DOI: 10.5061/dryad.54ht3
title: Data from: Effects of plant diversity on the concentration of secondary plant metabolites and the density of arthropods on focal plants in the field

description: 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.
description: Surrounding plant
 community and soil measurementsA detailed description
 of the field experiment is presented in the article. At the end of August
 2011, plant community measurements were made in each plot, to estimate the
 structural complexity of the community. For each plant community, the
 percentage cover of plant species was recorded in two 1 m2 quadrants along
 a diagonal transect within each plot. The total percentage cover can
 exceed 100% because plants in a community can overlap. The height of the
 vegetation was measured using the vertical drop disc method (Stewart et
 al. 2001). The disc weighed 200 g, had a diameter of 300 mm, and was
 released from a 1.5-m height. The height was measured at 10 random
 locations within each plot. One week after plant sampling, soil cores of
 15 cm depth and 2.5 cm diameter were collected from each experimental plot
 at five random positions. The soil samples were pooled per plot and used
 for chemical analysis. Mineral N content (NH4+ and NO3) in soil samples
 was determined colorimetrically in the CaCl2 extraction using a Traacs 800
 autoanalyser (TechniCon Systems Inc, USA). The C:N-ratio in soil samples
 was measured on a FlashEA 1112 Series NC soil analyser (Thermo
 Scientific). pH was measured in 2:5 dry soil : water suspensions. The
 percentage organic C was determined according to Nelson and Sommers (1982)
 and available P according to Olsen et al. (1954) and measured at 720
 nm.community and soil
 measurements.7zArthropod database
 2011A
 detailed description of the field experiment is presented in the article.
 Arthropods at all stages of their development (eggs, immature and adults)
 on each Jacobaea vulgaris plant were collected on four occasions from May
 to August 2011. During each collection, all plants were carefully
 inspected between 10:00 am and 16:00 pm and all arthropods that were
 observed on a plant were collected using an aspirator by three collectors
 distributed evenly over the field. Each collector inspected all 1324
 plants, spending an approximately equal amount of time at each plant at
 all diversity levels. All arthropods were stored individually in 70%
 ethanol in labelled Eppendorf tubes. Most arthropods were identified to
 species or family level. All arthropod species were assigned to feeding
 group (specialist herbivore, generalist herbivore, predator, pollinator,
 detritivore and omnivore) based on their feeding strategy and the degree
 of specialization.PA content J.
 vulgarisPA analysis of leaf and
 root samples was carried out using liquid chromatography-tandem mass
 spectrometry (LC-MS/MS) following the procedure outlined in Kostenko et
 al. (2013). In brief, 5 mg of freeze-dried ground plant material was
 extracted with 0.5 ml 2% formic acid solution containing heliotrine (1
 µg•ml-1) as internal standard. After centrifugation and filtration, 25 µl
 of the extracted filtrate was diluted with 975 µl of 10 mM ammonium
 hydroxide solution and 10 µl was injected in a Waters Acquity
 ultra-performance chromatographic system coupled to a Waters Quattro
 Premier tandem mass spectrometer (Waters, Milford, MA, USA). Separation
 and mass spectrometric detection of the PAs was as described in Cheng et
 al. (2011) and Appendix S2. Data were processed using Masslynx 4.1
 software.Jacobaea
 vulgaris_characteristics 2011A detailed description
 of the field experiment and plant measurements are presented in the
 article. Leaf carbon (C) and nitrogen (N) concentrations were determined
 using a Flash EA1112 CN analyser (Interscience, Breda, The
 Netherlands).