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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

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.

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

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.

Summary 1. For plant invaders, being different is often equated with being successful, yet the mechanistic connection remains unclear. 2. Classic niche theory predicts that invaders with niches distinct from the native flora should coexist with little interaction with native species, yet such invaders often have substantial impacts. Meanwhile, invaders that overlap in niche space with native species should either be repelled or dominate, yet these invaders often naturalize with little effect. Such discrepancies between theory and observation raise questions about how species differences influence invader establishment and impact. 3. Here, we review these issues in light of recent work on coexistence theory, which shows how niche and fitness differences between natives and invaders interact to determine invasion outcomes. We show how successful invader establishment depends on either a fitness advantage or niche difference from resident species, but that only the former allows invaders to become dominant. 4. By identifying the role of niche and fitness differences in leading invasion hypotheses, we unify their predictions for invasion success while highlighting new approaches for evaluating the importance of species differences for invasion. 5. Synthesis. Situating the invasion process within a recent coexistence framework broadens our understanding of invasion mechanisms and more tightly links problems in invasion ecology with our more general understanding of community dynamics.

Summary 1 We used seed additions to test experimentally whether long-term community assembly unfolds consistently (deterministic model) or whether different outcomes are possible depending on variations in climate and the presence of exotic dominants (stochastic model). The experiment was conducted in homogeneous semi-arid grassland on the northern Great Plains of North America. Native grass seed additions in each of 3 years (1994–96) were combined factorially with repeated selective herbicide applications to reduce the cover of the extant dominant, a Eurasian C3 grass (Agropyron cristatum). We assessed whether composition converged or diverged among treatments between 1996 and 2004. 2 The outcome of long-term community assembly varied by establishment year, in association with significant variations in monthly growing-season precipitation from 68-year averages during the years when seed was added. In 2004, the added native C4 grass Bouteloua gracilis dominated plots sprayed and seeded in 1994 and 1995, resisting re-invasion and having significantly higher plot diversity. In contrast, plots sown in 1996 – one of the driest years on record – reverted to the exotic C3 grass. Seeded but unsprayed plots maintained their pre-treatment dominance by exotic grass. Rainfall variability was also associated with the extent of exotic grass cover and bare soil in unsprayed plots, and natural recruitment by native species from nearby prairie. 3 Establishment success varied significantly among the added species. All seeded grasses, other than B. gracilis, were mostly absent by 2004. The failure of these native C3 species to establish appears consistent with deterministic assembly models where local abiotic conditions filter out unsuitable species. 4 Synthesis. Our results indicate that stochastic and deterministic processes operate simultaneously to influence community assembly, depending on interactions among climate, seed availability, species identity and disturbance during the initial stages of establishment. Multiple assembly trajectories developed, but the assembled communities did not include all the added species. Conversion of degraded grassland back to native-dominated grassland was possible, but only with the removal of the exotic dominant and seed additions during years that were suitably wet for establishment. Once formed, however, the assembled native community resisted re-invasion. This suggests that native grassland restoration will depend on establishment measures as intense as those used during initial cultivation almost a century earlier.

Summary 1 Invasive species dominate many ecosystems but the competitive strategies underlying this dominance are unclear. Are invasive species generalist competitors, or do they only thrive in certain environments? Do they occur mainly post-disturbance or can they persist throughout succession? 2 We tested the relative importance of resource acquisition (competitive suppression ability) and the ability to tolerate reduced resource levels (competitive tolerance ability) among four C3 perennial grass species in an invaded oak savanna. Two species ( Poa pratensis and Dactylis glomerata ) are exotic invaders and are thought to have replaced the two native species ( Bromus carinatus and Elymus glaucus ) as dominants. 3 Using glasshouse and field experiments we tested whether the two strategies were maintained with changing resource levels and successional conditions, and their relative roles in explaining exotic dominance. 4 The relative importance of suppression- and tolerance-based competition shifted with neighbour density, burning and planting order. Further, the relative importance of particular plant traits changed depending on the imposed conditions, and the exotic dominants were only competitively superior under certain circumstances. 5 Competitive suppression ability was maintained with changing resource levels but was confined to post-disturbance conditions. When planting of neighbours was delayed, the early establishing targets were dominant regardless of species, fertility and neighbour density. 6 Competitive tolerance ability determined long-term patterns of relative abundance and coexistence, but only under the current field conditions of low fertility and limited disturbance. Alteration of these conditions changed the relative abundance of the four grasses, and would probably reconfigure species patterns in the oak savanna community generally. 7 Exotic dominance is presently determined by tolerance-based competitive traits interacting with the long-term absence of disturbance. Dominance is therefore contingent on the interaction of competitive strategies, resource availability and disturbance history rather than any one factor alone. 8 Exotic flora dominate all stages of succession in this savanna because there exists both early (suppression) and late (tolerance) successional specialists. The identity of the dominant changes with succession based on the competitive strategy it employs. This result highlights the importance of examining the historical context of invaded communities and tracking their successional status over time.