Bottom-up effects of plant quantity and quality on arthropod diversity across multiple trophic levels in a semi-arid grassland

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.Excel software are required to open the data.Funding provided by: National Natural Science Foundation of ChinaCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001809Award Number: 32192461Funding provided by: Grant-in-Aid for Young Scientists*Crossref Funder Registry ID: Award Number: 25712036Funding provided by: National Natural Science Foundation of ChinaCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001809Award Number: 31630010Study 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), >10% of total herbivore biomass), and chewing herbivores were the rare feeding guild (RB, 10% of total predator biomass), and spiders were the rare predator feeding guild (RB,