1. We investigate where bottom-up and top-down control regulates ecological communities as a mechanism linking ecological gradients to the geography of consumer abundance and biomass. We use standardized surveys of 54 North American grasslands to test alternate hypotheses predicting 100-fold shifts in the biomass of four common grassland arthropod taxa-Auchenorrhyncha, sucking herbivores, Acrididae, chewing herbivores, Tettigoniidae, omnivores, and Araneae, predators.2. Bottom-up models predict that consumer biomass tracks plant quantity (e.g. productivity and standing biomass) and quality (nutrient content) and that ectotherm access to food increases with temperature. Each of the focal trophic groups responded differently to these drivers: the biomass of sucking herbivores and omnivores increased with plant biomass; that of chewing herbivores tracked plant quality; and predator biomass did not depend on plant quality, plant quantity or temperature. The Exploitation Ecosystem Hypothesis is a top-down hypothesis that predicts ashift from resource limitation of herbivores when plant production is low, to predator limitation when plant production is high. In grasslands where spider biomass was low, herbivore biomass increased with plant biomass, whereas bottom-up structuring was not evident when spiders were abundant. Furthermore, neither predator biomass nor trophic position (via stable isotope analysis) increased with plant biomass, suggesting predators themselves are top-down limited.4. Stable isotope analysis revealed that trophic position of the chewing herbivore and omnivore increased significantly with plant biomass, suggesting these groups increased scavenging and meat consumption in grasslands with higher carbohydrate availability. 5. Taken together, our snapshot sampling documents gradients of food web structure across 54 grasslands, consistent with multiple hypotheses of bottom-up and top-down regulation.

2020. Abiotic factors and plant biomass, not plant diversity, strongly shape grassland arthropods under drought conditions. Ecology 101(6):Abstract. Arthropod abundance and diversity often track plant biomass and diversity at the local scale. However, under altered precipitation regimes and anthropogenic disturbances, plant-arthropod relationships are expected to be increasingly controlled by abiotic, rather than biotic, factors. We used an experimental precipitation gradient combined with human management in a temperate mixed-grass prairie to examine (1) how two drivers, altered precipitation and biomass removal, can synergistically affect abiotic factors and plant communities and (2) how these effects can cascade upward, impacting the arthropod food web. Both drought and hay harvest increased soil surface temperature, and drought decreased soil moisture. Arthropod abundance decreased with low soil moisture and, contrary to our predictions, decreased with increased plant biomass. Arthropod diversity increased with soil moisture, decreased with high surface temperatures, and tracked arthropod abundance but was unaffected by plant diversity or quality. Our experiment demonstrates that arthropod abundance is directly constrained by abiotic factors and plant biomass, in turn constraining local arthropod diversity. If robust, this result suggests climate change in the southern Great Plains may directly reduce arthropod diversity.

As ecosystems warm, ectotherm consumer activity should also change. Here we use principles from metabolic and thermal ecology to explore how seasonal and diel temperature change shapes a prairie ant community's foraging rate and its demand for two fundamental resources: salt and sugar. From April through October 2016 we ran transects of vials filled with solutions of 0.5% NaCl and 1% sucrose. We first confirm a basic prediction rarely tested: the discovery rate of both food resources accelerated with soil temperature, but this increase was typically capped at midday due to extreme surface temperatures. We then tested the novel prediction that sodium demand accelerates with temperature, premised on a key thermal difference between sugar and sodium: sugar is stored in cells, while salt is pumped out of cells proportional to metabolic rate, and hence temperature. We found strong support for the resulting prediction that recruitment to NaCl baits accelerates with temperature more steeply than recruitment to 1% sucrose baits. A follow up experiment in 2017 verified that temperature-dependent recruitment to sucrose concentrations of 20% (mimicking rich extrafloral nectaries), while noisy, was still only half as temperature dependent as recruitment recorded for 0.5% NaCl. These results demonstrate how ecosystem warming accelerates then curtails the work done by a community of ectotherms, and how the demand and use of fundamental nutrients can be differentially temperature dependent.

Habitat heterogeneity affects both biotic and abiotic factors important in determining arthropod community composition. In a sandy, mixed‐grass prairie in the southern Great Plains, we used clipping and NPK fertilization to manipulate plant biomass, habitat heterogeneity, and plant quality to quantify their relative effects on the abundance and diversity of its arthropod community. Both clipping and fertilization treatments affected plant biomass and microclimate, including light availability, temperature, and humidity. By decreasing plant biomass, clipping simplified habitat structure and resulted in reduced arthropod abundance and diversity and increased arthropod activity. This reduction appeared to be mediated by fertilizer addition, which increased total plot carbon, plant biomass, and habitat volume, resulting in lower average surface temperature and higher average humidity. By itself, increasing plant biomass through fertilization increased arthropod abundance, activity, and richness. In addition, we show that changing microclimate and plant biomass promoted shifts in arthropod community composition. These results demonstrate the role of habitat heterogeneity and plant quality in structuring arthropod community composition, specifically by regulating microclimate and providing habitat space.

Aim: Ongoing alterations to Earth's biogeochemical cycles (e.g., via fertilization, burning of fossil fuels, and pollution) are expected to impact plants, plant consumers and all subsequent trophic levels. While fertilization experiments often reveal arthropod nutrient limitation by nitrogen and phosphorus via effects on plant nutrient density and biomass, these macronutrients are only two of many nutrients important to arthropod fitness. Micronutrients are key to osmoregulation and enzyme function and can interact synergistically with macronutrients to shape the geography of arthropod abundance. We examine arthropod response to macro-and micronutrient fertilization as a function of nutrient type, application amount, duration, frequency, and plant responses to fertilization with the goal of addressing how ongoing alterations to biogeochemical cycles will shape future grassland food webs.