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Introduced species threaten native biodiversity, but whether exotic species can competitively displace native species remains contested. Building on theory that predicts multi-species coexistence based on a competition-colonisation tradeoff, we derive a mechanistic basis by which human-mediated species invasions could cause extinctions through competitive displacement. In contrast to past invasions, humans principally introduce modern invaders, repeatedly and in large quantities, and in ways that can facilitate release from enemies and competitors. Associated increases in exotic species’ propagule rain, survival and competitive ability could enable some introduced species to overcome the tradeoffs that constrain all other species. Using evidence from metacommunity models, we show how species introductions could disrupt species coexistence, generating extinction debts, especially when combined with other forms of anthropogenic environmental change. Even though competing species have typically coexisted following past biogeographic migrations, the multiplicity and interactive impacts of today’s threats could change some exotic species into agents of extinction. Introduced species may displace ecologically similar native species, but mechanisms are still to be established. Here, Catford et al. provide theoretical evidence of how human-mediated species invasions may overcome competition-colonisation tradeoffs, leading to the local extinction of native species.

CONTENTS: 19 I. 19 II. 20 III. 24 IV. 26 V. 30 VI. 32 33 References 33 SUMMARY: Riparian vegetation is exposed to stress from inundation and hydraulic disturbance, and is often rich in native and alien plant species. We describe 35 traits that enable plants to cope with riparian conditions. These include traits for tolerating or avoiding anoxia and enabling underwater photosynthesis, traits that confer resistance and resilience to hydraulic disturbance, and attributes that facilitate dispersal, such as floating propagules. This diversity of life‐history strategies illustrates that there are many ways of sustaining life in riparian zones, which helps to explain high riparian biodiversity. Using community assembly theory, we examine how adaptations to inundation, disturbance and dispersal shape plant community composition along key environmental gradients, and how human actions have modified communities. Dispersal‐related processes seem to explain many patterns, highlighting the influence of regional processes on local species assemblages. Using alien plant invasions like an (uncontrolled) experiment in community assembly, we use an Australian and a global dataset to examine possible causes of high degrees of riparian invasion. We found that high proportions of alien species in the regional species pools have invaded riparian zones, despite not being riparian specialists, and that riparian invaders disperse in more ways, including by water and humans, than species invading other ecosystems.

Little is currently known about how climate modulates the relationship between plant diversity and soil organic carbon and the mechanisms involved. Yet, this knowledge is of crucial importance in times of climate change and biodiversity loss. Here, we show that plant diversity is positively correlated with soil carbon content and soil carbon-to-nitrogen ratio across 84 grasslands on six continents that span wide climate gradients. The relationships between plant diversity and soil carbon as well as plant diversity and soil organic matter quality (carbon-to-nitrogen ratio) are particularly strong in warm and arid climates. While plant biomass is positively correlated with soil carbon, plant biomass is not significantly correlated with plant diversity. Our results indicate that plant diversity influences soil carbon storage not via the quantity of organic matter (plant biomass) inputs to soil, but through the quality of organic matter. The study implies that ecosystem management that restores plant diversity likely enhances soil carbon sequestration, particularly in warm and arid climates. Soil carbon content is positively related with plant diversity in global grasslands, and this relationship is particularly strong in warm and arid climates. Plant diversity is related to soil carbon via the quality of organic matter.

The biodiversity–ecosystem functioning (BEF) literature provides strong evidence of the biophysical basis for the potential profitability of greater diversity but does not address questions of optimal management. BEF studies typically focus on the ecosystem outputs produced by randomly assembled communities that only differ in their biodiversity levels, measured by indices such as species richness. Landholders, however, do not randomly select species to plant; they choose particular species that collectively maximize profits. As such, their interest is not in comparing the average performance of randomly assembled communities at each level of biodiversity but rather comparing the best-performing communities at each diversity level. Assessing the best-performing mixture requires detailed accounting of species’ identities and relative abundances. It also requires accounting for the financial cost of individual species’ seeds, and the economic value of changes in the quality, quantity, and variability of the species’ collective output—something that existing multifunctionality indices fail to do. This study presents an assessment approach that integrates the relevant factors into a single, coherent framework. It uses ecological production functions to inform an economic model consistent with the utility-maximizing decisions of a potentially risk-averse private landowner. We demonstrate the salience and applicability of the framework using data from an experimental grassland to estimate production relationships for hay and carbon storage. For that case, our results suggest that even a risk-neutral, profit-maximizing landowner would favor a highly diverse mix of species, with optimal species richness falling between the low levels currently found in commercial grasslands and the high levels found in natural grasslands.

Riparian forests are structured and maintained by their hydrology. Woody riparian plants typically adapt to the local flood regime to maximise their likelihood of survival and reproductive success. Understanding how extant trees form and reproduce in response to flood disturbance is crucial for predicting vegetation changes and informing restoration. Working in a temperate evergreen riparian forest, we aimed to determine whether disturbance-based responses of plants found in other ecosystems also typify woody plants in riparian forests where disturbances are often mild or chronic, non-lethal, annual events. Using plant surveys and 20-year modelled hydrological data, we examined whether (1) the morphology (main stem diameter, height, crown width, crown extent, stem leaning) and (2) reproduction type (sexual and asexual reproduction) and extent of three dominant woody species ( Eucalyptus camphora , Leptospermum lanigerum and Melaleuca squarrosa ) vary with flood regime (flood frequency and flood duration); and (3) whether different morphology is associated with different reproductive strategies. Increased flooding generally resulted in increased stem numbers and greater stem leaning—morphologies associated with asexual reproduction—of our study species. More frequent flooding also reduced plant size and sexual reproduction in E. camphora . Sexual reproduction in the studied species was more common in taller plants with single, more upright stems in good condition. Flexible morphology and plastic reproductive strategy may constitute an adaptation of trees to mild or chronic disturbance in floodplains. Our findings suggest that flood regime (i.e. variable frequency and duration of flooding events) is critical to the structural integrity and self-maintenance of species-diverse riparian forests.

Plant productivity varies due to environmental heterogeneity, and theory suggests that plant diversity can reduce this variation. While there is strong evidence of diversity effects on temporal variability of productivity, whether this mechanism extends to variability across space remains elusive. Here we determine the relationship between plant diversity and spatial variability of productivity in 83 grasslands, and quantify the effect of experimentally increased spatial heterogeneity in environmental conditions on this relationship. We found that communities with higher plant species richness (alpha and gamma diversity) have lower spatial variability of productivity as reduced abundance of some species can be compensated for by increased abundance of other species. In contrast, high species dissimilarity among local communities (beta diversity) is positively associated with spatial variability of productivity, suggesting that changes in species composition can scale up to affect productivity. Experimentally increased spatial environmental heterogeneity weakens the effect of plant alpha and gamma diversity, and reveals that beta diversity can simultaneously decrease and increase spatial variability of productivity. Our findings unveil the generality of the diversity-stability theory across space, and suggest that reduced local diversity and biotic homogenization can affect the spatial reliability of key ecosystem functions. The insurance hypothesis posits that more diverse communities are more stable through time. Here, the authors show that plant biodiversity reduces the spatial variability of productivity in grassland communities, demonstrating that the insurance hypothesis applies also across space.

1. River regulation and exotic plant invasion threaten riverine ecosystems, and the two often co-occur. By altering water regimes, flow regulation can facilitate plant invasion by providing conditions that directly benefit invading species, or by reducing competition from native species unsuited to the modified conditions. Integrating water and weed management has the potential to limit riparian plant invasion and maximize the ecological benefit of environmental flows. 2. We surveyed plant communities and modelled flood histories of 24 riparian wetlands along the regulated River Murray, south-eastern Australia. There were no suitable control rivers, so we compared modelled pre- and post-regulation hydrological data to quantify hydrological change in the study wetlands. Regression analyses revealed relationships between hydrological modification and cover of native non-weed, native weed and exotic weed groups and 10 individual species. 3. Exotic cover was highest and native non-weed cover lowest in wetlands that had experienced the greatest change in hydrology — a reduction in peak flow. Native weeds did not respond to hydrological modification indicating that exotic species' success was not reliant on their generalist characteristics. 4. By altering habitat filters, hydrological modification caused a decline in amphibious native non-weed species cover and simultaneously provided drier conditions that directly favoured the exotic species group dominated by terrestrial species. Exotic species were potentially further assisted by human-mediated dispersal. 5. Species and functional diversity was inversely related to exotic cover. By shifting the balance between native and exotic taxa and changing community functional composition, flow regulation may disrupt the ecological function and ecosystem services of floodplain wetlands. 6. Synthesis and applications. Worldwide, flow regulation has led to riverine ecosystems becoming more terrestrial. The success of most introduced plants relies on minimal inundation. In this study, flood magnitude was more important than frequency, timing, or duration for wetland flora because it reflects spatial extent and depth of flooding. Augmenting natural spring floods with environmental flows will kill terrestrial weeds and facilitate native macrophyte growth. Combined with strategies for managing particular amphibious weeds, we recommend flows of 117 000—147 000 ML day⁻¹ for at least 2 days every 10 years for River Murray wetland weed management.

Charles Darwin posited two alternative hypotheses to explain the success of nonnative species based on their relatedness to natives: nonnative species that are closely related to native species could experience (1) higher invasion success because of an increased probability of habitat suitability (conferred by trait similarity) or (2) lower invasion success due to biotic interference, such as competition and limiting similarity. The paradox raised by the opposing predictions of these two hypotheses has been termed \"Darwin's naturalization conundrum\" (DNC). Using plant communities measured repeatedly across an experimental fire gradient in an oak savanna (Minnesota, USA) over 31 yr, we evaluated the DNC by incorporating taxonomic, functional, and phylogenetic information. We used a \"focal-species\" approach, in which the taxonomic, functional, and phylogenetic structure of species co-occurring with a given nonnative (focal) species in local communities was quantified. We found three main results: first, nonnative species tended to co-occur most with closely related natives, except at the extreme ends of the fire gradient (i.e., in communities with no fire and those subjected to high fire frequencies); second, with increasing fire frequency, nonnative species were functionally more similar to native species in recipient communities; third, functional similarity between co-occurring nonnatives and natives was stable over time, but their phylogenetic similarity was not, suggesting that dynamic external forces (e.g., climate variability) influenced the phylogenetic relatedness of nonnatives to natives. Our results provide insights for understanding invasion dynamics across environmental gradients and highlight the importance of evaluating different dimensions of biodiversity in order to draw stronger inferences regarding species co-occurrence at different spatial and temporal scales.

The extent of alien plant invasion and numbers of invasive species are increasing, exacerbating invasion impacts. Effective and efficient management requires understanding the drivers and distribution of plant invasions at the landscape scale. In this study, we used a species distribution modelling approach to determine whether the patterns and correlates of alien invasion vary by plant growth form. Focusing on the occupancy and proportional cover of forbs, graminoids and woody vegetation, we used boosted regression trees (BRTs) to characterise alien plant invasion risk in two major catchment regions in Victoria, Australia. Of 7,630 quadrats surveyed between 1970 and 2019, 69% contained alien plants, with forbs being the most prevalent growth form. Alien plants constituted 22% of the total number of plant species recorded. Alien species cover varied widely, with alien species contributing between 0.2% to 100% of vegetation cover in a plot. Alien forbs and graminoids had higher mean cover compared to alien woody plants. Abiotic conditions, particularly temperature and precipitation, had the greatest influence on alien plant invasion overall, explaining 41–76% of observed variation. Summer mean maximum temperature was a strong predictor across all growth forms. Areas with higher vegetation cover (native + alien species) were predicted to have higher occupancy, but lower proportional cover, of alien forbs and graminoids. In contrast, alien woody plants had a negative relationship with woody vegetation cover. High levels of invasion were predicted in areas with intensive land use, such as urban and agricultural zones. Forbs had a high probability of occupancy throughout the region, even in higher elevations, while graminoids and woody vegetation were more restricted to lower elevations and areas characterized by human activity. The study highlights that alien plant invasion is influenced by a complex interplay of abiotic factors, propagule pressure, human activity and biotic conditions. The findings underscore that, while there are common drivers across growth forms, specific patterns and influences vary. For instance, alien forbs were more widespread but less dominant in areas with high vegetation cover, while alien woody plants were less common and more constrained by vegetation than the other two growth forms. Management strategies should prioritise maintaining and restoring native vegetation to limit the dominance of alien species and controlling invasive plants after disturbance. Although single-species models remain valuable, our study shows that species distribution models based on growth form offer a practical approach for assessing plant invasions across diverse landscapes.

Eutrophication usually impacts grassland biodiversity, community composition, and biomass production, but its impact on the stability of these community aspects is unclear. One challenge is that stability has many facets that can be tightly correlated (low dimensionality) or highly disparate (high dimensionality). Using standardized experiments in 55 grassland sites from a globally distributed experiment (NutNet), we quantify the effects of nutrient addition on five facets of stability (temporal invariability, resistance during dry and wet growing seasons, recovery after dry and wet growing seasons), measured on three community aspects (aboveground biomass, community composition, and species richness). Nutrient addition reduces the temporal invariability and resistance of species richness and community composition during dry and wet growing seasons, but does not affect those of biomass. Different stability measures are largely uncorrelated under both ambient and eutrophic conditions, indicating consistently high dimensionality. Harnessing the dimensionality of ecological stability provides insights for predicting grassland responses to global environmental change. Anthropogenic eutrophication is a driver of plant community shifts in many grassland ecosystems. Here, the authors use data from a globally distributed experiment to assess how nutrient addition affects multiple facets of grassland ecological stability and their correlations.