Widespread Plant Invader Research Articles

Selective method for invasive plant removal enhances restoration

Effective and sustainable restoration of habitats invaded by non‐native plant species requires both invader removal and recovery of native species. However, efficacy of removal methods and native species responses commonly depend on site conditions, spatial scale, and time, indicating invader removal and restoration approaches must consider environmental context. To better understand how selectivity of invader control methods and site conditions determine invader removal efficacy and resident community responses, we conducted an experiment with a widespread plant invader in the southeast United States (cogongrass, Imperata cylindrica) and co‐occurring resident species across nine sites that varied widely in environmental context. We found that three applications of the commonly used broad‐spectrum herbicide glyphosate reduced invader cover by 97%, on average, across the sites over 2 years. Fluazifop‐P‐butyl, a selective grass‐specific herbicide was equally effective (95% reduction in cover) in removing the invader when only two additional treatments were applied (five treatments total over 2 years) but it also helped preserve or allow for recovery of resident plant cover. Furthermore, removal efficacy was similar across sites regardless of differences in environmental conditions. Higher resource availability, as estimated by initial invader biomass at the sites, resulted in greater resident species recovery, but only under the grass‐specific herbicide treatment. Our results suggest that refocusing invasive plant management efforts from removal alone to community‐wide native species preservation and recovery, in part by using more selective invader removal strategies, has higher initial management costs but could simultaneously ease invader impacts and enhance ecosystem restoration.

Domestication does not alter invasion risk of a non-native legume.

Most non-native domesticated plants provide benefits without escaping cultivation, but others have become prominent invaders. A better understanding of how domestication might alter plant traits that influence the ability of species to overcome barriers to invasion could improve invasion risk predictions. We explored how variation in foliar chemistry among cultivars of a widespread invader in the U.S. (Lespedeza cuneata) might influence invasion risk through differences in herbivore interactions and a potential tradeoff with competitive ability. In a no-choice feeding bioassay a generalist herbivore performed better when fed cultivars compared to wild genotypes and native congeners, suggesting domesticated L. cuneata may be more vulnerable to herbivory compared to wild genotypes. However, in a factorial greenhouse experiment, with treatments of herbivory and competition, all cultivars had similar seed and biomass production as the wild genotype. Competition with native species reduced productivity of all L. cuneata types, while experimental herbivory alone did not. We also found far less variation in foliar chemistry among L. cuneata types than expected based on domestication history. While our findings from the bioassay suggest potentially lower invasion risk for cultivars if herbivore populations expand more rapidly when feeding on cultivars, we did not find evidence that cultivars had different responses to herbivory or competition with native species in the greenhouse experiment. Altogether, the findings from our multi-pronged approach for evaluating potential factors underlying invasion success of a domesticated species indicate that domestication has not altered invasion risk of this widespread plant invader.

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader.

Predicting plant distributions under climate change is constrained by our limited understanding of potential rapid adaptive evolution. In an experimental evolution study with the invasive common ragweed (Ambrosia artemisiifolia L.) we subjected replicated populations of the same initial genetic composition to simulated climate warming. Pooled DNA sequencing of parental and offspring populations showed that warming populations experienced greater genetic divergence from their parents, than control populations. In a common environment, offspring from warming populations showed more convergent phenotypes in seven out of nine plant traits, with later flowering and larger biomass, than plants from control populations. For both traits, we also found a significantly higher ratio of phenotypic to genetic differentiation across generations for warming than for control populations, indicating stronger response to selection under warming conditions. As a measure for evolutionary rate, the phenotypic and sequence divergence between generations were assessed using the Haldane metric. Our approach combining comparisons between generations (allochronic) and between treatments (synchronic) in an experimental evolutionary field study, and linking population genomic data with phenotyping analyses provided a powerful test to detect rapid responses to selection. Our findings demonstrate that ragweed populations can rapidly evolve in response to climate change within a single generation. Short-term evolutionary responses to climate change may aggravate the impact of some plant invaders in the future and should be considered when making predictions about future distributions and impacts of plant invaders.

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Rapid genomic and phenotypic change in response to climate warming in a widespread plant invader

Plant-Soil Feedback Effects on Germination and Growth of Native and Non-Native Species Common across Southern California

Changes in plant assemblages can influence biotic and abiotic soil conditions. These changes can cause plant–soil feedbacks that can inhibit or facilitate plant germination and growth. Here, we contribute to a growing literature examining plant–soil feedbacks in the endangered sage scrub ecosystem by examining the germination and growth of Artemisia californica, the dominant native shrub species in the ecosystem, in soil conditioned by two widespread plant invaders (Brassica nigra, Bromus madritensis ssp. rubens), and the germination and growth of these invasive species in conspecific and heterospecific soils. Our findings suggest that: (i) A. californica soils can limit establishment of some species (B. nigra) but not others (B. madritensis), (ii) A. californica soil conditions reduce growth of all plant species, and (iii) non-natives are negatively impacted by soil microbes, but in some contexts can do better in heterospecific soil. As our findings were often incongruent with other studies that examined interactions among similar species at other sites, we suggest that we are at our infancy of understanding these complex interactions, and that developing a predictive framework for understanding plant soil feedbacks in the sage scrub ecosystem involves understanding how various plant species respond in different soil contexts within the ecosystem.

The role of plant-mycorrhizal mutualisms in deterring plant invasions: Insights from an individual-based model.

Understanding the factors that determine invasion success for non‐native plants is crucial for maintaining global biodiversity and ecosystem functioning. One hypothesized mechanism by which many exotic plants can become invasive is through the disruption of key plant–mycorrhizal mutualisms, yet few studies have investigated how these disruptions can lead to invader success. We present an individual‐based model to examine how mutualism strengths between a native plant (Impatiens capensis) and mycorrhizal fungus can influence invasion success for a widespread plant invader, Alliaria petiolata (garlic mustard). Two questions were investigated as follows: (a) How does the strength of the mutualism between the native I. capensis and a mycorrhizal fungus affect resistance (i.e., native plant maintaining >60% of final equilibrium plant density) to garlic mustard invasion? (b) Is there a non‐linear relationship between initial garlic mustard density and invasiveness (i.e., garlic mustard representing >60% of final equilibrium plant density)? Our findings indicate that either low (i.e., facultative) or high (i.e., obligate) mutualism strengths between the native plant and mycorrhizal fungus were more likely to lead to garlic mustard invasiveness than intermediate levels, which resulted in higher resistance to garlic mustard invasion. Intermediate mutualism strengths allowed I. capensis to take advantage of increased fitness when the fungus was present but remained competitive enough to sustain high numbers without the fungus. Though strong mutualisms had the highest fitness without the invader, they proved most susceptible to invasion because the loss of the mycorrhizal fungus resulted in a reproductive output too low to compete with garlic mustard. Weak mutualisms were more competitive than strong mutualisms but still led to garlic mustard invasion. Furthermore, we found that under intermediate mutualism strengths, the initial density of garlic mustard (as a proxy for different levels of plant invasion) did not influence its invasion success, as high initial densities of garlic mustard did not lead to it becoming dominant. Our results indicate that plants that form weak or strong mutualisms with mycorrhizal fungi are most vulnerable to invasion, whereas intermediate mutualisms provide the highest resistance to an allelopathic invader.

Do novel weapons that degrade mycorrhizal mutualisms promote species invasion?

Non-native plants often dominate novel habitats where they did not co-evolve with the local species. The novel weapons hypothesis suggests that non-native plants bring competitive traits against which native species have not adapted defenses. Novel weapons may directly affect plant competitors by inhibiting germination or growth, or indirectly by attacking competitor plant mutualists (degraded mutualisms hypothesis). Japanese knotweed (Fallopia japonica) and European buckthorn (Rhamnus cathartica) are widespread plant invaders that produce potent secondary compounds that negatively impact plant competitors. We tested whether their impacts were consistent with a direct effect on the tree seedlings (novel weapons) or an indirect attack via degradation of seedling mutualists (degraded mutualism). We compared recruitment and performance using three Ulmus congeners and three Betula congeners treated with allelopathic root macerations from allopatric and sympatric ranges. Moreover, given that the allelopathic species would be less likely to degrade their own fungal symbiont types, we used arbuscular mycorrhizal (AMF) and ectomycorrhizal (ECM) tree species to investigate the effects of F. japonica (no mycorrhizal association) and Rhamnus cathartica (ECM association) on the different fungal types. We also investigated the effects of F. japonica and R. cathartica exudates on AMF root colonization. Our results suggest that the allelopathic plant exudates impact seedlings directly by inhibiting germination and indirectly by degrading fungal mutualists. Novel weapons inhibited allopatric seedling germination but sympatric species were unaffected. However, seedling survivorship and growth appeared more dependent on mycorrhizal fungi, and mycorrhizal fungi were inhibited by allopatric species. These results suggest that novel weapons promote plant invasion by directly inhibiting allopatric competitor germination and indirectly by inhibiting mutualist fungi necessary for growth and survival.

Erratum to: The impact of Carpobrotus cfr. acinaciformis (L.) L. Bolus on soil nutrients, microbial communities structure and native plant communities in Mediterranean ecosystems

Carpobrotus spp. are amongst the most impactful and widespread plant invaders of Mediterranean habitats. Despite the negative ecological impacts on soil and vegetation that have been documented, information is still limited about the effect by Carpobrotus on soil microbial communities. We aimed to assess the changes in the floristic, soil and microbial parameters following the invasion by Carpobrotus cfr. acinaciformis within an insular Mediterranean ecosystem. Within three study areas a paired-site approach, comparing an invaded vs. a non-invaded plot, was established. Within each plot biodiversity indexes, C and N soil content, pH and microbial biomass and structure (bacterial and fungal) were assessed. Invaded plots showed a decrease of α-species richness and diversity. The least represented plant species in invaded plots were those related to grassland habitats. In all invaded soils, a significant increase of carbon and nitrogen content and a significant decrease of pH were registered. Carpobrotus significantly increased bacterial and fungal biomass and altered soil microbial structure, particularly favoring fungal growth. Carpobrotus may deeply impact edaphic properties and microbial communities and, in turn, these strong modifications probably increase its invasive potential and its ability to overcome native species, by preventing their natural regeneration.

Geographic structuring and transgenerational maternal effects shape germination in native, but not introduced, populations of a widespread plant invader.

Germination is critical in determining species distributions and invasion dynamics. However, is it unclear how often invasive populations evolve germination characteristics different from native populations, because few studies have isolated genetic variation by using seed from garden-grown plants. Additionally, while herbivore-induced transgenerational effects are common, it is unknown whether maternal herbivory differentially shapes germination in native and introduced offspring. We explored germination in native and introduced populations of the North American invader Verbascum thapsus using seed from garden-grown maternal plants, half of which were protected from herbivores. To elucidate (1) germination niche breadth and (2) whether germination conditions affected expression of genetic structuring among populations, we germinated seed under four ecologically relevant temperature regimes. Native populations had a wide germination niche breadth, germinating as well as or better than introduced populations. At cooler temperatures, native populations exhibited a genetically based environmental cline indicative of local adaptation, with populations from warmer locales germinating better than populations from cooler locales. However, this cline was obscured when maternal plants were attacked by herbivores, revealing that local stressors can override the expression of geographic structuring. Introduced populations did not exhibit clinal variation, suggesting its disruption during the introduction process. Native and introduced populations have evolved genetic differences in germination. The result of this difference manifests in a wider germination niche breadth in natives, suggesting that the invasive behavior of V. thapsus in North America is attributable to other factors.

A recolonization record of the invasive Poa annua in Paradise Bay, Antarctic Peninsula: modeling of the potential spreading risk

Antarctica is one of the most extreme environments for vascular plants occurrence worldwide, and only two native vascular plants have colonized this continent: Deschampsia antarctica and Colobanthus quitensis. Nevertheless, in recent years, several alien plant species has been found in Antarctica with negative effects on the native flora. In this study, we show a recolonization record of the most widespread plant invader in Antarctica (Poa annua) and the risk of a potential spreading in a highly visited site on the Antarctic Peninsula. Overall, two new P. annua individuals were recorded, where four specimens were previously reported and removed in 2010, suggesting that either a propagule load is continuous, or that a seed bank prevailed in the site. On the other hand, the spreading modeling suggests that the probability to colonize and spreading of P. annua increases notoriously with the possibility of dispersion of propagules, with consequent risk of displacement for the native flora. Biological invasions are a major threat to the integrity of native biodiversity in all biomes, and they have the potential to change irreversibly Antarctica’s fragile ecosystems.

Consistent performance of invasive plant species within and among islands of the Mediterranean basin

Since the success of an invasive species depends not only upon its intrinsic traits but also on particular characteristics of the recipient habitat, assessing the performance of an invader across habitats provides a more realistic analysis of risk. Such an analysis will not only provide insights into the traits related to invasiveness, but also the habitat characteristics that underpin vulnerability to invasion that, taken together, will facilitate the selection of management strategies to mitigate the invader’s effect. In the present study, we considered the Mediterranean basin islands as an excellent study region to test how the same invasive species perform in different habitats within a single island, and to scale up differences among islands with similar climate. We tested how the performance of three widespread plant invaders with clonal growth but contrasting life-history traits, a deciduous tree Ailanthus altissima, a succulent subshrub Carpobrotus spp., and an annual geophyte Oxalis pes-caprae, varied depending upon the species identity, habitat, and invaded island. The environmental parameters considered were habitat type, elevation, species diversity in the invaded plot, and several soil traits (% C, % N, C/N, pH, and relative humidity). The study documents that the performance of these three important and widespread plant invaders is dependent mainly on species identity, and less upon the invaded island’s general features. Likewise, differences in performance among habitats were only significant in the case of Ailanthus, whereas Carpobrotus and Oxalis appear to perform equally well in different environments. Ailanthus thus appears to have a broader spectrum of invasiveness, being able to invade a larger number of habitat types. On the contrary, Carpobrotus spp. have not yet invaded habitats different from those where the species have been originally introduced and where they are still commonly spread by humans. Oxalis distribution is mainly related to agricultural activities and disturbed sites, and the total area infested by this geophyte may be more reflection of the extent of suitable habitats than of invasiveness or ecological impact. Our results confirm the potential for these species to significantly alter the functioning of ecosystems in the Mediterranean islands and highlight the risk to other islands not yet invaded.

Local and regional assessments of the impacts of plant invaders on vegetation structure and soil properties of Mediterranean islands

AbstractAims Although biological invasions occur throughout the world, and some invaders are widespread in many habitats, few studies on the ecological impact of invaders have examined multiple sites. We tested how the impact of three widespread plant invaders changed depending on the identity of the species and the invaded island. We also tested whether relative species loss was lower in species‐rich communities than in species‐poor ones.Location We conducted floristic surveys and soil analyses in eight Mediterranean Basin islands: Crete and Lesbos (Greece), Sardinia (Italy), Corsica, Bagaud and Porquerolles (France), and Mallorca and Menorca (Spain).Methods We compared native species richness and diversity, proportion of life forms, soil percentage nitrogen, percentage organic carbon, C/N, and soil pH in nearby paired plots of 2 × 2 m: one control and one invaded by either the deciduous tree Ailanthus altissima, the succulent subshrubs Carpobrotus spp. or the annual geophyte Oxalis pes‐caprae, across eight Mediterranean Basin islands.Results On average, the presence of invaders reduced species diversity, Carpobrotus spp. exhibiting the largest impact and Oxalis the least. However, the relative impact was island‐dependent, and was positively but weakly associated with the species richness of the recipient community. Therophytes were the life form that experienced the largest decrease across islands. The effects of invasion on soil properties were very variable. Total N changed (increased) only in plots invaded by Ailanthus, significantly decreasing the C/N ratio. The presence of this tree increased soil pH, whereas the opposite was found in plots invaded by the other two species. Organic C increased in plots invaded by Ailanthus and Carpobrotus species.Main conclusions By conducting an analysis at multiple sites, we found that the three plant invaders had an impact on plant community structure not entirely concordant with changes in soil properties. The impacts depended on the identity of the species and of the invaded island, suggesting that impact of invaders is context‐specific. The impact in terms of species loss was not lower in species‐rich than in species‐poor communities.