Ayub M. O. Oduor

Non-native plant invasions pose a global challenge, yet how nutrient enrichment and artificial light at night (ALAN) interact with native-community species richness to shape invasive-plant performance and community-level dominance remains unclear. We conducted a multispecies greenhouse experiment in which target invasive species and native communities were established simultaneously under fully crossed factorial combinations of ALAN (No-ALAN or ALAN), nutrient enrichment (low or high) and native species richness (two, four or eight species). We quantified target invasive-plant above-ground biomass, flowering probability and proportional above-ground biomass, as well as native-community and total-community above-ground biomass. Nutrient enrichment increased invader above-ground biomass by 272.88%, flowering probability by 30.39% and native-community above-ground biomass by 97.58%, although its effect on proportional invader biomass remained uncertain. ALAN did not clearly affect invader biomass or flowering but increased its proportional biomass under high-nutrient conditions by 5.74% and reduced it under low nutrients by 4.75%. Native species richness showed no clear effects on invader performance, community biomass or responses to ALAN and nutrients. Within this experimental context, invasive-plant performance was driven primarily by nutrient enrichment, with ALAN facilitating invader dominance under high-nutrient conditions, indicating that invasion outcomes were more influenced by resource availability than by native-community species richness.

Climate warming is seasonally asymmetric, yet its impact on plant invasions is typically studied using uniform year‐round warming, which may obscure season‐specific mechanisms. In a pristine grassland, we used a four‐year 2 × 2 factorial field‐warming experiment coupled with a greenhouse bioassay to test how growing‐ and nongrowing‐season warming influence soil nitrogen, fungal communities, and subsequent invasive plant performance. Growing‐season warming increased soil NH ₄ ⁺ ‐N, NO ₃ ⁻ ‐N, and available nitrogen, and elevated the effective species number of pathogenic fungi. Nongrowing‐season warming increased soil NO ₃ ⁻ ‐N and generated soil legacies that enhanced invasive plant biomass and root allocation. Conversely, year‐round warming reduced soil NO ₃ ⁻ ‐N and suppressed invasive biomass relative to nongrowing‐season warming. Moreover, invasive plant biomass was positively related to soil NO ₃ ⁻ ‐N but showed no correlation with the effective species number of pathogenic fungi. Our findings show that warming produces seasonally contingent soil legacies mediated by specific nitrogen pathways. Predicting invasion dynamics under climate change requires seasonally explicit frameworks, rather than relying on annual mean temperature increases alone.

Herbivory shapes plant invasion outcomes, yet its role in aquatic plant invasions under artificial light at night (ALAN) remains poorly understood. We conducted three experiments using an invasive macrophyte Myriophyllum aquaticum , a native macrophyte community comprising Vallisneria natans , Hydrilla verticillata , Myriophyllum spicatum , as well as an invasive snail Pomacea canaliculata , and a native snail Cipangopaludina chinensis , in our study area, to test the combined effects of ALAN and herbivory on nonnative macrophyte invasions. In the absence of grazing, ALAN increased M. aquaticum height and total biomass, but had no effect on native species. In 4‐day feeding assays, P. canaliculata consumed all native species but consistently avoided M. aquaticum under both light treatments. In community mesocosms, the invasive snail reduced native macrophyte biomass by 48.0% under No‐ALAN and by 87.2% under ALAN without affecting M. aquaticum , thereby increasing proportional biomass of M. aquaticum within the community, with a stronger effect under ALAN. Therefore, ALAN may indirectly facilitate nonnative macrophyte invasions by amplifying their relative biomass within native communities, particularly in the presence of invasive herbivores, and may promote invasional meltdown through selective herbivory.

• Nutrient addition, allelopathy, soil biota, and soil legacy effects impacted plant growth. • Allelopathy and soil microbes were key factors in invasion resistance. • A high diversity of native plants suppressed the growth of invasive species, possibly by promoting a richer community of soil microbes. • Addition of activated carbon increased biomass of the low-diversity plant communities, indicating strong allelopathic interactions in such communities. • Invasive species exhibited similar growth in soils conditioned by low- and high-diversity native communities.

In this chapter, we summarise the level of biological invasion in grasslands around the world. For naturalised plants in grasslands, we use the SynHab database of invasions in different habitats to identify: the most globally widespread naturalised species, the known number of naturalised and invasive plant species per continent, the most represented plant families, and the representation of perennial versus annual/biennial plants. We then review notable invasive plant and animal species with high impacts in grasslands per continental region. The most widespread plant species are mainly herbaceous and perennial. All continental regions except the Antarctic and Pacific have more than 500 naturalised plant species in grasslands. However, the grass family is poorly represented by naturalised species compared to other families. Annuals/biennials are well represented among naturalised plants, while impactful invasive plants include annual and perennial herbs and grasses, trees and shrubs. Grasses feature heavily among invasive plants on multiple continents, but not in Africa. On several continents, examples of invasive animals include ants and multiple mammal herbivores, and the same species can exert similar ecological and livelihood impacts wherever they invade. Management of invasive species in grasslands will, therefore, be enhanced by sharing of management knowledge across continents.

Rising tropospheric ozone (O ₃ ) and invasion of exotic plant species are two major global environmental challenges. While elevated O ₃ is known to directly influence plant invasion by asymmetrically inhibiting the growth of exotic and native species, its indirect effects on invasions via biotic interactions remain poorly understood. We conducted a common‐garden experiment to test the effects of elevated O ₃ and herbivory on invasion of exotic plant species. We established replicate native plant communities, each with the same composition of five native species and separately invaded them with 14 exotic plant species under factorial combinations of O ₃ (ambient versus elevated) and herbivory (present versus absent). Above‐ground biomass of invasive species was significantly influenced by Herbivory × O ₃ interactions, whereas native biomass and the proportional biomass of invaders responded only to herbivory. Under ambient O ₃ , herbivory strongly suppressed biomass of invasive plants, but this effect largely disappeared under elevated O ₃ . By contrast, herbivory consistently reduced biomass of native plant communities and lowered proportional biomass of invasive plants regardless of O ₃ . Invasive species showed strong interspecific variation in biomass responses to herbivory: Datura stramonium, Lolium perenne, Medicago sativa, Senecio vulgaris, Sesbania cannabina and Trifolium repens showed strong reductions in above‐ground biomass, with S. cannabina being most severely affected. Structural equation models confirmed direct negative effects of O ₃ and herbivory on invaders, with herbivory exerting the strongest suppressive influence. Our multispecies experiment supports previous evidence that elevated O ₃ can hinder exotic plant growth but provides novel evidence that it can also reduce the suppressive effect of herbivores, thereby facilitating invasion under O ₃ ‐polluted conditions.

Premise: The diversity–invasibility hypothesis suggests that native plant communities with high species diversity are more resistant to invasions by exotic species compared to those with fewer species. This resistance stems from more complete resource use and stronger biotic interactions in diverse communities, which limit opportunities for invaders to establish. However, this resistance could potentially be weakened by environmental stressors, including elevated tropospheric ozone. Ozone levels have increased globally from 10 ppb in pre‐industrial times to around 50 ppb today. Elevated ozone affects plant species differently, depending on their functional traits and physiological tolerances. Yet, no prior study has tested how elevated ozone affects the link between native plant community diversity and invasion resistance. We thus set up an open‐top chamber experiment to assess the impact of ozone enrichment on invasive plant growth in native communities of differing species richness.Methods: We established 16 native plant communities: eight low‐diversity and eight high‐diversity. Each community was invaded by a single individual from one of nine exotic species and grown in either ambient (40.72 ppb) or elevated (83.10 ppb) ozone.Results: Elevated ozone reduced the mean aboveground biomass of invasive species by 9.64% overall. The decline was greater in low‐diversity communities (–18.46%) than in high‐diversity ones (–12.54%). Elevated ozone enhanced the mean proportional aboveground biomass of invasive species in low‐diversity communities (11.24%), but not in high‐diversity communities.Conclusions: These findings support the diversity–invasibility hypothesis and suggest that species‐rich communities maintain stronger resistance to invasion even in elevated ozone.

Biological invasions impact ecosystems worldwide, including through changing the behaviour of native species. Here, we used camera traps to investigate the effects of invasive Opuntia spp. on the habitat use of 12 mammal species in Laikipia County, Kenya, an internationally important region of mammalian biodiversity. We found that Opuntia impacted mammal occupancy and activity. These effects were evident when Opuntia was considered at both site level and landscape scales; however, some mammal species showed different responses to increasing Opuntia at these two scales. The effects of Opuntia were generally context dependent, with their strength and direction varying among mammal species and between seasons. As well as having important implications for mammal conservation, ecosystem functioning and the future spread of Opuntia, our findings highlight behavioural changes in large mammals as a potentially important pathway through which invasive species impact ecosystems.

Artificial light at night (ALAN) can influence plant growth, defence, interactions with herbivores and invasion by exotic plants. However, studies assessing the interactive effects of ALAN, nutrient enrichment and herbivory on invasive and native plant species remain limited. We conducted a greenhouse experiment to investigate the interactive effects of ALAN (no-ALAN versus ALAN), nutrient enrichment (low versus high) and herbivory by a generalist Spodoptera litura (without versus with) on the growth, root allocation and defence of six congeneric pairs of invasive and native plant species. Nutrient enrichment increased total biomass more in native plants than in invasive plants. ALAN enhanced total biomass in both groups of plants without herbivory, but increased native plant biomass and decreased that of invasive plants under herbivory. ALAN reduced root mass fraction, especially under low-nutrient conditions without herbivory. Native plants exhibited greater tolerance to herbivory and higher net photosynthetic rates under ALAN compared to invasive species. Moreover, S. litura larvae grew faster on invasive plants but slower on native plants under ALAN. These findings suggest that ALAN may weaken the competitive advantage of invasive species by enhancing the defence mechanisms and physiological performance of native plants, potentially shifting competitive dynamics in favour of native flora.

Woody plant encroachment into graminoid‐dominated freshwater marshes is a widespread phenomenon globally, yet the mechanisms driving it are not well understood. It is often presumed that the establishment of woody plants—encompassing seed germination, seedling survival, and seedling growth—is influenced by environmental changes such as alterations in water regimes and nutrient availability. However, this hypothesis has not been rigorously tested through empirical studies. In this study, we conducted a 3‐year common garden experiment in a freshwater marsh in northeastern China to investigate the separate and combined effects of water and nitrogen levels on woody plant establishment in a graminoid‐dominated vegetation. The experiment involved manipulating water levels (flooded vs. drained) and nitrogen concentrations (ambient vs. added) in a fully crossed factorial design to examine their individual and combined effects. We sowed seeds of seven woody species under each treatment combination and tracked their germination, seedling survival, growth and potential competitive interactions with the resident vegetation. This approach allowed us to assess the direct effects of water and nitrogen on woody plant establishment, as well as any indirect effects mediated by changes in the resident plant community. The results show that drainage in the first year significantly improved both seed germination and seedling survival of woody plants, and these survival benefits persisted over the following years. The structural equation model revealed that drainage indirectly facilitated seedling survival by reducing light asymmetry. In contrast, continuous flooding over 3 years resulted in complete seedling mortality. However, after 3 years of drainage, the addition of nitrogen significantly increased the leaf count, maximum height, and stem diameter of the surviving woody plants. Synthesis. This multispecies introduction experiment has demonstrated that water regime is the primary factor driving the establishment of woody plant seedlings in freshwater marshes. As seedlings mature, nutrient supplementation can accelerate their development, potentially facilitating their encroachment into freshwater marshes. By demonstrating the distinct and varying impacts of water and nitrogen treatments across different stages of woody plant establishment, our study offers valuable insights into the complex interplay of environmental factors shaping this phenomenon.