The quantity (e.g. biomass production) and quality (e.g. leaf nutrient content) of plants can strongly influence arthropod diversity, but few studies have tried to disentangle such effects.

In this study, we examined the independent effects of plant productivity and leaf traits on the taxon richness and abundance of entire arthropod communities and multiple arthropod orders in replicated monocultures of 15 herbaceous species in the Inner Mongolian grassland.

Total taxon richness of arthropod communities increased with plant productivity and an increase in a high nutrient content indicator (PC1) of plant leaf traits (e.g. high leaf nitrogen, phosphorus and water contents), but decreased with an increase in a poor nutrient content indicator (PC2) of plant leaf traits (e.g. high leaf lignin content but low specific leaf area).  Total abundance of arthropod communities increased with increasing plant productivity but decreased with increasing PC2.

Many common, rather than rare arthropod orders, exhibited strong responses to the changes in plant quantity or quality.  Taxon richness of Diptera, Neuroptera and Coleoptera responded positively to the increase in plant productivity and PC1, while taxon richness of Hemiptera and Coleoptera responded negatively to the increase in PC2. Abundances of Diptera and Coleoptera responded positively to the increased plant productivity, whereas abundances of Hymenoptera and Hemiptera responded negatively to the increased PC2.  The order-specific responses of arthropod richness and abundance to plant quantity or quality reflected the different food requirements and feeding behaviors of arthropods.

Our findings demonstrate that plant quantity and quality can independently control richness and abundance of arthropod communities.  The changes in plant productivity and nutrient content of different plant species may alter arthropod diversity and community structure, and these changes in turn may have strong cascading effects on multiple functions (e.g. prey, decomposers, pollinators and predators) in terrestrial ecosystems.

Arthropod richness and abundance, plant leaf traits, and aboveground net primary productivity (ANPP) data used in the analyses are included in this file.

Arthropod richness and abundance

Arthropods were collected by using sweep net sampling between 10 a.m. and 4 p.m. on days with no rainfall in August 2018. For each monoculture plot, we conducted 50 sweeps using muslin net. We sampled arthropods by sweeping in 180° arcs through vegetation canopy, quickly turning and reversing direction at the end of each arc. At the end of each arc, a quick but fluid upturn of the sweep-net was used to prevent the escape of captured arthropods. The contents of the sweep net were preserved in bottles containing ethyl acetate. In the lab, all arthropod individuals were manually sorted into eight orders: Hymenoptera, Diptera, Hemiptera, Neuroptera, Coleoptera, Araneae, Lepidoptera and Orthoptera. Then, arthropods were identified by optical microscopy at the category of genus and species as far as possible. Finally, we summarized the information of arthropods in each plot and measured the total taxon richness and abundance of arthropods.

Plant leaf traits

Leaf area was calculated from photographs of fresh leaves using IMAGEJ software. The sampled leaves were then weighed and dried for 24 h at 65°C and the water content of leaf was expressed as the difference between fresh and dry weight, divided by dry weight. SLA was calculated as the ratio of total leaf area divided by total leaf dry mass per sample (cm² g^-1). Dry leaf material was then ground to a fine powder with a ball mill, and samples of 10 mg were analyzed for leaf N and P contents with an elemental analyzer (VarioEL Element Analyzer, Hanau, Germany). Leaf lignin content was measured following a sequential extraction analysis of acid detergent lignin. Finally, the data of each leaf trait was averaged for each plant species.

Aboveground net primary productivity (ANPP)

ANPP was calculated based on dry biomass measurement (g m^-2) of each plot, as the standing aboveground biomass of these steppe communities reached the annual peak at middle August.