DOI: 10.5061/dryad.jq2bvq8c7
title: Bottom-up effects of plant quantity and quality on arthropod diversity across multiple trophic levels in a semi-arid grassland

description: 1. Plant quantity and quality can independently affect the diversity of
 the entire arthropod communities and multiple arthropod taxa in grassland
 ecosystems. However, it remains unclear how these effects on arthropod
 taxa at one trophic level propagate through food web to influence the
 diversity of higher trophic levels. 2. We performed a monoculture
 experiment with 15 herbaceous species in the Inner Mongolian grassland to
 investigate how natural variations in plant productivity and host leaf
 traits affect herbivore taxon richness, which in turn affects predator
 taxon richness. 3. For herbivores, plant productivity indirectly promoted
 herbivore taxon richness by increasing herbivore biomass, which was
 attributed to the increases in the richness of dominant sucking herbivores
 and endophytes with high food requirements. However, the high plant
 quality indicator (e.g. high leaf protein, phosphorus and water contents,
 and high leaf protein to carbohydrate ratio) directly increased, whereas
 the low plant quality indicator (e.g. high leaf lignin content) directly
 decreased herbivore taxon richness. Taxon richness of chewing and sucking
 herbivores with specific feeding modes (tearing or sucking mouthparts)
 showed strong positive responses to increasing plant quality. 4. For
 predators, herbivore taxon richness, rather than herbivore biomass, mainly
 mediated the positive effects of plant productivity and the high plant
 quality indicator, but the negative effect of the low plant quality
 indicator, on predator taxon richness. At the feeding guild level, the
 taxon richness of parasitoids, other predators and spiders exhibited
 positive responses to different herbivores, which was attributed to their
 different diet preferences. Predator diversity could be promoted by prey
 partitioning among predator guilds facilitating species coexistence. At
 the family level, the taxon richness of most predator families was
 positively correlated with that of more than one herbivore family,
 suggesting that high predator diversity may be caused by balanced diets
 owing to high prey diversity. 5. Synthesis. Natural variations in plant
 quantity and quality can substantially affect the diversity of herbivores
 and cascade up the food web to affect predators. Specificity and
 mechanisms of feeding have a large impact on the responses of arthropod
 guilds at each trophic level.
description: Study
 site This study was conducted at the Inner Mongolia
 Grassland Ecosystem Research Station (IMGERS, 116°42′E, 43°38′N) of the
 Chinese Academy of Sciences, which is located in the Xilin River Basin of
 Inner Mongolia, China (Bai et
 al. 2004). The
 study area has a semi-arid continental climate and is characterized by a
 mean annual precipitation of 346.1 mm and a mean annual temperature of
 0.3°C. Precipitation mainly occurs in the growing season (June–August),
 which coincides with high temperatures. The soil is a loamy sand texture
 (Calcic Chernozem according to the ISSS Working Group RB,
 1998). The monoculture experiment was conducted in June
 2014, with a fenced area of 40 × 40 m near the IMGERS (Fig.1a). The soil
 seed bank was reduced by bulldozing and manually ploughing and harrowing
 this area to remove the top 10 cm of the soil horizon. Before the seeds
 were sown, this area was divided into four blocks. We selected 20 native
 grassland plant species that represent more than 95% of the biomass and
 plant coverage of the natural community at our study site. These 20 plant
 species were randomly assigned to each plot (1.2 × 1.2 m) within each
 block, resulting in a random block design and a total of 80 monoculture
 plots. In 2013, seeds of all plant species were collected by hand from
 natural communities and stored at 4°C until use. Seeds were sown in each
 plot in the middle of June 2014 at a density of 340 seeds per m2,
 according to the density data for the natural communities at the
 experimental site (Sasaki et
 al. 2019). After
 the seeds were sown, the plots were regularly watered until the end of
 July to encourage germination and plant establishment, and weeds (unwanted
 plants germinating from the seed bank) were removed manually. Because of
 high mortality of herbaceous species in 2018, only 15 of the 20 planted
 species (belonging to four plant families) were investigated in this
 study. All necessary permits were obtained from IMGERS prior to
 establishing of the monoculture experiment.  Plant
 productivity and leaf trait measurements For each
 monoculture plot, the standing biomass of plants was sampled from a 50 cm
 × 50 cm area in mid-August 2018. Plant material was sampled by cutting all
 plants in each quadrant at the soil surface and then was oven-dried at 65
 °C for 48 h and weighed. Plant biomass was calculated from dry biomass
 measurements (g m-2) of each plot, as the standing above-ground biomass of
 these steppe communities reached the annual peak at
 mid-August (Bai et
 al. 2004). In August
 2018, five undamaged, fully expanded leaves were collected from each of
 the 15 plant species from each monoculture plot. Plant leaf traits,
 including leaf non-structural carbohydrate, protein, phosphorus, water and
 lignin contents were measured. These leaf traits are important
 determinants of plant nutrient quality for
 arthropods (Awmack & Leather
 2002; Joern, Provin & Behmer
 2012; Forbes, Watson & Steinbauer
 2017). Fresh
 leaves were weighed and dried for 24 h at 65°C, and leaf water content was
 expressed as the difference between fresh and dry weight, divided by dry
 weight. A ball mill was used to grind dry leaf material to a fine powder,
 and samples of 10 mg were analyzed for leaf phosphorus content using an
 elemental analyzer (VarioEL Element Analyzer; Hanau, Germany). We measured
 plant protein content with using the Bradford assay and analyzed
 non-structural carbohydrate content by using the phenol–sulphuric acid
 method, following the protocol of Clissold, Sanson and Read
 (2006). The leaf
 protein: carbohydrate was defined as the ratio of leaf protein to leaf
 non-structural carbohydrate content. Leaf lignin content was measured
 following sequential extraction analysis of acid detergent
 lignin (Forbes, Watson & Steinbauer
 2017). We
 averaged the data of each leaf trait for each plant species and used the
 mean leaf traits values for data analysis. These 15 plant species
 encompassed a broad range of leaf trait variation.
 Arthropod sampling and
 identification Using the
 sweep-net sampling, we collected arthropods from the monoculture plots
 between 10 a.m. and 4 p.m. on days with no rainfall in August 2018. The
 sampling period corresponded with our collection of data on plant biomass
 and functional traits. Sweep-net sampling facilitates the capture of
 numerous vegetation-dwelling arthropods by dislodging them from
 vegetation (Andersen, Cambrelin &
 Steidl 2019),
 and the large intake diameter of the sweep net (32.0 cm) also allows the
 capture of stronger flying insects such as wasps. Although some
 grasshoppers have different phenology and show seasonal differences in
 peak abundance (Guo et
 al. 2009), our
 sampling period typically coincided with the peak abundance of many
 arthropod taxa in the study area (Wang et al. 2020). We did not use pitfall trap sampling
 method to collect the ground-dwelling arthropods, which
 may have led to the omission of some predators (e.g. carabid beetles)
 playing key roles in grassland ecosystems (Pringle & Fox-Dobbs
 2008; Andersen, Cambrelin & Steidl
 2019).
 Therefore, arthropod diversity may be underestimated in this study.
 However, other studies have found that the number of
 arthropod species obtained from sweep-net sampling is highly obtained with
 that sampled using vacuum sampling for both vegetation-dwelling and
 ground-dwelling arthropods (Siemann 1998). We conducted 50 sweeps by using a muslin net
 for each monoculture plot. We sampled the arthropods by sweeping at 180°
 arcs through the vegetation canopy, quickly turning, and reversing the
 direction at the end of each arc (Doxon, Davis & Fuhlendorf
 2011). At the
 end of each arc, a quick but fluid upturn of the sweep net was used to
 prevent the escape of the captured arthropods. The contents of the sweep
 net were preserved in bottles containing ethyl
 acetate. In the laboratory, all arthropod individuals
 were identified by optical microscopy at the genus and species levels as
 far as possible. Some species were treated as reference specimens because
 they could not be identified to the genus or species level during the
 initial identification. These reference specimens were placed in vials
 containing 75% ethanol and sent to taxonomists for accurate identification
 to morphospecies. Each morphospecies was further placed into one of two
 trophic categories (Table S2): herbivores and
 predators (Perner et
 al. 2005), and
 then carefully assigned to one of the three feeding guilds based on
 published accounts for the taxonomic guilds (Carmona, Lajeunesse & Johnson
 2011; Pratt et al. 2017). Herbivorous feeding guilds consisted of: (1)
 sucking/piercing herbivore (species or genus from Pentatomidae,
 Rhyparochromidae, Coreidae, Tingidae, Piesmatidae, Mordellidae, Miridae,
 Curculionidae, Aphididae, and Cicadellidae); (2) chewing herbivore
 (species or genus from Anthicidae, Acrididae, and Chrysomelidae), and (3)
 endophytes (species or genus from Sphecidae, Tephritidae, Tachinidae,
 Anthomyiidae, and Trypetidae) (Carmona, Lajeunesse & Johnson
 2011). Predatory
 guilds consisted of: (1) parasitoids (species or genus from Dryinidae,
 Ichneumonidae, Mellinidae, Crabronidae, Bethylidae, Tiphiidae, and
 Scoliidae); (2) other predators (species or genus from Chrysopidae,
 Anthocoridae, Asilidae, Syrphidae, Chironomida, Dolichopodidae, and
 Bombyliidae); and (3) spiders (species or genus from
 Thomisidae) (Pratt et
 al. 2017). We
 also measured the biomass of each arthropod family collected from the
 sampled monoculture plots. For herbivores, sucking and endophyte
 herbivores were the dominant feeding guilds (relative biomass (RB),
 &gt;10% of total herbivore biomass), and chewing herbivores were the
 rare feeding guild (RB, &lt;10% of total herbivore biomass). For
 predators, parasitoids and other predators were the two dominant feeding
 guilds (RB, &gt;10% of total predator biomass), and spiders were the
 rare predator feeding guild (RB, &lt;10% of total predator biomass).
 Consequently, we could establish gradients in taxon richness and biomass
 of the entire community and each herbivore and predator feeding guild
 across the monoculture plots of different plant
 species.
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