal plant）上昆虫的种群丰度，同时可调控该目标植物的个体大小与品质。然而，植物多样性对目标植物上节肢动物群落（arthropod community）的影响，在多大程度上由宿主植物品质介导，抑或是由周边植物多样性直接驱动，目前仍未得到明确解答。2. 本研究在野外实验中，对种植于已连续维持3年的不同多样性梯度（1、2、4或9个物种）的实验植物群落中的目标植物——欧洲千里光（Jacobaea vulgaris）上的节肢动物群落进行了采样；同时将目标植物种植于无伴生植被的对照样地中。我们记录了各周边植物群落的结构特征，并测定了目标植物的生长状况、初生与次生代谢化学成分（吡咯里西啶生物碱，pyrrolizidine alkaloids, PAs），以解析引发植物多样性效应的潜在机制。3. 种植两年后，处于营养生长阶段的目标植物上的节肢动物丰度随植物多样性提升而降低；而处于生殖生长阶段的目标植物上的节肢动物丰度，则未受周边群落多样性的显著影响。无论目标植物处于营养生长还是生殖生长阶段，其个体大小均未受周边群落多样性的显著影响，但营养生长阶段目标植物的吡咯里西啶生物碱（pyrrolizidine alkaloids, PAs）含量与叶片氮浓度均随植物多样性升高而降低。结构方程模型（structural equation modelling）分析显示，植物多样性对目标植物上节肢动物群落的影响，并非通过植物品质的变化介导。4. 综合分析表明：植物品质可显著影响昆虫的寄主偏好与生长表现。然而在自然条件下，周边植物群落的效应可抵消该群落中单个宿主植物的品质对其关联昆虫丰度的影响。