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The effects of altered nutrient supplies and herbivore density on species diversity vary with spatial scale, because coexistence mechanisms are scale dependent. This scale dependence may alter the shape of the species–area relationship (SAR), which can be described by changes in species richness (S) as a power function of the sample area (A): S = cAz, where c and z are constants. We analysed the effects of experimental manipulations of nutrient supply and herbivore density on species richness across a range of scales (0.01–75 m²) at 30 grasslands in 10 countries. We found that nutrient addition reduced the number of species that could co-occur locally, indicated by the SAR intercepts (log c), but did not affect the SAR slopes (z). As a result, proportional species loss due to nutrient enrichment was largely unchanged across sampling scales, whereas total species loss increased over threefold across our range of sampling scales.

Plant‐soil feedbacks (PSFs) have been shown to strongly affect plant performance under controlled conditions, and PSFs are thought to have far reaching consequences for plant population dynamics and the structuring of plant communities. However, thus far the relationship between PSF and plant species abundance in the field is not consistent. Here, we synthesize PSF experiments from tropical forests to semiarid grasslands, and test for a positive relationship between plant abundance in the field and PSFs estimated from controlled bioassays. We meta‐analyzed results from 22 PSF experiments and found an overall positive correlation (0.12 ≤ r¯ ≤ 0.32) between plant abundance in the field and PSFs across plant functional types (herbaceous and woody plants) but also variation by plant functional type. Thus, our analysis provides quantitative support that plant abundance has a general albeit weak positive relationship with PSFs across ecosystems. Overall, our results suggest that harmful soil biota tend to accumulate around and disproportionately impact species that are rare. However, data for the herbaceous species, which are most common in the literature, had no significant abundance‐PSFs relationship. Therefore, we conclude that further work is needed within and across biomes, succession stages and plant types, both under controlled and field conditions, while separating PSF effects from other drivers (e.g., herbivory, competition, disturbance) of plant abundance to tease apart the role of soil biota in causing patterns of plant rarity versus commonness., Across studies, we detected a small but significant positive correlation between plant‐soil feedbacks measured during controlled experiments and plant field abundance. Plants that are rare are seemingly burdened more by the negative effects of harmful soil biota than common plants.

Almost 50 years ago, Michael Rosenzweig pointed out that nutrient addition can destabilise food webs, leading to loss of species and reduced ecosystem function through the paradox of enrichment. Around the same time, David Tilman demonstrated that increased nutrient loading would also be expected to cause competitive exclusion leading to deleterious changes in food web diversity. While both concepts have greatly illuminated general diversity-stability theory, we currently lack a coherent framework to predict how nutrients influence food web stability across a landscape. This is a vitally important gap in our understanding, given mounting evidence of serious ecological disruption arising from anthropogenic displacement of resources and organisms. Here, we combine contemporary theory on food webs and meta-ecosystems to show that nutrient additions are indeed expected to drive loss in stability and function in human-impacted regions. Our models suggest that destabilisation is more likely to be caused by the complete loss of an equilibrium due to edible plant species being competitively excluded. In highly modified landscapes, spatial nutrient transport theory suggests that such instabilities can be amplified over vast distances from the sites of nutrient addition. Consistent with this theoretical synthesis, the empirical frequency of these distant propagating ecosystem imbalances appears to be growing. This synthesis of theory and empirical data suggests that human modification of the Earth is strongly connecting distantly separated ecosystems, causing rapid, expansive and costly nutrient-driven instabilities over vast areas of the planet. Similar to existing food web theory, the corollary to this spatial nutrient theory is that slowing down spatial nutrient pathways can be a potent means of stabilising degraded ecosystems.

Aim: Climate variability threatens to destabilize production in many ecosystems. Asynchronous species dynamics may buffer against such variability when a decrease in performance by some species is offset by an increase in performance of others. However, high climatic variability can eliminate species through stochastic extinctions or cause similar stress responses among species that reduce buffering. Local conditions, such as soil nutrients, can also alter production stability directly or by influencing asynchrony. We test these hypotheses using a globally distributed sampling experiment. Location: Grasslands in North America, Europe and Australia. Time period: Annual surveys over 5 year intervals occurring between 2007 and 2014. Major taxa studied: Herbaceous plants. Methods: We sampled annually the per species cover and aboveground community biomass [net primary productivity (NPP)], plus soil chemical properties, in 29 grasslands. We tested how soil conditions, combined with variability in precipitation and temperature, affect species richness, asynchrony and temporal stability of primary productivity. We used bivariate relationships and structural equation modelling to examine proximate and ultimate relationships. Results: Climate variability strongly predicted asynchrony, whereas NPP stability was more related to soil conditions. Species richness was structured by both climate variability and soils and, in turn, increased asynchrony. Variability in temperature and precipitation caused a unimodal asynchrony response, with asynchrony being lowest at low and high climate variability. Climate impacted stability indirectly, through its effect on asynchrony, with stability increasing at higher asynchrony owing to lower inter-annual variability in NPP. Soil conditions had no detectable effect on asynchrony but increased stability by increasing the mean NPP, especially when soil organic matter was high. Main conclusions: We found globally consistent evidence that climate modulates species asynchrony but that the direct effect on stability is low relative to local soil conditions. Nonetheless, our observed unimodal responses to variability in temperature and precipitation suggest asynchrony thresholds, beyond which there are detectable destabilizing impacts of climate on primary productivity.

Nutrient enrichment can simultaneously increase and destabilise plant biomass production, with co-limitation by multiple nutrients potentially intensifying these effects. Here, we test how factorial additions of nitrogen (N), phosphorus (P) and potassium with essential nutrients (K+) affect the stability (mean/standard deviation) of aboveground biomass in 34 grasslands over 7 years. Destabilisation with fertilisation was prevalent but was driven by single nutrients, not synergistic nutrient interactions. On average, N-based treatments increased mean biomass production by 21-51% but increased its standard deviation by 40-68% and so consistently reduced stability. Adding P increased interannual variability and reduced stability without altering mean biomass, while K+ had no general effects. Declines in stability were largest in the most nutrient-limited grasslands, or where nutrients reduced species richness or intensified species synchrony. We show that nutrients can differentially impact the stability of biomass production, with N and P in particular disproportionately increasing its interannual variability.

Fertilisation experiments have demonstrated that nutrient availability is a key determinant of biomass production and carbon sequestration in grasslands. However, the influence of nutrients in explaining spatial variation in grassland biomass production has rarely been assessed. Using a global dataset comprising 72 sites on six continents, we investigated which of 16 soil factors that shape nutrient availability associate most strongly with variation in grassland aboveground biomass. Climate and N deposition were also considered. Based on theory-driven structural equation modelling, we found that soil micronutrients (particularly Zn and Fe) were important predictors of biomass and, together with soil physicochemical properties and C:N, they explained more unique variation (32%) than climate and N deposition (24%). However, the association between micronutrients and biomass was absent in grasslands limited by NP. These results highlight soil properties as key predictors of global grassland biomass production and point to serial co-limitation by NP and micronutrients.

Spatial rarity is often used to predict extinction risk, but rarity can also occur temporally. Perhaps more relevant in the context of global change is whether a species is core to a community (persistent) or transient (intermittently present), with transient species often susceptible to human activities that reduce niche space. Using 5–12 yr of data on 1,447 plant species from 49 grasslands on five continents, we show that local abundance and species persistence under ambient conditions are both effective predictors of local extinction risk following experimental exclusion of grazers or addition of nutrients; persistence was a more powerful predictor than local abundance. While perturbations increased the risk of exclusion for low persistence and abundance species, transient but abundant species were also highly likely to be excluded from a perturbed plot relative to ambient conditions. Moreover, low persistence and low abundance species that were not excluded from perturbed plots tended to have a modest increase in abundance following perturbance. Last, even core species with high abundances had large decreases in persistence and increased losses in perturbed plots, threatening the long-term stability of these grasslands. Our results demonstrate that expanding the concept of rarity to include temporal dynamics, in addition to local abundance, more effectively predicts extinction risk in response to environmental change than either rarity axis predicts alone. © 2021 by the Ecological Society of America

Arctic plants are adapted to climatic variability, but their long-term responses to warming remain unclear. Responses may occur by range shifts, phenological adjustments in growth and reproduction, or both. Here, we compare distribution and phenology of 83 arctic and boreal mountain species, sampled identically in the early 20th (1917–1919) and 21st centuries (2017–2018) from a region of northern Sweden that has warmed significantly. We test two compensatory hypotheses to high-latitude warming—upward shifts in distribution, and earlier or extended growth and reproduction. For distribution, we show dramatic upward migration by 69% of species, averaging 6.1 m per decade, especially boreal woodland taxa whose upward expansion has reduced arctic montane habitat by 30%. Twenty percent of summit species showed distributional shifts but downward, especially moisture-associated snowbed flora. For phenology, we detected wide inter-annual variability in the onset of leafing and flowering in both eras. However, there was no detectable change in growing-season length, relating to two mechanisms. First, plot-level snow melt data starting in 1917 demonstrated that melt date, rather than vernal temperatures, better predicts plant emergence, with snow melt influenced by warmer years having greater snowfall—warmer springs did not always result in earlier emergence because snowbeds can persist longer. Second, the onset of reproductive senescence between eras was similar, even when plant emergence was earlier by a month, possibly due to intensified summer heat stress or hard-wired ‘canalization’ where senescence occurs regardless of summer temperature. Migrations in this system have possibly buffered arctic species against displacement by boreal expansion and warming, but ongoing temperature increases, woody plant invasion, and a potential lack of flexibility in timing of senescence may foreshadow challenges.

Abandoned agricultural lands often accumulate soil carbon (C) following depletion of soil C by cultivation. The potential for this recovery to provide significant C storage benefits depends on the rate of soil C accumulation, which, in turn, may depend on nutrient supply rates. We tracked soil C for almost four decades following intensive agricultural soil disturbance along an experimentally imposed gradient in nitrogen (N) added annually in combination with other macro- and micro-nutrients. Soil %C accumulated over the course of the study in unfertilized control plots leading to a gain of 6.1 Mg C ha-1 in the top 20 cm of soil. Nutrient addition increased soil %C accumulation leading to a gain of 17.8 Mg C ha-1 in fertilized plots, nearly a threefold increase over the control plots. These results demonstrate that substantial increases in soil C in successional grasslands following agricultural abandonment occur over decadal timescales, and that C gain is increased by high supply rates of soil nutrients. In addition, soil %C continued to increase for decades under elevated nutrient supply, suggesting that short-term nutrient addition experiments underestimate the effects of soil nutrients on soil C accumulation.

Nutrients and herbivores have independent effects on the temporal stability of aboveground biomass in grasslands; however, their joint effects may not be additive and may also depend on spatial scales. In an experiment adding nutrients and excluding herbivores in 34 globally distributed grasslands, we found that nutrients and herbivores mainly had additive effects. Nutrient addition consistently reduced stability at the local and larger spatial scales (aggregated local communities), while herbivore exclusion weakly reduced stability at these scales. Moreover, nutrient addition reduced stability primarily by causing changes in local community composition over time and by reducing local species richness and evenness. In contrast, herbivore exclusion weakly reduced stability at the larger scale mainly by decreasing asynchronous dynamics among local communities, but also by weakly decreasing local species richness. Our findings indicate disentangling the influences of processes operating at different spatial scales may improve conservation and management in stabilizing grassland biomass.

Biodiversity—both above‐ and belowground—influences multiple functions in terrestrial ecosystems. Yet, it is unclear whether differences in above‐ and belowground species composition (β‐diversity) are associated with differences in multiple ecosystem functions (e.g., spatial turnover in ecosystem function). Here, we partitioned the contributions of above‐ and belowground β‐diversity and abiotic factors (geographic distance, differences in environments) on the spatial turnover of multiple grassland ecosystem functions. We compiled a dataset of plant and soil microbial communities and six indicators of grassland ecosystem functions (i.e., plant aboveground live biomass, plant nitrogen [N], plant phosphorus [P], root biomass, soil total N, and soil extractable P) from 18 grassland sites on four continents contributing to the Nutrient Network experiment. We used Mantel tests and structural equation models to disentangle the relationship between above‐ and belowground β‐diversity and spatial turnover in grassland ecosystem functions. We found that the effects of abiotic factors on the spatial turnover of ecosystem functions were largely indirect through their influences on above‐ and belowground β‐diversity, and that spatial turnover of ecosystem function was more strongly associated with plant β‐diversity than with soil microbial β‐diversity. These results indicate that changes in above‐ and belowground species composition are one mechanism that interacts with environmental change to determine variability in multiple ecosystem functions across spatial scales. As grasslands face global threats from shrub encroachment, conversion to agriculture, or are lost to development, the functions and services they provide will more strongly converge with increased aboveground community homogenization than with soil microbial community homogenization.

Soil nitrogen (N) availability is critical for grassland functioning. However, human activities have increased the supply of biologically limiting nutrients, and changed the density and identity of mammalian herbivores. These anthropogenic changes may alter net soil N mineralization (soil net Nmin), that is, the net balance between N mineralization and immobilization, which could severely impact grassland structure and functioning. Yet, to date, little is known about how fertilization and herbivore removal individually, or jointly, affect soil net Nmin across a wide range of grasslands that vary in soil and climatic properties. Here we collected data from 22 grasslands on five continents, all part of a globally replicated experiment, to assess how fertilization and herbivore removal affected potential (laboratory‐based) and realized (field‐based) soil net Nmin. Herbivore removal in the absence of fertilization did not alter potential and realized soil net Nmin. However, fertilization alone and in combination with herbivore removal consistently increased potential soil net Nmin. Realized soil net Nmin, in contrast, significantly decreased in fertilized plots where herbivores were removed. Treatment effects on potential and realized soil net Nmin were contingent on site‐specific soil and climatic properties. Fertilization effects on potential soil net Nmin were larger at sites with higher mean annual precipitation (MAP) and temperature of the wettest quarter (T.q.wet). Reciprocally, realized soil net Nmin declined most strongly with fertilization and herbivore removal at sites with lower MAP and higher T.q.wet. In summary, our findings show that anthropogenic nutrient enrichment, herbivore exclusion and alterations in future climatic conditions can negatively impact soil net Nmin across global grasslands under realistic field conditions. This is an important context‐dependent knowledge for grassland management worldwide.

AimNutrient enrichment is associated with plant invasions and biodiversity loss. Functional trait advantages may predict the ascendancy of invasive plants following nutrient enrichment but this is rarely tested. Here, we investigate (a) whether dominant native and non‐native plants differ in important morphological and physiological leaf traits, (b) how their traits respond to nutrient addition, and (c) whether responses are consistent across functional groups.LocationAustralia, Europe, North America and South Africa.Time period2007–2014.Major taxa studiedGraminoids and forbs.MethodsWe focused on two types of leaf traits connected to resource acquisition: morphological features relating to light‐foraging surfaces and investment in tissue (specific leaf area, SLA) and physiological features relating to internal leaf chemistry as the basis for producing and utilizing photosynthate. We measured these traits on 503 leaves from 151 dominant species across 27 grasslands on four continents. We used an identical nutrient addition treatment of nitrogen (N), phosphorus (P) and potassium (K) at all sites. Sites represented a broad range of grasslands that varied widely in climatic and edaphic conditions.ResultsWe found evidence that non‐native graminoids invest in leaves with higher nutrient concentrations than native graminoids, particularly at sites where native and non‐native species both dominate. We found little evidence that native and non‐native forbs differed in the measured leaf traits. These results were consistent in natural soil fertility levels and nutrient‐enriched conditions, with dominant species responding similarly to nutrient addition regardless of whether they were native or non‐native.Main conclusionsOur work identifies the inherent physiological trait advantages that can be used to predict non‐native graminoid establishment, potentially because of higher efficiency at taking up crucial nutrients into their leaves. Most importantly, these inherent advantages are already present at natural soil fertility levels and are maintained following nutrient enrichment.

Eutrophication is a widespread environmental change that usually reduces the stabilizing effect of plant diversity on productivity in local communities. Whether this effect is scale dependent remains to be elucidated. Here, we determine the relationship between plant diversity and temporal stability of productivity for 243 plant communities from 42 grasslands across the globe and quantify the effect of chronic fertilization on these relationships. Unfertilized local communities with more plant species exhibit greater asynchronous dynamics among species in response to natural environmental fluctuations, resulting in greater local stability (alpha stability). Moreover, neighborhood communities that have greater spatial variation in plant species composition within sites (higher beta diversity) have greater spatial asynchrony of productivity among communities, resulting in greater stability at the larger scale (gamma stability). Importantly, fertilization consistently weakens the contribution of plant diversity to both of these stabilizing mechanisms, thus diminishing the positive effect of biodiversity on stability at differing spatial scales. Our findings suggest that preserving grassland functional stability requires conservation of plant diversity within and among ecological communities., Eutrophication has been shown to weaken diversity-stability relationships in grasslands, but it is unclear whether the effect depends on scale. Analysing a globally distributed network of grassland sites, the authors show a positive role of beta diversity and spatial asynchrony as drivers of stability but find that nitrogen enrichment weakens the diversity-stability relationships at different spatial scales.

Soil stores approximately twice as much carbon as the atmosphere and fluctuations in the size of the soil carbon pool directly influence climate conditions. We used the Nutrient Network global change experiment to examine how anthropogenic nutrient enrichment might influence grassland soil carbon storage at a global scale. In isolation, enrichment of nitrogen and phosphorous had minimal impacts on soil carbon storage. However, when these nutrients were added in combination with potassium and micronutrients, soil carbon stocks changed considerably, with an average increase of 0.04 KgCm 2 year 1 (standard deviation 0.18 KgCm 2 year 1 ). These effects did not correlate with changes in primary productivity, suggesting that soil carbon decomposition may have been restricted. Although nutrient enrichment caused soil carbon gains most dry, sandy regions, considerable absolute losses of soil carbon may occur in high-latitude regions that store the majority of the world’s soil carbon. These mechanistic insights into the sensitivity of grassland carbon stocks to nutrient enrichment can facilitate biochemical modelling efforts to project carbon cycling under future climate scenarios. Fil: Crowther, Thomas W.. Univerität strasse; Suiza Fil: Riggs, C.. University of Minnesota; Estados Unidos Fil: Lind, E. M.. University of Minnesota; Estados Unidos Fil: Borer, Elizabeth. University of Minnesota; Estados Unidos Fil: Seabloom, Eric. University of Minnesota; Estados Unidos Fil: Hobbie, S. E.. University of Minnesota; Estados Unidos Fil: Wubs, E. R. Jasper. Netherlands Institute of Ecology; Países Bajos Fil: Adler, Peter. University of Utah; Estados Unidos Fil: Firn, Jennifer. Queensland University of Technology; Australia Fil: Gherardi, Laureano. Arizona State University; Estados Unidos Fil: Hagenah, N.. University of Pretoria; Sudáfrica Fil: Hofmockel, K. S.. University of Iowa; Estados Unidos Fil: Knops, Johannes M. H.. Earth and Biological Sciences Directorate. Pacific Northwest National Laboratory; Estados Unidos Fil: McCulley, R. L.. University of Kentucky; Estados Unidos Fil: MacDougall, A. S.. University of Guelph; Canadá Fil: Peri, Pablo Luis. Instituto Nacional de Tecnología Agropecuaria; Argentina. Universidad Tecnológica Nacional; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Prober, S.M.. CSIRO Land and Water; Australia Fil: Stevens, C. J.. Lancaster University; Reino Unido Fil: Routh, D.. Univerität strasse; Suiza

Almost 50 years ago, Michael Rosenzweig pointed out that nutrient addition can destabilize food webs, leading to loss of species and reduced ecosystem function through the paradox of enrichment. Around the same time, David Tilman demonstrated that increased nutrient loading would also be expected to cause competitive exclusion leading to deleterious changes in food web diversity. While both concepts have greatly illuminated general diversity-stability theory, we currently lack a coherent framework to predict how nutrients influence food web stability across a landscape. This is a vitally important gap in our understanding, given mounting evidence of serious ecological disruption arising from anthropogenic displacement of resources and organisms. Here, we combine contemporary theory on food webs and meta-ecosystems to show that nutrient additions are indeed expected to drive loss in stability and function in human-impacted regions. However, this loss in stability occurs not just from wild oscillations in population abundance, but more frequently from the complete loss of an equilibrium due to edible plant species being competitively excluded. In highly modified landscapes, spatial nutrient transport theory suggests that such instabilities can be amplified over vast distances from the sites of nutrient addition. Consistent with this theoretical synthesis, the empirical frequency of these distant propagating ecosystem imbalances appears to be growing. This synthesis of theory and empirical data suggests that human modification of the Earth’s ecological connectivity is “entangling” once distantly separated ecosystems, causing rapid, expansive, and costly nutrient-driven instabilities over vast areas of the planet. The corollary to this spatial nutrient theory, though -- akin to weak interaction theory from food web networks -- is that slow spatial nutrient pathways can be potent stabilizers by moderating flows across a landscape.

The paradigmatic hypothesis for the effect of fertilisation on plant diversity represents a one-dimensional trade-off for plants competing for below-ground nutrients (generically) and above-ground light: fertilisation reduces competition for nutrients while increasing biomass and thereby shifts competition for depleted available light. The essential problem of this simple paradigm is that it misses both the multivariate and mechanistic nature of the factors that determine biodiversity as well as their causal relationships. We agree that light limitation, as DeMalach and Kadmon argue, can indeed be an important factor associated with diversity loss, and we presented it as an integral part of our tests of the niche dimension hypothesis. We disagree with DeMalach and Kadmon that light is the ‘main’ factor explaining diversity, because this misrepresents the causal structure represented in the design of our experiment in which multiple nutrient addition was the ultimate causal driver of a suite of correlated responses that included diversity and light, and especially live and dead biomass, which are the factors that control light depletion. Our findings highlight that multiple nutrient limitations can structure plant diversity and composition independently of changes in light and biomass. For example, approximately one-third of our sites showed no significant increase in biomass with greater number of added nutrients yet still lost diversity when nutrients were added. The important message is that while light limitation can be an important contributor to diversity loss, it is not a necessary mechanism. Fil: Harpole, W. Stanley. Helmholtz Center for Environmental Research; Alemania Fil: Sullivan, Lauren L.. University of Minnesota; Estados Unidos Fil: Lind, Eric M.. University of Minnesota; Estados Unidos Fil: Firn, Jennifer. Queensland University of Technology; Australia Fil: Adler, Peter B.. State University of Utah; Estados Unidos Fil: Borer, Elizabeth T.. University of Minnesota; Estados Unidos Fil: Chase, Jonathan. Martin Luther University Halle Wittenberg; Alemania Fil: Fay, Philip A.. USDA-ARS Grassland Soil and Water Research Lab; Estados Unidos Fil: Hautier, Yann. Utrecht University; Países Bajos Fil: Hillebrand, Helmut. University of Oldenburg; Alemania Fil: MacDougall, Andrew S.. University of Guelph; Canadá Fil: Seabloom, Eric W.. University of Minnesota; Estados Unidos Fil: Bakker, Jonathan D.. University of Washington; Estados Unidos Fil: Cadotte, Marc W.. University of Toronto; Canadá Fil: Chaneton, Enrique Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía; Argentina Fil: Chu, Chengjin. Sun Yat-sen University; China Fil: Hagenah, Nicole. University of KwaZulu-Natal; Sudáfrica Fil: Kirkman, Kevin. University of KwaZulu-Natal; Sudáfrica Fil: La Pierre, Kimberly J.. Smithsonian Environmental Research Center; Estados Unidos Fil: Moore, Joslin L.. Monash University; Australia Fil: Morgan, John W.. La Trobe University; Australia Fil: Prober, Suzanne M.. CSIRO Land and Water; Australia Fil: Risch, Anita C.. Swiss Federal Institute for Forest, Snow and Landscape Research; Suiza Fil: Schuetz, Martin. Swiss Federal Institute for Forest, Snow and Landscape Research; Suiza Fil: Stevens, Carly J.. Lancaster University; Reino Unido

Summary Clarifying what species are being consumed at what times can improve our understanding of how anthropogenic change affects food web dynamics, with implications for community assembly including restoration. This includes human-based changes to plant communities via species introductions, which can interact with consumer feeding preferences to indirectly alter assembly outcomes including reduced restoration success if planted species are preferentially targeted. We used DNA barcoding of plant material in rodent scat, combined with field-based feeding trials, rodent trapping, and rodent exclosures to test for dietary preferences and assembly impacts of native rodents on a restored and regionally rare tallgrass prairie of central North America. We examined whether native rodents preferred non-targeted and mostly non-native oldfield plants that are more locally abundant, thus protecting the rarer native planted species from consumption, or if rodents preferred native plants regardless of abundance. Our results supported the latter outcome. Barcoding revealed that native rodents consumed mostly non-planted oldfield species for ten months of the year (92% of the diet). The exceptions were August–September, when planted prairie species accounted for 87% of plants consumed and coincided with their peak seed production. Cafeteria trials suggested diet seasonality to be explained by food limitation – native prairie seeds were consumed year-round when made available. Non-targeted oldfield species thus appeared to be of sufficient quality to support rodent populations (peaking at 23 individuals ha−1 in summer), but without rescuing rarer prairie species from targeted granivory and resulting in reduced prairie diversity outside of experimental exclosures. Synthesis and applications. Our work illustrates how anthropogenic-based changes to producer communities may affect feeding pathways in grassland food webs, potentially facilitating establishment by non-target species via indirect consumer effects that can be difficult to detect. These findings suggest that rodents may reduce the restoration success of tallgrass prairie in our region, with some planted species likely to suffer future recruitment difficulties due to granivory. Managers may need to consider multiple trophic levels with restoration, not just the commonly planted producer communities but also the consumers and predators associated with them. This article is protected by copyright. All rights reserved.

Protected areas are a cornerstone of the global strategy for conserving biodiversity, and yet their efficacy in comparison to unprotected areas is rarely tested. In the highly fragmented forests of temperate regions, landscape context and forest history may be more important than protection status for plant species diversity. To determine whether there are differences in plant diversity between protected areas and private lands while controlling for landscape context, forest age, and other important factors. We used a database of 156 one-hectare forest plots distributed over 120,000 km2 in the fragmented forests of southern Ontario to test whether protected areas and private forests differed in native species richness, relative abundance of exotic species, and the probability of finding species of conservation concern. Plots with more forest on the surrounding landscape had higher native species richness, lower abundance of exotic species, and greater probability of supporting at least one species of conservation concern. Young forests tended to have higher abundance of exotics, and were less likely to support species of conservation concern. Surprisingly, privately owned forests had greater native species richness and were more likely to support species of conservation concern once these other factors were accounted for. In addition, there were significant interactions between ownership type, forest history, and landscape context. Our results highlight the importance of privately owned forests in this region, and the need to consider forest history and landscape context when comparing the efficacy of protected areas versus private land for sustaining biodiversity.

Globally, lake fish communities are being subjected to a range of scale-dependent anthropogenic pressures, from climate change to eutrophication, and from overexploitation to species introductions. As a consequence, the composition of these communities is being reshuffled, in most cases leading to a surge in taxonomic similarity at the regional scale termed homogenization. The drivers of homogenization remain unclear, which may be a reflection of interactions between various environmental changes. In this study, we investigate two potential drivers of the recent changes in the composition of freshwater fish communities: recreational fishing and climate change. Our results, derived from 524 lakes of Ontario, Canada sampled in two periods (1965-1982 and 2008-2012), demonstrate that the main contributors to homogenization are the dispersal of gamefish species, most of which are large predators. Alternative explanations relating to lake habitat (e.g., area, phosphorus) or variations in climate have limited explanatory power. Our analysis suggests that human-assisted migration is the primary driver of the observed compositional shifts, homogenizing freshwater fish community among Ontario lakes and generating food webs dominated by gamefish species.

A long-standing hypothesis is that many European plants invade temperate grasslands globally because they are introduced simultaneously with pastoralism and cultivation, to which they are ‘preadapted’ after millennia of exposure dating to the Neolithic era (‘Neolithic Plant Invasion Hypothesis’ (NPIH)). These ‘preadaptations’ are predicted to maximize their performance relative to native species lacking this adaptive history. Here, we discuss the explanatory relevance of the NPIH, clarifying the importance of evolutionary context vs other mechanisms driving invasion. The NPIH makes intuitive sense given established connections between invasion and agricultural-based perturbation. However, tests are often incomplete given the need for performance contrasts between home and away ranges, while controlling for other mechanisms. We emphasize six NPIH-based predictions, centring on trait similarity of invaders between home vs away populations, and differing perturbation responses by invading and native plants. Although no research has integrated all six predictions, we highlight studies suggesting preadaptation influences on invasion. Given that many European grasslands are creations of human activity from the past, current invasions by these flora may represent the continuation of processes dating to the Neolithic. Ironically, European Neolithic-derived grasslands are becoming rarer, reflecting changes in management and illustrating the importance of human influences on these species. Fil: MacDougall, Andrew S.. University of Guelph; Canadá Fil: McCune, Jenny L.. University of Guelph; Canadá. Carleton University; Canadá Fil: Eriksson, Ove. Stockholm Universitet; Suecia Fil: Cousins, Sara A. O.. Stockholm Universitet; Suecia Fil: Pärtel, Meelis. University Of Tartu.; Estonia Fil: Firn, Jennifer. Queensland University of Technology; Australia Fil: Hierro, Jose Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Ciencias de la Tierra y Ambientales de La Pampa. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales. Instituto de Ciencias de la Tierra y Ambientales de La Pampa; Argentina. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales; Argentina

Plant stoichiometry, the relative concentration of elements, is a key regulator of ecosystem functioning and is also being altered by human activities. In this paper we sought to understand the global drivers of plant stoichiometry and compare the relative contribution of climatic vs. anthropogenic effects. We addressed this goal by measuring plant elemental (C, N, P and K) responses to eutrophication and vertebrate herbivore exclusion at eighteen sites on six continents. Across sites, climate and atmospheric N deposition emerged as strong predictors of plot-level tissue nutrients, mediated by biomass and plant chemistry. Within sites, fertilization increased total plant nutrient pools, but results were contingent on soil fertility and the proportion of grass biomass relative to other functional types. Total plant nutrient pools diverged strongly in response to herbivore exclusion when fertilized; responses were largest in ungrazed plots at low rainfall, whereas herbivore grazing dampened the plant community nutrient responses to fertilization. Our study highlights the importance of climate in determining plant nutrient concentrations mediated through effects on plant biomass, that eutrophication affects grassland nutrient pools via both soil and atmospheric pathways and that interactions among soils, herbivores and eutrophication drive plant nutrient responses at small scales, especially at water-limited sites. Fil: Anderson, T. Michael. University Wake Forest; Estados Unidos Fil: Griffith, Daniel M.. State University of Oregon; Estados Unidos Fil: Grace, James B.. United States Geological Survey; Estados Unidos Fil: Lind, Eric M.. University of Minnesota; Estados Unidos Fil: Adler, Peter B.. State University of Utah; Estados Unidos Fil: Biederman, Lori A.. IOWA STATE UNIVERSITY (ISU); Fil: Blumenthal, Dana M.. United States Department Of Agriculture; Estados Unidos Fil: Daleo, Pedro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina Fil: Firn, Jennifer. Queensland University Of Technology Qut; Australia Fil: Hagenah, Nicole. University Of Kwazulu-natal; Sudáfrica Fil: Harpole, W. Stanley. Helmholtz Zentrum Für Umweltforschung; Alemania Fil: MacDougall, Andrew S.. University of Guelph; Canadá Fil: McCulley, Rebecca L.. University of Kentucky; Estados Unidos Fil: Prober, Suzanne M.. Csiro Land And Water; Australia Fil: Risch, Anita C.. Eidgenössische Forschungsanstalt Fur Wald, Schnee Und Landschaft Eth-bereichs; Suiza Fil: Sankaran, Mahesh. International Centre Of Theoretical Science. Tata Institute Of Fundamental Research; España Fil: Schütz, Martin. Eidgenössische Forschungsanstalt Fur Wald, Schnee Und Landschaft Eth-bereichs; Suiza Fil: Seabloom, Eric W.. University of Minnesota; Estados Unidos Fil: Stevens, Carly J.. Lancaster Environment Centre; Reino Unido Fil: Sullivan, Lauren L.. University of Minnesota; Estados Unidos Fil: Wragg, Peter D.. University of Minnesota; Estados Unidos Fil: Borer, Elizabeth T.. University of Minnesota; Estados Unidos

Species richness is regulated by a complex network of scale-dependent processes. This complexity can obscure the influence of limiting species interactions, making it difficult to determine if abiotic or biotic drivers are more predominant regulators of richness. Using integrative modeling of freshwater fish richness from 721 lakes along an 11o latitudinal gradient, we find negative interactions to be a relatively minor independent predictor of species richness in lakes despite the widespread presence of predators. Instead, interaction effects, when detectable among major functional groups and 231 species pairs, were strong, often positive, but contextually dependent on environment. These results are consistent with the idea that negative interactions internally structure lake communities but do not consistently ‘scale-up’ to regulate richness independently of the environment. The importance of environment for interaction outcomes and its role in the regulation of species richness highlights the potential sensitivity of fish communities to the environmental changes affecting lakes globally., Species richness patterns are driven by biotic and abiotic factors, the relative strengths of which are unclear. Here, the authors test how species interactions or environmental traits influence fish richness across over 700 Canadian lakes, showing a surprisingly small role of negative interactions.

Climate change is asymmetrically altering environmental conditions in space, from local to global scales, creating novel heterogeneity. Here, we argue that this novel heterogeneity will drive mobile generalist consumer species to rapidly respond through their behaviour in ways that broadly and predictably reorganize — or rewire — food webs. We use existing theory and data from diverse ecosystems to show that the rapid behavioural responses of generalists to climate change rewire food webs in two distinct and critical ways. First, mobile generalist species are redistributing into systems where they were previously absent and foraging on new prey, resulting in topological rewiring — a change in the patterning of food webs due to the addition or loss of connections. Second, mobile generalist species, which navigate between habitats and ecosystems to forage, will shift their relative use of differentially altered habitats and ecosystems, causing interaction strength rewiring — changes that reroute energy and carbon flows through existing food web connections and alter the food web’s interaction strengths. We then show that many species with shared traits can exhibit unified aggregate behavioural responses to climate change, which may allow us to understand the rewiring of whole food webs. We end by arguing that generalists’ responses present a powerful and underutilized approach to understanding and predicting the consequences of climate change and may serve as much-needed early warning signals for monitoring the looming impacts of global climate change on entire ecosystems. Climate change is spatially asymmetrical and so will alter the behaviour of generalist consumer species, affecting food webs in two ways. Movement into novel ecosystems will affect the topology of food webs, while changes within an ecosystem will affect interaction strengths.

Environmental change can result in substantial shifts in community composition. The associated immigration and extinction events are likely constrained by the spatial distribution of species. Still, studies on environmental change typically quantify biotic responses at single spatial (time series within a single plot) or temporal (spatial beta diversity at single time points) scales, ignoring their potential interdependence. Here, we use data from a global network of grassland experiments to determine how turnover responses to two major forms of environmental change - fertilisation and herbivore loss - are affected by species pool size and spatial compositional heterogeneity. Fertilisation led to higher rates of local extinction, whereas turnover in herbivore exclusion plots was driven by species replacement. Overall, sites with more spatially heterogeneous composition showed significantly higher rates of annual turnover, independent of species pool size and treatment. Taking into account spatial biodiversity aspects will therefore improve our understanding of consequences of global and anthropogenic change on community dynamics.

Plant invasion often increases stand biomass, but higher tissue quality (for example, less lignin and more nutrients) in invasive species might accelerate litter decomposition. This mechanism may minimize increases in soil carbon (C) sequestration despite higher production. Our knowledge about invasion and tissue quality is based on shoots, but roots contribute 50–90% of biomass in vegetation types such as semiarid grasslands. Here we investigate root decomposition rates and tissue quality in the widespread invasive grass Agropyron cristatum, which doubles root mass but not soil C in the Great Plains of North America. Root length was significantly greater beneath Agropyron than native grassland 7 years after minirhizotron installation. However, CO2 evolution from decomposing roots was twice as much for Agropyron roots as for native grass roots (P

Humans dominate many important Earth system processes including the nitrogen (N) cycle. Atmospheric N deposition affects fundamental processes such as carbon cycling, climate regulation, and biodiversity, and could result in changes to fundamental Earth system processes such as primary production. Both modelling and experimentation have suggested a role for anthropogenically altered N deposition in increasing productivity, nevertheless, current understanding of the relative strength of N deposition with respect to other controls on production such as edaphic conditions and climate is limited. Here we use an international multiscale data set to show that atmospheric N deposition is positively correlated to aboveground net primary production (ANPP) observed at the 1-m 2 level across a wide range of herbaceous ecosystems. N deposition was a better predictor than climatic drivers and local soil conditions, explaining 16% of observed variation in ANPP globally with an increase of 1 kg Nha � 1 � yr � 1 increasing ANPP by 3%. Soil pH explained 8% of observed variation in ANPP while climatic drivers showed no significant relationship. Our results illustrate that the incorporation of global N deposition patterns in Earth system models are likely to substantially improve estimates of primary production in herbaceous systems. In herbaceous systems across the world, humans appear to be partially driving local ANPP through impacts on the N cycle.

Summary1. Population decline is associated with increased vulnerability to extinction, but also with possibledensity-, frequency- or distance-related ‘rarity advantages’ that increase recruitment success as indi-viduals become isolated from their congeners. Distinguishing between these alternatives (risk vs.recovery of rare populations via demographic processes) has become critical, given how anthropo-genic disturbances are causing population declines globally.2. Here, we demonstrate how distance-related rarity advantages are evident in spatially isolatedrecruits of a canopy-dominant but regionally rare species of oak that appears to be suffering recruit-ment collapse. As distance from parent trees increased, seedlings had significantly more leaves andexperienced reduced insect browsing and intraspecific competition. Long-term field-based experi-mental treatments revealed these advantages to be associated with rapid rates of juvenile maturationand survival that are unobserved in natural settings.3. The discrepancy between the experimental and natural settings was explained by trophic collapseand habitat loss – two changes ubiquitous to many terrestrial ecosystems – that combine to concen-trate vertebrate herbivores in habitat remnants and cause 100% juvenile mortality via the browsingof taller juveniles. Exotic grass cover, long associated with oak recruitment failure, significantly sup-pressed seedling height and leaf production, but appeared to delay mortality by hiding shorter seed-lings from vertebrate herbivores.4. Synthesis. Our work demonstrates how rarity advantages have the potential to positively influencethe population performance of a declining species, but are short-circuited by intense herbivory asso-ciated with human-based environmental change. Regionally, there appear to be few existing condi-tions on the contemporary landscape that favour juvenile survival, suggesting ongoing recruitmentdifficulties without intervention. Our work clarifies how extinction risk can in some cases be bestdefined by how anthropogenic disturbances affect, and are offset by, demographic-based persistencemechanisms, than simply by present-day abundance or distribution.Key-words: grass invasion, oak recruitment failure, oak savanna, plant population and communitydynamics, rarity advantages, species coexistence, trophic collapseIntroduction

Biodiversity is declining in many local communities while also becoming increasingly homogenized across space. Experimentsshow that local plant species loss reduces ecosystem functioning and services, but the role of spatial homogenization of communitycomposition and the potential interaction between diversity at different scales in maintaining ecosystem functioningremains unclear, especially when many functions are considered (ecosystem multifunctionality). We present an analysis ofeight ecosystem functions measured in 65 grasslands worldwide. We find that more diverse grasslands?those with both species-rich local communities (α-diversity) and large compositional differences among localities (β-diversity)?had higher levelsof multifunctionality. Moreover, α- and β-diversity synergistically affected multifunctionality, with higher levels of diversity at one scale amplifying the contribution to ecological functions at the other scale. The identity of species influencing ecosystem functioning differed among functions and across local communities, explaining why more diverse grasslands maintained greater functionality when more functions and localities were considered. These results were robust to variation in environmental drivers. Our findings reveal that plant diversity, at both local and landscape scales, contributes to the maintenance of multiple ecosystem services provided by grasslands. Preserving ecosystem functioning therefore requires conservation of biodiversity both within and among ecological communities. Fil: Hautier, Yann. University of Utrecht; Países Bajos Fil: Isbell, Forest. University of Minnesota; Estados Unidos Fil: Borer, Elizabeth T.. University of Minnesota; Estados Unidos Fil: Seabloom, Eric W.. University of Minnesota; Estados Unidos Fil: Harpole, Stanley. Martin Luther University Halle-Wittenberg; Alemania Fil: Lind, Eric M.. University of Minnesota; Estados Unidos Fil: MacDougall, Andrew. University of Guelph; Canadá Fil: Stevens, Carly. Lancaster University, ; Reino Unido Fil: Adler, Peter. Utah State University; Estados Unidos Fil: Alberti, Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina Fil: Bakker, Jonathan D. University of Washington, ; Estados Unidos Fil: Brudvig, Lars A. Michigan State University; Estados Unidos Fil: Buckley, Yvonne M. Trinity College Dublin; Irlanda Fil: Cadotte, Marc. University of Toronto Scarborough,; Canadá Fil: Caldeira, María C. University of Lisbon; Portugal Fil: Chaneton, Enrique Jose. Universidad de Buenos Aires; Argentina Fil: Chu, Chengjin. Sun Yat-sen University; China Fil: Daleo, Pedro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina Fil: Dickman, Christopher R. The University of Sydney; Australia Fil: Dwyer, John M. The University of Queensland; Australia Fil: Eskelinen, Anu. University of Oulu; Finlandia Fil: Fay, Philip. USDA-ARS, Grassland Soil and Water Lab; Estados Unidos Fil: Firn, Jennifer. Queensland University of Technology; Australia Fil: Hagenah, Nicole. University of KwaZulu-Natal; Sudáfrica Fil: Hillebrand, Helmut. University Oldenburg; Alemania Fil: Iribarne, Oscar Osvaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina Fil: Kirkman, Kevin P.. University of KwaZulu-Natal; Sudáfrica Fil: Knops, Johannes M. H. University of Nebraska, Lincoln; Estados Unidos Fil: La Pierre, Kimberly J. Smithsonian Environmental Research Center; Estados Unidos Fil: McCulley, Rebecca L. University of Kentucky; Estados Unidos Fil: Morgan, John W.. La Trobe University; Australia Fil: Pärtel, Meelis. University of Tartu; Estonia Fil: Pascual, Jesus Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina Fil: Price, Jodi N. Charles Sturt University; Australia Fil: Prober, Suzanne M. CSIRO Land and Water; Australia Fil: Risch, Anita C. Swiss Federal Institute of Forest, Snow and Landscape Research; Suiza Fil: Sankaran, Mahesh. National Centre for Biological Sciences; India Fil: Schuetz, Martin. Swiss Federal Institute of Forest, Snow and Landscape Research; Suiza Fil: Standish, Rachel J. Murdoch University; Australia Fil: Virtanen, Risto. University of Oulu; Finlandia Fil: Wardle, Glenda M. The University of Sydney; Australia Fil: Yahdjian, María Laura. UBA; Argentina Fil: Hector, Andy. University of Oxford; Reino Unido

Earth's biodiversity and carbon uptake by plants, or primary productivity, are intricately interlinked, underlie many essential ecosystem processes, and depend on the interplay among environmental factors, many of which are being changed by human activities. While ecological theory generalizes across taxa and environments, most empirical tests of factors controlling diversity and productivity have been observational, single-site experiments, or meta-analyses, limiting our understanding of variation among site-level responses and tests of general mechanisms. A synthesis of results from ten years of a globally distributed, coordinated experiment, the Nutrient Network (NutNet), demonstrates that species diversity promotes ecosystem productivity and stability, and that nutrient supply and herbivory control diversity via changes in composition, including invasions of non-native species and extinction of native species. Distributed experimental networks are a powerful tool for tests and integration of multiple theories and for generating multivariate predictions about the effects of global changes on future ecosystems.

Food webs assemble via the interacting constraints of dispersal limitation and bottom-up trophic dependencies where consumers can only establish after their resources have arrived. These factors can affect assembly by influencing the expression of the regional species pool in local patches, but how this unfolds mechanistically remains unclear. Here, we use a large-scale grassland meta-community experiment to demonstrate how the independent influences of spatial factors and bottom-up constraints on insect trophic guilds interact to create divergent local insect communities. Bottom-up trophic dependencies tightly controlled the composition and abundance of specialist trophic guilds, resulting in different communities among islands because producer communities were mainly determined by patch size contingencies. Spatial isolation controlled the composition and abundance of generalist trophic guilds, resulting in different insect communities among islands based on distance from mainland. These results demonstrate that neither diet-based constraints nor the spatial characteristics of islands can predict the early structure of locally assembling food webs, with their establishment deriving instead from interactions between both processes.

1. Grasslands cover similar to 30% of the Earth's terrestrial surface and provide many ecosystem services. Many grasslands are heavily managed to maximize these services for human benefit, but the outcome of management is anticipated to be increasingly influenced by various aspects of climate change and elevated atmospheric CO2. The relative importance of global change vs. land management on grasslands is largely unknown. 2. A meta-analysis is used here to examine drivers at both scales primarily targeting services provided by grasslands relating to plant productivity (above- and below-ground biomass) and soil processes (nutrients and soil respiration) in 38 manipulative experiments published in the last decade. We specifically target effects of (i) single and combined land management practices (LMs), (ii) single and combined factors relating to broad-scale climate change and elevated CO2, and (iii) combined management practices and changes to climate and CO2. Collectively, this examines the general efficacy of global change models in predicting changes to grassland functioning. 3. We found that combinations of management practices had approximately double the explanatory power for variation in grassland services compared with individual or interactive effects of factors associated with climate change and CO2. These interacting management practices such as nutrient additions and defoliation predominantly influenced functions associated with productivity or biomass both below and above ground. The effects of interacting factors of climate and CO2 influenced a wider range of ecosystem functions, but the magnitude of these effects was relatively smaller. 4. Interactions between management practices or between climate change/CO2 factors always had higher explanatory power than any factor in isolation indicating that multivariate synergistic models of environmental change can better describe impacts on ecosystem function in plant communities (e.g. relative to univariate climate-based models). Given that the magnitude and direction (positive or negative) of the interactions varied widely, this also implies that the outcomes of these multivariate interactions can vary spatially, temporally or by immediate context (e.g. management prescriptions). 5. Synthesis. Although our work confirms how climate change and CO2 can affect many ecosystem-based functional attributes, it suggests that combinations of LMs remain the dominant set of factors in determining the performance of grassland plant communities. Land management may thus be critical for influencing projected responses to future climate change and elevated CO2 in models of grassland function at least for factors relating to primary production.

Summary 1. Plant invasions can be promoted by environmental heterogeneity, but the opposite effect, the impact of plant invasion on heterogeneity, has received little attention. Grassland invasions might contribute to decreased spatial heterogeneity because invaders tend to be larger than native vegetation. Lowered heterogeneity may contribute to the low diversity of invaded communities, as well as to the persistence of invasive populations. 2. We compared the spatial heterogeneity of roots and resources in uninvaded native grassland and in stands invaded by a relatively large exotic grass (Agropyron cristatum), in four combinations of mowing and nitrogen (N) addition. We focused on roots because they account for the majority of primary production in grasslands. 3. The spatial heterogeneity of root length (m root m � 2 rhizotron image) and root production was significantly lower beneath A. cristatum than uninvaded grassland. This result was consistent in all combinations of mowing and N addition. 4. Beneath the invader, root length was significantly greater, and the proportion of samples that contained roots was significantly higher. This suggests that the invader decreased spatial heterogeneity by more completely filling the soil volume with roots. 5. Resource heterogeneity varied significantly between vegetation types in just one of four cases examined, suggesting that invader effects on resource heterogeneity were small relative to its effects on root heterogeneity. 6. These results suggest a novel mechanism promoting invader success and persistence: high root heterogeneity, lower root length and empty soil volumes in native grassland may make it relatively vulnerable to invasion, while reduced heterogeneity and greater root length in invaded grasslands may sustain stable, low-diversity communities dominated by the invader. Lowered heterogeneity accompanying invasion may partly account for the wide-spread occurrence of low-diversity, invader-dominated grasslands in North America.

Aim Evidence linking the accumulation of exotic species to the suppression of native diversity is equivocal, often relying on data from studies that have used different methods. Plot-level studies often attribute inverse relationships between native and exotic diversity to competition, but regional abiotic filters, including anthropogenic influences, can produce similar patterns. We seek to test these alternatives using identical scale-dependent sampling protocols in multiple grasslands on two continents. Location Thirty-two grassland sites in North America and Australia. Methods We use multiscale observational data, collected identically in grain and extent at each site, to test the association of local and regional factors with the plot-level richness and abundance of native and exotic plants. Sites captured environmental and anthropogenic gradients including land-use intensity, human population density, light and soil resources, climate and elevation. Site selection occurred independently of exotic diversity, meaning that the numbers of exotic species varied randomly thereby reducing potential biases if only highly invaded sites were chosen. Results Regional factors associated directly or indirectly with human activity had the strongest associations with plot-level diversity. These regional drivers had divergent effects: urban-based economic activity was associated with high exotic : native diversity ratios; climate- and landscape-based indicators of lower human population density were associated with low exotic : native ratios. Negative correlations between plot-level native and exotic diversity, a potential signature of competitive interactions, were not prevalent; this result did not change along gradients of productivity or heterogeneity. Main conclusion We show that plot-level diversity of native and exotic plants are more consistently associated with regional-scale factors relating to urbanization and climate suitability than measures indicative of competition. These findings clarify the long-standing difficulty in resolving drivers of exotic diversity using single-factor mechanisms, suggesting that multiple interacting anthropogenic-based processes best explain the accumulation of exotic diversity in modern landscapes.

Grassland root responses to mowing and fertility are less well known than shoot responses, even though as much as 90% of productivity in semiarid grasslands occurs belowground. Thus, understanding root responses may aid the management of invasive grassland species such as Agropyron cristatum (L.) Gaerth (crested wheatgrass). We asked whether root responses reflect shoot responses to mowing and fertility in native grassland with and without a major component of crested wheatgrass. We subjected grasslands in northern Montana to 5 yr of mowing at two nitrogen (N) levels and followed root responses with minirhizotrons. Surprisingly, the roots of both native and invaded grasslands were unaffected by mowing and N addition, despite significant changes in shoot mass across both vegetation types. Root length was significantly greater beneath areas heavily occupied by crested wheatgrass (363 m · m -2 image ± 200, mean ± standard deviation [SD]) than areas comprising largely native grassland (168 m · m -2 image ± 128 SD). Also, no interactions occurred between year and any other factor, indicating that there were no changes in belowground responses over the 5 yr examined. In contrast, shoot mass was significantly reduced by mowing (not mowed, 612 g · m -2 ± 235 SD; mowed, 239 g · m -2 ± 81 SD) and was significantly increased by N addition (no added N, 380 g · m -2 ± 215 SD; added N, 488 g · m -2 ± 287 SD). In conclusion, 5 yr of mowing decreased shoot mass, but not root mass. On the other hand, 5 yr of N addition increased shoot mass, but not root mass. Given that most production and competition in grasslands occurs belowground, this suggests that mowing may not be a successful tool for reducing crested wheatgrass root length, regardless of soil fertility.

High diversity is often poorly explained by trait-based deterministic models, in part because stochastic processes also influence community assembly. Testing how deterministic and stochastic processes combine to regulate diversity, however, has been limited by the spatial complexity of these interactions. Here, we demonstrate how spatial variability in small-mammal predation on plants, mostly by granivory, results in fine-scale switching between deterministically and stochastically regulated plant community assembly in an otherwise environmentally homogeneous tallgrass prairie. We initiated assembly with the uniform application of a 24-species mixture of prairie grasses and forbs, thereby setting the maximum level of diversity (γ-diversity). In field edges with higher densities of small mammals, traits reducing seed palatability deterministically produced homogeneous subsets of less palatable plant species within the first few months after planting (low α and β diversity). As small-mammal densities decreased in more open areas, assembly unfolded stochastically on the basis of which planted species happened to land at a given location (high α and β diversity). We used randomization models to validate that this higher β diversity was explained by true differences in community structure among plots rather than by the hidden effects of increasing α diversity. The net effect at the site level was a spatially structured array of prairie species, including a positive relationship between diversity and environmental suitability relating to reduced predator intensity.

1. Plant species can influence soil biota, which in turn can influence the relative performance of plant species. These plant–soil feedbacks (PSFs) have been hypothesized to affect many community-level dynamics including species coexistence, dominance and invasion. 2. The importance of PSFs in exotic species invasion, although widely hypothesized, has been difficult to determine because invader establishment necessarily precedes invader-mediated PSFs. Here, we combine a spatial simulation model of invasion that incorporates PSFs with a meta-analysis that synthesizes published case studies describing feedbacks between pairs of native and exotic species. 3. While our spatial model confirmed the link between positive soil feedbacks (‘home’ advantage) for exotic species and exotic species spread, results were dependent on the initial abundance of the exotic species and the equivalence of dispersal and life history characteristics between exotic and native species. 4. The meta-analysis of 52 native–exotic pairwise feedback comparisons in 22 studies synthesized measures of native and exotic performance in soils conditioned by native and exotic species. The analysis indicated that the growth responses of native species were often greater in soil conditioned by native species than in soil conditioned exotic species (a ‘home’ advantage). The growth responses of exotic species were variable and not consistently related to species soil-conditioning effects. 5. Synthesis. Overlaying empirical estimates of pairwise PSFs with spatial simulations, we conclude that the empirically measured PSFs between native and exotic plant species are often not consistent with predictions of the spread of exotic species and mono-dominance. This is particularly the case when exotic species are initially rare and share similar dispersal and average fitness characteristics with native species. However, disturbance and other processes that increase the abundance of exotic species as well as the inclusion of species dispersal and life history differences can interact with PSF effects to explain the spread of invasive species

Given the regulatory impact of resources and consumers on plant production, decomposition, and soil carbon sequestration, anthropogenic changes to nutrient inputs and grazing have likely transformed how grasslands process atmospheric CO2. The direction and magnitude of these changes, however, remain unclear in this system, whose soils contain -20% of the world's carbon pool. Nutrients stimulate production but can also increase tissue palatability and decomposition. Grazing variously affects tissue quality and quantity, decreasing, standing biomass, but potentially increasing leaf nutrient concentrations, root production, or investment in tissue defenses that slow litter decay. Here, we quantified individual and interactive impacts of nutrient addition and simulated grazing (mowing) on above- and belowground production, tissue quality, and soil carbon inputs in a western North American grassland with globally distributed agronomic species. Given that nutrients and grazing are often connected with increased root production and higher foliar tissue quality, we hypothesized that these treatments would combine to reduce inputs of recalcitrant-rich litter critical for C storage. This hypothesis was unsupported. Nutrients and defoliation combined to significantly increase belowground production but did not affect root tissue quality. There were no significant interactions between nutrients and defoliation for any measured response. Three years of nutrient addition increased root and shoot biomass by 37% and 23%, respectively, and had no impact on decomposition, resulting in a -15% increase in soil organic matter and soil carbon. Defoliation triggered a significant burst of short-lived lignin-rich roots, presumably a compensatory response to foliar loss, which increased root litter inputs by 33%. The majority of root and shoot responses were positively correlated, with aboveground biomass a reasonable proxy for whole plant responses. The exceptions were decomposition, with roots six times more decay resistant, and grazing impacts on tissue chemistry, with shoots undergoing significant alterations, while roots were unaffected. Because neither treatment affected concentrations of decay-resistant compounds in roots, the implied net effect is higher soil C inputs with potentially longer residency times. Areas managed with nutrients and moderate grazing in our study system could thus accumulate significantly more soil C than unmanaged areas, with a greater capacity to serve as sinks for atmospheric CO2.

Niche dimensionality provides a general theoretical explanation for biodiversity-more niches, defined by more limiting factors, allow for more ways that species can coexist. Because plant species compete for the same set of limiting resources, theory predicts that addition of a limiting resource eliminates potential trade-offs, reducing the number of species that can coexist. Multiple nutrient limitation of plant production is common and therefore fertilization may reduce diversity by reducing the number or dimensionality of belowground limiting factors. At the same time, nutrient addition, by increasing biomass, should ultimately shift competition from belowground nutrients towards a one-dimensional competitive trade-off for light. Here we show that plant species diversity decreased when a greater number of limiting nutrients were added across 45 grassland sites from a multi-continent experimental network. The number of added nutrients predicted diversity loss, even after controlling for effects of plant biomass, and even where biomass production was not nutrient-limited. We found that elevated resource supply reduced niche dimensionality and diversity and increased both productivity and compositional turnover. Our results point to the importance of understanding dimensionality in ecological systems that are undergoing diversity loss in response to multiple global change factors. Fil: Harpole, W. Stanley. Martin Luther University Halle-Wittenberg; Alemania Fil: Sullivan, Lauren L.. University of Minnesota; Estados Unidos Fil: Lind, Eric M.. University of Minnesota; Estados Unidos Fil: Firn, Jennifer. Queensland University of Technology; Australia Fil: Adler, Peter B.. State University of Utah; Estados Unidos Fil: Borer, Elizabeth T.. University of Minnesota; Estados Unidos Fil: Chase, Jonathan. Martin Luther University Halle-Wittenberg; Alemania Fil: Fay, Philip A.. USDA-ARS Grassland Soil and Water Research Lab; Estados Unidos Fil: Hautier, Yann. Utrecht University; Países Bajos Fil: Hillebrand, Helmut. University of Oldenburg; Alemania Fil: MacDougall, Andrew S.. University of Guelph; Canadá Fil: Seabloom, Eric W.. University of Minnesota; Estados Unidos Fil: Williams, Ryan. University of Iowa; Estados Unidos Fil: Bakker, Jonathan D.. University of Washington; Estados Unidos Fil: Cadotte, Marc W.. University of Toronto; Canadá Fil: Chaneton, Enrique Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía; Argentina Fil: Chu, Chengjin. Sun Yat-sen University; China Fil: Cleland, Elsa E.. University of California at San Diego; Estados Unidos Fil: D'Antonio, Carla. University of California; Estados Unidos Fil: Davies, Kendi F.. University of Colorado; Estados Unidos Fil: Gruner, Daniel S.. University of Maryland; Estados Unidos Fil: Hagenah, Nicole. University of KwaZulu-Natal; Sudáfrica Fil: Kirkman, Kevin. University of KwaZulu-Natal; Sudáfrica Fil: Knops, Johannes M. H.. Universidad de Nebraska - Lincoln; Estados Unidos Fil: La Pierre, Kimberly J.. University of California at Berkeley; Estados Unidos Fil: McCulley, Rebecca L.. University of Kentucky; Estados Unidos Fil: Moore, Joslin L.. Monash University; Australia Fil: Morgan, John W.. La Trobe University; Australia Fil: Prober, Suzanne M.. CSIRO Land and Water; Australia Fil: Risch, Anita C.. Swiss Federal Institute for Forest, Snow and Landscape Research; Suiza Fil: Schuetz, Martin. Swiss Federal Institute for Forest, Snow and Landscape Research; Suiza Fil: Stevens, Carly J.. Lancaster University; Reino Unido Fil: Wragg, Peter D.. University of Yale; Estados Unidos

Summary 1. Plant establishment is critical for community assembly, but mechanisms regulating establishment can be obscured by covarying influences of incoming seed diversity and fine-scale spatial environmental heterogeneity (microhabitat heterogeneity). Both can maximize establishment, depending on whether species differences or environmental variability more fundamentally structures plant communities. 2. We experimentally assembled limestone-pavement herb communities to examine the relative effects of seed diversity and microhabitat heterogeneity on establishment. This included testing (i) whether effects of seed diversity strengthen with heterogeneity, as would be expected if potential niche differences are more strongly expressed in more heterogeneous environments and (ii) whether a greater number of incoming species can establish in more heterogeneous environments due to environmental filtering. Species interaction theory predicts that increased facilitation and niche complementarity with realized diversity has the potential to increase overall community density. 3. Heterogeneity operated independently of seed biodiversity and explained establishment even when spatially averaged microhabitat conditions were accounted for. Homogeneous plots sown with six species supported establishment of plant density that could be increased c. 10% by doubling added seed diversity (while holding heterogeneity constant), but increased c. 40% by holding seed diversity constant and maximizing heterogeneity. 4. Ordinations revealed that species establishment was sorted by gradients in soil pH and surface cover by moss, litter and open bedrock. Regressions indicated more species established in plots featuring a greater diversity of surface cover types and ⁄ or greater soil depth variability. Community density increased with established richness, and heterogeneity ceased to explain variance in density once established richness was included as an explanatory variable. All but one of the sown species exhibited increased population density with increased plot richness. 5. Synthesis: Community density in a high-stress environment increased with both fine-scale spatial heterogeneity and added seed diversity. However, these effects were independent of one another, and impacts of heterogeneity were stronger than those of seed diversity. Our results suggest heterogeneity promotes density indirectly, through downstream effects of enhanced establishment diversity such as facilitation. These findings confirm establishment-stage interrelationships among biodiversity, density and heterogeneity as overlooked determinants of community structure by providing important field support for ideas primarily tested in the greenhouse.

Allelopathic phytochemicals have been linked to invasion success, but their role in the invasion process remains unclear. Toxicity effects demonstrated with lab bioassays may be neutralized in soils, and their role in population expansion can be intertwined with nonallelopathic processes that also influence dispersal and establishment. Here, we use greenhouse experiments to test the soil-based impacts of invasive fine fescue ( Festuca rubra L.) on recruitment in tallgrass prairie. Fescue roots release the growth inhibitor m-tyrosine. Using root washes and fescue-conditioned soils to mimic field potency, we determined allelopathic impacts on recruitment, including intraspecific limitation. We also tested whether nonallelopathic factors (propagule pressure, disturbance, and fertility) influence invasion into constructed fescue and prairie mesocosms, and whether root washes inhibit arbuscular mycorrhizal (AM) fungi. We observed significant negative effects of fescue soils and root washes on germination and seedling survival, including on fescue itself. Mesocosm invasion, however, was determined more by nonallelopathic mechanisms (propagule pressure and rapid growth). In prairie mesocosms, fescue invasion was higher than its own understory, with no effects of disturbance or fertility. Tallgrass species had difficulty establishing in all environments, regardless of propagule pressure. Impacts on AM fungal hyphal length and spore production were insignificant. Our results suggest that nonallelopathic traits may be sufficient to explain fescue invasion, with allelopathy likely emerging as a final "coup de grâce" for recruiting native grasses once dominance has been attained. Allelopathic species, including fine fescue, may thus not necessarily be invasive unless nonallelopathic traits facilitate establishment prior to the accumulation of soil-based toxins.

Root dynamics are among the largest knowledge gaps in determining how terrestrial carbon (C) cycles will respond to environmental change. Increases in productivity accompanying plant invasions and introductions could increase ecosystem C storage, but belowground changes are unknown, even though roots may account for 50-90% of production in temperate ecosystems. We examined whether the introduction of a widespread invasive grass with relatively high shoot production also increased belowground productivity and soil C storage, using a multiyear rhizotron study in 50-year-old stands dominated either by the invasive C3 grass Agropyron cristatum or by largely C4 native grasses. Relative to native vegetation, stands dominated by the invader had doubled root productivity. Soil carbon isotope values showed that the invader had made detectable contributions to soil C. Soil C content, however, was not significantly different between invader-dominated stands (0.42 mg C/g soil) and native vegetation (0.45 mg C/g soil). The discrepancy between enhanced production and lack of soil C changes was attributable to differences in root traits between invader-dominated stands and native vegetation. Relative to native vegetation, roots beneath the invader had 59% more young white tissue, with 80% higher mortality and 19% lower C:N ratios (all P < 0.05). Such patterns have previously been reported for aboveground tissues of invaders, and we show that they are also found belowground. If these root traits occur in other invasive species, then the global phenomenon of increased productivity following biological invasion may not increase soil C storage.

Many ecosystems worldwide are dominated by introduced plant species, leading to loss of biodiversity and ecosystem function. A common but rarely tested assumption is that these plants are more abundant in introduced vs. native communities, because ecological or evolutionary-based shifts in populations underlie invasion success. Here, data for 26 herbaceous species at 39 sites, within eight countries, revealed that species abundances were similar at native (home) and introduced (away) sites – grass species were generally abundant home and away, while forbs were low in abundance, but more abundant at home. Sites with six or more of these species had similar community abundance hierarchies, suggesting that suites of introduced species are assembling similarly on different continents. Overall, we found that substantial changes to populations are not necessarily a pre-condition for invasion success and that increases in species abundance are unusual. Instead, abundance at home predicts abundance away, a potentially useful additional criterion for biosecurity programmes.

A number of North American oaks are experiencing recruitment failure, with explanatory hypotheses including a range of consumer- or resource-based limitations. Using a factorial experiment with transplanted seedlings, we demonstrate how direct and indirect consumer effects prevent recruitment by Quercus garryana, a keystone savanna species of northwestern North America. Foremost, intense herbivory by mostly exotic small mammals severely damaged or killed 100% of unprotected seedlings during winter. Many damaged seedlings survived but were 64% smaller in size and produced 75% fewer leaves. Herbivory by deer had no detectable impact despite being long hypothesized as a major contributor to oak decline. Indirectly, herbivory altered the outcome of moisture limitation and competition. Without small mammals, summer drought and a dense exotic grass cover associated with fire suppression significantly reduced growth but caused little mortality. With small mammals, both significantly increased mortality of herbivore-damaged seedlings. Herbivore damage also increased the likelihood of severe insect attack, possibly due to reduced investment in foliar defense by recovering seedlings. These results show that no single factor necessarily prevents seedling establishment by oaks, but that the combination of herbivory, undisturbed exotic grass swards, and summer drought creates an almost insurmountable barrier for recruitment.