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  1. Abstract

    Nonnative species are a key agent of global change. However, nonnative invertebrates remain understudied at the community scales where they are most likely to drive local extirpations. We use the North American NEON pitfall trapping network to document the number of nonnative species from 51 invertebrate communities, testing four classes of drivers. We sequenced samples using the eDNA from the sample's storage ethanol. We used AICc informed regression to evaluate how native species richness, productivity, habitat, temperature, and human population density and vehicular traffic account for continent‐wide variation in the number of nonnative species in a local community. The percentage of nonnatives varied 3‐fold among habitat types and over 10‐fold (0%–14%) overall. We found evidence for two types of constraints on nonnative diversity. Consistent with Capacity rules (i.e., how the number of niches and individuals reflect the number of species an ecosystem can support) nonnatives increased with existing native species richness and ecosystem productivity. Consistent with Establishment Rules (i.e., how the dispersal rate of nonnative propagules and the number of open sites limits nonnative species richness) nonnatives increased with automobile traffic—a measure of human‐generated propagule pressure—and were twice as common in pastures than native grasslands. After accounting for drivers associated with a community's ability to support native species (native species richness and productivity), nonnatives are more common in communities that are regularly seasonally disturbed (pastures and, potentially deciduous forests) and those experiencing more vehicular traffic. These baseline values across the US North America will allow NEON's monitoring mission to document how anthropogenic change—from disturbance to propagule transport, from temperature to trends in local extinction—further shape biotic homogenization.

     
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  2. Abstract This paper synthesizes the contemporary challenges for the sustainability of the social-environmental system (SES) across a geographically, environmentally, and geopolitically diverse region—the Asian Drylands Belt (ADB). This region includes 18 political entities, covering 10.3% of global land area and 30% of total global drylands. At the present time, the ADB is confronted with a unique set of environmental and socioeconomic changes including water shortage-related environmental challenges and dramatic institutional changes since the collapse of the Union of Soviet Socialist Republics. The SES of the ADB is assessed using a conceptual framework rooted in the three pillars of sustainability science: social, economic, and ecological systems. The complex dynamics are explored with biophysical, socioeconomic, institutional, and local context-dependent mechanisms with a focus on institutions and land use and land cover change (LULCC) as important drivers of SES dynamics. This paper also discusses the following five pressing, practical challenges for the sustainability of the ADB SES: (a) reduced water quantity and quality under warming, drying, and escalating extreme events, (b) continued, if not intensifying, geopolitical conflicts, (c) volatile, uncertain, and shifting socioeconomic structures, (d) globalization and cross-country influences, and (e) intensification and shifts in LULCC. To meet the varied challenges across the region, place-based, context-dependent transdisciplinary approaches are needed to focus on the human-environment interactions within and between regional landscapes with explicit consideration of specific forcings and regulatory mechanisms. Future work focused on this region should also assess the role of the following mechanisms that may moderate SES dynamics: socioeconomic regulating mechanisms, biophysical regulating mechanisms, regional and national institutional regulating mechanisms, and localized institutional regulating mechanisms. 
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  3. Evidence for global insect declines mounts, increasing our need to understand underlying mechanisms. We test the nutrient dilution (ND) hypothesis—the decreasing concentration of essential dietary minerals with increasing plant productivity—that particularly targets insect herbivores. Nutrient dilution can result from increased plant biomass due to climate or CO2enrichment. Additionally, when considering long-term trends driven by climate, one must account for large-scale oscillations including El Niño Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), and Pacific Decadal Oscillation (PDO). We combine long-term datasets of grasshopper abundance, climate, plant biomass, and end-of-season foliar elemental content to examine potential drivers of abundance cycles and trends of this dominant herbivore. Annual grasshopper abundances in 16- and 22-y time series from a Kansas prairie revealed both 5-y cycles and declines of 2.1–2.7%/y. Climate cycle indices of spring ENSO, summer NAO, and winter or spring PDO accounted for 40–54% of the variation in grasshopper abundance, mediated by effects of weather and host plants. Consistent with ND, grass biomass doubled and foliar concentrations of N, P, K, and Na—nutrients which limit grasshopper abundance—declined over the same period. The decline in plant nutrients accounted for 25% of the variation in grasshopper abundance over two decades. Thus a warming, wetter, more CO2-enriched world will likely contribute to declines in insect herbivores by depleting nutrients from their already nutrient-poor diet. Unlike other potential drivers of insect declines—habitat loss, light and chemical pollution—ND may be widespread in remaining natural areas.

     
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  4. Abstract

    Plant elemental content can vary up to 1,000‐fold across grasslands, with implications for the herbivores the plants feed. We contrast the regulation, in grasses and forbs, of 12 elements essential to plants and animals (henceforth plant‐essential), 7 essential to animals but not plants (animal‐essential) and 6 with no known metabolic function (nonessential). Four hypotheses accounted for up to two thirds of the variation in grass and forb ionomes across 54 North American grasslands. Consistent with the supply‐side hypothesis, the plant‐essential ionome of both forbs and grasses tracked soil availability. Grass ionomes were more likely to harvest even nonessential elements like Cd and Sr. Consistent with the grazing hypothesis, cattle‐grazed grasslands also accumulated a handful of metals like Cu and Cr. Consistent with the NP‐catalysis hypothesis, increases in the macronutrients N and P in grasses were associated with higher densities of cofactors like Zn and Cu. The plant‐essential elements of forbs, in contrast, consistently varied as per the nutrient‐dilution hypothesis—there was a decrease in elemental parts per million with increasing local carbohydrate production. Combined, these data fit a working hypothesis that grasses maintain lower elemental densities and survive on nutrient‐poor patches by opportunistically harvesting soil nutrients. In contrast, nutrient‐rich forbs use episodes of high precipitation and temperature to build new carbohydrate biomass, raising leaves higher to compete for light, but diluting the nutrient content in every bite of tissue. Herbivores of forbs may thus be particularly prone to increases inpCO2via nutrient dilution.

     
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  5. Abstract

    Across the globe, temperatures are predicted to increase with consequences for many taxonomic groups. Arthropods are particularly at risk as temperature imposes physiological constraints on growth, survival, and reproduction. Given that arthropods may be disproportionately affected in a warmer climate—the question becomes which taxa are vulnerable and can we predict the supposed winners and losers of climate change? To address this question, we resurveyed 33 ant communities, quantifying 20‐yr differences in the incidence of 28 genera. Each North American ant community was surveyed with 30 1‐m2plots, and the incidence of each genus across the 30 plots was used to estimate change. From the original surveys in 1994–1997 to the resurveys in 2016–2017, temperature increased on average 1°C (range, −0.4°C to 2.5°C) and ~64% of ant genera increased in more than half of the sampled communities. To test Thermal Performance Theory's prediction that genera with higher average thermal limits will tend to accumulate at the expense of those with lower limits, we quantified critical thermal maxima (CTmax: the high temperatures at which they lose muscle control) and minima (CTmin: the low temperatures at which ants first become inactive) for common genera at each site. Consistent with prediction, we found a positive decelerating relationship between CTmaxand the proportion of sites in which a genus had increased. CTmin, by contrast, was not a useful predictor of change. There was a strong positive correlation (r = 0.85) between the proportion of sites where a genus was found with higher incidence after 20 yr and the average difference in number of plots occupied per site, suggesting genera with high CTmaxvalues tended to occupy more plots at more sites after 20 yr. Thermal functional traits like CTmaxhave thus proved useful in predicting patterns of long‐term community change in a dominant, diverse insect taxon.

     
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  6. Abstract

    Sugar and sodium are essential nutrients to above‐ and below‐ground consumers. Unlike other properties of ecological communities such as abundance and richness, we know relatively little about nutritional geography—the sources and supply rates of nutrients, and how and why they vary across communities and ecosystems.

    Towards a remedy, we present a suite of hypotheses for how sodium and sugary exudate availability should vary for a common omnivore—the ants—and test them using a survey of 53 North American grasslands.

    We do so by running transects of salt and sugar baits and inferring the magnitude of environmental supplies as the inverse of their use as exogenous baits. We then use estimates of potential drivers of the availability of salt and sugary exudates—plant and soil nutrients, and bioclimatic variables—to test the best predictors of sodium and salt use by ant communities.

    Beyond a baseline of ant activity, salt use increased as an inverse of the amount of sodium in an ecosystem's plant tissue, but not its soils. Plant sodium varied by two orders of magnitude in grasslands across 16° latitude. This suggests that plant exudates are an important source of sodium for grassland consumers. The three drivers that best predict exogenous sugar use by ants all point to factors constraining sugar production: net above‐ground productivity, how far the community is into that year's growing season (both reflecting the rates of photosynthesis) and, intriguingly, the potassium content of plant tissue, which is likely linked to exudate production via plant turgor.

    These data suggest that ants and other consumers across a range of grasslands and climate vary significantly in the demand and supply of sugar and salt. This nutritional geography ultimately arises from gradients of climate and biogeochemistry with implications for the geography of plant–consumer interactions.

     
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  7. Abstract. Relationships between land use and water quality are complex with interdependencies, feedbacks, and legacy effects. Most river water quality studies have assessed catchment land use as areal coverage, but here, we hypothesize and test whether land use intensity – the inputs (fertilizer, livestock) and activities (vegetation removal) of land use – is a better predictor of environmental impact. We use New Zealand (NZ) as a case study because it has had one of the highest rates of agricultural land intensification globally over recent decades. We interpreted water quality state and trends for the 26 years from 1989 to 2014 in the National Rivers Water Quality Network (NRWQN) – consisting of 77 sites on 35 mostly large river systems. To characterize land use intensity, we analyzed spatial and temporal changes in livestock density and land disturbance (i.e., bare soil resulting from vegetation loss by either grazing or forest harvesting) at the catchment scale, as well as fertilizer inputs at the national scale. Using simple multivariate statistical analyses across the 77 catchments, we found that median visual water clarity was best predicted inversely by areal coverage of intensively managed pastures. The primary predictor for all four nutrient variables (TN, NOx, TP, DRP), however, was cattle density, with plantation forest coverage as the secondary predictor variable. While land disturbance was not itself a strong predictor of water quality, it did help explain outliers of land use–water quality relationships. From 1990 to 2014, visual clarity significantly improved in 35 out of 77 (34∕77) catchments, which we attribute mainly to increased dairy cattle exclusion from rivers (despite dairy expansion) and the considerable decrease in sheep numbers across the NZ landscape, from 58 million sheep in 1990 to 31 million in 2012. Nutrient concentrations increased in many of NZ's rivers with dissolved oxidized nitrogen significantly increasing in 27∕77 catchments, which we largely attribute to increased cattle density and legacy nutrients that have built up on intensively managed grasslands and plantation forests since the 1950s and are slowly leaking to the rivers. Despite recent improvements in water quality for some NZ rivers, these legacy nutrients and continued agricultural intensification are expected to pose broad-scale environmental problems for decades to come.

     
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  8. Abstract

    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.

    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 a shift 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.

    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.

    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.

     
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  9. Abstract Aim

    Multiple hypotheses predict how gradients of nutrient availability, plant biomass, and temperature shape trophic pyramids. We aim to disentangle the simultaneous influence of those factors and their indirect effects on trophic structure and individual trophic levels.

    Location

    United States.

    Time period

    2017.

    Major taxa studied

    Invertebrates.

    Methods

    To examine differences in trophic pyramid shape and abundance within trophic levels and across ecological gradients, we conducted 54 standardized surveys of invertebrate communities in North American grasslands. We tested for the direct and indirect effects of plant biomass, temperature, sodium (Na), other essential elements (e.g. N, P, and K), and toxic heavy metals, (e.g. Ar and Pb) in plant tissue on both individual trophic levels, and trophic pyramid shape, estimated as the community trophic mean (CTM).

    Results

    Plant sodium increased CTM, indicating that high plant sodium concentrations are associated with top‐heavy invertebrate trophic pyramids. Sites with higher plant biomass had higher proportions of herbivores compared to higher trophic levels. Finally, increasing temperature resulted in more top‐heavy trophic pyramids. Overall, plant biomass, temperature, and plant chemistry directly and indirectly affected the abundances within different trophic levels, highlighting the complexity of factors regulating trophic structure.

    Main conclusions

    Trophic structure of grassland invertebrate communities is strongly influenced by plant sodium, plant biomass, and to a lesser extent, temperature. Grasslands occupy 30% of the Earth’s terrestrial surface and are an imperiled ecosystem due to conversion to row crop agriculture. As biogeochemistry and temperature in the Anthropocene are increasingly modified, our results have considerable implications for the trophic structure of future grassland communities.

     
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