Seed bank survival of an invasive species,but not of two native species,declines with invasion

Soil-borne seed pathogens may play an important role in either hindering or facilitating the spread of invasive exotic plants. We examined whether the invasive shrub Lonicera maackii (Caprifoliaceae) affected fungi-mediated mortality of conspecific and native shrub seeds in a deciduous forest in eastern Missouri. Using a combination of L. maackii removal and fungicide treatments, we found no effect of L. maackii invasion on seed viability of the native Symphoricarpos orbiculatus (Caprifoliaceae) or Cornus drummondii (Cornaceae). In contrast, fungi were significant agents of L. maackii seed mortality in invaded habitats. Losses of L. maackii to soil fungi were also significant in invaded habitats where L. maackii had been removed, although the magnitude of the effect of fungi was lower, suggesting that changes in soil chemistry or microhabitat caused by L. maackii were responsible for affecting fungal seed pathogens. Our work suggests that apparent competition via soil pathogens is not an important factor contributing to impacts of L. maackii on native shrubs. Rather, we found that fungal seed pathogens have density-dependent effects on L. maackii seed survival. Therefore, while fungal pathogens may provide little biotic resistance to early invasion by L. maackii, our study illustrates that more work is needed to understand how changes in fungal pathogens during the course of an invasion contribute to the potential for restoration of invaded systems. More generally, our study suggests that increased rates of fungal pathogen attack may be realized by invasive plants, such as L. maackii, that change the chemical or physical environment of the habitats they invade.     

Local and landscape correlates of non‐native species invasion in restored wetlands

Vulnerability of natural communities to invasion by non‐native plants has been linked to factors such as recent disturbance and high resource availability, suggesting that recently restored habitats may be especially invasible. Because non‐native plants can interfere with restoration goals, monitoring programs should anticipate which sites are most susceptible to invasion and which species are likely to become problematic at a site. Restored sites of larger area and those with high rates of propagule input should have higher species richness of both natives and non‐natives, leading to a positive correlation between the two. However, in restored wetlands, urbanization, riparian landscape settings, and nitrogen enrichment likely favor non‐native relative to native species. We sampled 28 restored wetlands in Illinois, USA, modeled the responses of native richness, non‐native richness and non‐native cover to local and landscape predictors with linear regression, and modeled the presence/absence of 21 non‐native species with logistic regressions. Unexpectedly, native and non‐native richness were uncorrelated, suggesting different responses to environmental factors. Native richness declined with increasing available soil nitrogen and urbanization in the surrounding landscape. Non‐native richness, the richness of non‐natives relative to natives, and the likelihood of invasion by several individual invasive species decreased with increasing distance from the city of Chicago, likely in response to decreasing non‐native propagule pressure. Total cover of non‐natives, however, as well as cover by non‐native Phalaris arundinacea, increased with nitrogen availability. Our results indicate that although non‐native richness was better predicted by factors related to propagule pressure, non‐native species dominance was more closely related to local abiotic factors. Non‐native richness in restoration sites may be beyond the control of restoration practitioners, and furthermore, may be of limited relevance for conservation goals. In contrast, limiting the relative dominance of non‐natives should be a restoration priority and may be achievable through management of nutrient availability.     

Polyploidy and invasion success: trait trade-offs in native and introduced cytotypes of two Asteraceae species

Invasion success is favoured by the introduction of pre-adapted genotypes. In addition, novel pressures in the introduced range may lead to phenotypic changes related to fitness or competitive ability of introduced plants. Polyploidy appears to be over-represented in invasive plants, but differences between cytotypes in growth strategies including trade-offs among plant traits have received little attention so far in the context of biological invasions. We grew Centaurea stoebe L. and Senecio inaequidens D.C. in a greenhouse experiment to test for differences in fitness (shoot biomass, reproductive output) and competitive ability (vegetative size, specific leaf area, leaf dry matter content, root–shoot ratio) between diploid and polyploid cytotypes as well as between native and introduced plants. For both species, diploid and tetraploid genotypes occur in the native range, whereas only tetraploids are present in the introduced range. In the native range of both species, diploid and tetraploid genotypes had different growth strategies. Tetraploid genotypes of C. stoebe and S. inaequidens had, respectively, higher specific leaf area and stem height than diploid ones. Thus, for both species, native tetraploids appeared more competitive than native diploids, which could explain, at least partially, the invasion success of the pre-adapted tetraploid genotypes. The comparison of native and introduced tetraploid genotypes revealed differences in traits linked to competitive ability, which could be linked to novel selection in the new environment. In S. inaequidens, we found evidence for a competition-colonisation trade-off, whereas persistence of C. stoebe in the new range seemed to be linked to a competition-defence trade-off.     

Local soil,but not commercial AMF inoculum,increases native and non‐native grass growth at a mine restoration site

Soil communities are often degraded in mined sites, and facilitating the recovery of soil mutualists such as arbuscular mycorrhizal fungi (AMF) may assist with the restoration of native plants. At a grassland mine restoration site, I compared a commercial AMF inoculum with soil collected from beneath native grasses as a source of inoculum, as well as a control treatment. Field plots were broadcast‐inoculated and seeded with native grasses, and biomass of native and non‐native species was measured in three consecutive years. In addition, greenhouse‐grown seedlings of a native bunchgrass (Stipa pulchra) were inoculated with similar treatments, transplanted into the field, and assessed after 18 months. When broadcast inoculation was used, the local soil inoculum tended to increase non‐native grass biomass, and marginally decreased non‐native forb biomass in the second year of study, but did not significantly affect native grass biomass. Broadcast commercial inoculum had no detectable effects on biomass of any plant group. Stipa pulchra transplants had greater N content and mycorrhizal colonization, and marginally higher shoot mass and K content, when pre‐inoculated with local soil (relative to controls). Pre‐inoculation with commercial AMF increased AMF colonization of the S. pulchra transplants, but did not significantly affect biomass or nutrient content. The findings indicate that at this site, the use of local soil as an inoculum had greater effects on native and non‐native plants than the commercial product used. In order to substantially increase native grass performance, inoculation of transplanted plugs may be one potential strategy.     

Boron bridging of rhamnogalacturonan‐II,monitored by gel electrophoresis,occurs during polysaccharide synthesis and secretion but not post‐secretion

The cell‐wall pectic domain rhamnogalacturonan‐II (RG‐II) is cross‐linked via borate diester bridges, which influence the expansion, thickness and porosity of the wall. Previously, little was known about the mechanism or subcellular site of this cross‐linking. Using polyacrylamide gel electrophoresis (PAGE) to separate monomeric from dimeric (boron‐bridged) RG‐II, we confirmed that Pb²⁺ promotes H₃BO₃‐dependent dimerisation in vitro. H₃BO₃ concentrations as high as 50 mm did not prevent cross‐linking. For in‐vivo experiments, we successfully cultured ‘Paul's Scarlet’ rose (Rosa sp.) cells in boron‐free medium: their wall‐bound pectin contained monomeric RG‐II domains but no detectable dimers. Thus pectins containing RG‐II domains can be held in the wall other than via boron bridges. Re‐addition of H₃BO₃ to 3.3 μm triggered a gradual appearance of RG‐II dimer over 24 h but without detectable loss of existing monomers, suggesting that only newly synthesised RG‐II was amenable to boron bridging. In agreement with this, Rosa cultures whose polysaccharide biosynthetic machinery had been compromised (by carbon starvation, respiratory inhibitors, anaerobiosis, freezing or boiling) lost the ability to generate RG‐II dimers. We conclude that RG‐II normally becomes boron‐bridged during synthesis or secretion but not post‐secretion. Supporting this conclusion, exogenous [³H]RG‐II was neither dimerised in the medium nor cross‐linked to existing wall‐associated RG‐II domains when added to Rosa cultures. In conclusion, in cultured Rosa cells RG‐II domains have a brief window of opportunity for boron‐bridging intraprotoplasmically or during secretion, but secretion into the apoplast is a point of no return beyond which additional boron‐bridging does not readily occur.     

Native species richness buffers invader impact in undisturbed but not disturbed grassland assemblages

Many systems are prone to both exotic plant invasion and frequent natural disturbances. Native species richness can buffer the effects of invasion or disturbance when imposed in isolation, but it is largely unknown whether richness provides substantial resistance against invader impact in the face of disturbance. We experimentally examined how disturbance (drought/burning) influenced the impact of three exotic invaders (Centaurea stoebe, Linaria dalmatica, or Potentilla recta) on native abundance across a gradient of species richness, using previously constructed grassland assemblages. We found that invaders had higher cover in experimentally disturbed plots than in undisturbed plots across all levels of native species richness. Although exotic species varied in cover, all three invaders had significant impacts on native cover in disturbed plots. Regardless of disturbance, however, invader cover diminished with increasing richness. Invader impacts on native cover also diminished at higher richness levels, but only in undisturbed plots. In disturbed plots, invaders strongly impacted native cover across all richness levels, as disturbance favoured invaders over native species. By examining these ecological processes concurrently, we found that disturbance exacerbated invader impacts on native abundance. Although diversity provided a buffering effect against invader impact without disturbance, the combination of invasion and disturbance markedly depressed native abundance, even in high richness assemblages.     

Pre‐ and post‐mating reproductive barriers drive divergence of five sympatric species of Naryciinae moths (Lepidoptera: Psychidae)

The biological species concept suggests that species can be separated on the basis of reproductive isolation. However, because natural interbreeding capabilities are often unknown, differences in morphology are generally used to separate species. Alternatively, genetic dissimilarity is used to separate morphologically similar species. Many genetic markers, including the maternally inherited mitochondrial cytochrome oxidase I sequence, cannot show interbreeding and therefore species status of groups may remain unresolved. In species of the genera Dahlica and Siederia (Lepidoptera: Psychidae: Naryciinae) the lack of morphological distinction and unknown interbreeding has led to unclear and unresolved taxonomic status. Mitochondrial DNA sequences suggest five sexual species to occur in Finland. However, their species status remains unconfirmed, due to a lack of knowledge on interbreeding, unclear morphological distinction and the limited variation in mitochondrial DNA. We combine three methods, a cross‐mating experiment, an analysis of mitochondrial and nuclear DNA, and a detailed male genital morphological examination, to establish the species status of the five suspected species. All suspected species exhibit intraspecies mating preference, although several interspecies pairs readily produce offspring. The genetic analysis, however, fails to show hybrids or introgression, suggesting that both pre‐ and post‐copulation mechanisms isolate the species reproductively. Morphological analysis of the male genitalia confirms that the species have diverged. Our results highlight the need of combining behavioural, morphological and genetic methods to determine species status in challenging taxonomic groups. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112 , 584–605.     

Is there evidence for the post‐invasion evolution of increased size among invasive plant species?

Many plant species grow taller and have higher reproductive capacity where they are nonindigenous invaders than where they are native components of the flora. Traditionally, it has been accepted that this is a plastic response to a benign environment, though recently this assumption has been challenged and a genetic basis for increased plant size has been invoked. We tested the hypothesis that the increased size of certain weed species is genetically, rather than environmentally, based. A common environment growth experiment revealed no significant differences in the size of Carduus nutans , Digitalis purpurea , Echium vulgare or Senecio jacobaea sampled from alien (Australia and New Zealand) or native (Britain and continental Europe) habitats. We conclude that post-invasion genetic changes associated with increased size may be unusual and that the phenomenon, where it occurs, generally reflects a plastic response to a novel environment.     

Abiotic barriers limit tree invasion but do not hamper native shrub recruitment in invaded stands

The interplay between the invasion of alien plant species and re-colonization of native plant species is important for conservation. Sandy coastal plains (called restinga in Brazil) were used as a model system to explore the abiotic barriers that potentially limit the initial establishment of alien and native woody plants in invaded and non-invaded areas. The study tested the influence of light availability, soil type and litter layer on recruitment of a wind-dispersed alien tree (Casuarina equisetifolia) and two bird-dispersed native shrubs under a Casuarina stand and in the preserved restinga. The effect of soil type and the physical and allelopathic effects of Casuarina litter on seedling emergence of the three species were also evaluated under greenhouse conditions. Low dispersal associated with low seedling emergence and zero survival of young plants (mainly due to microhabitat conditions) apparently prevents the spread of Casuarina in the preserved restinga. The main cause of low recruitment of native species in the Casuarina stand was the physical barrier of the litter. However, if seeds overcome this physical barrier, the presence of litter improves seedling emergence and survival of young plants, mainly because the litter increases soil moisture. Sowing seeds below litter and planting young plants of native shrubs on litter can improve the re-colonization of native plants in invaded areas. In conclusion, Casuarina invasion on sandy coastal plains is strongly limited by abiotic barriers, but anthropogenic disturbances are altering the key processes that naturally make the restinga resistant to invasion.     

Phenotypic plasticity,but not adaptive tracking,underlies seasonal variation in post‐cold hardening freeze tolerance of Drosophila melanogaster

In temperate regions, an organism's ability to rapidly adapt to seasonally varying environments is essential for its survival. In response to seasonal changes in selection pressure caused by variation in temperature, humidity, and food availability, some organisms exhibit plastic changes in phenotype. In other cases, seasonal variation in selection pressure can rapidly increase the frequency of genotypes that offer survival or reproductive advantages under the current conditions. Little is known about the relative influences of plastic and genetic changes in short‐lived organisms experiencing seasonal environmental fluctuations. Cold hardening is a seasonally relevant plastic response in which exposure to cool, but nonlethal, temperatures significantly increases the organism's ability to later survive at freezing temperatures. In the present study, we demonstrate seasonal variation in cold hardening in Drosophila melanogaster and test the extent to which plasticity and adaptive tracking underlie that seasonal variation. We measured the post‐cold hardening freeze tolerance of flies from outdoor mesocosms over the summer, fall, and winter. We bred outdoor mesocosm‐caught flies for two generations in the laboratory and matched each outdoor cohort to an indoor control cohort of similar genetic background. We cold hardened all flies under controlled laboratory conditions and then measured their post‐cold hardening freeze tolerance. Comparing indoor and field‐caught flies and their laboratory‐reared G1 and G2 progeny allowed us to determine the roles of seasonal environmental plasticity, parental effects, and genetic changes on cold hardening. We also tested the relationship between cold hardening and other factors, including age, developmental density, food substrate, presence of antimicrobials, and supplementation with live yeast. We found strong plastic responses to a variety of field‐ and laboratory‐based environmental effects, but no evidence of seasonally varying parental or genetic effects on cold hardening. We therefore conclude that seasonal variation in post‐cold hardening freeze tolerance results from environmental influences and not genetic changes.     

Invasive plants negatively impact native,but not exotic,animals

Despite our growing understanding of the impacts of invasive plants on ecosystem structure and function, important gaps remain, including whether native and exotic species respond differently to plant invasion. This would elucidate basic ecological interactions and inform management. We performed a meta‐analytic review of the effects of invasive plants on native and exotic resident animals. We found that invasive plants reduced the abundance of native, but not exotic, animals. This varied by animal phyla, with invasive plants reducing the abundance of native annelids and chordates, but not mollusks or arthropods. We found dissimilar impacts among “wet” and “dry” ecosystems, but not among animal trophic levels. Additionally, the impact of invasive plants increased over time, but this did not vary with animal nativity. Our review found that no studies considered resident nativity differences, and most did not identify animals to species. We call for more rigorous studies of invaded community impacts across taxa, and most importantly, explicit consideration of resident biogeographic origin. We provide an important first insight into how native and exotic species respond differently to invasion, the consequences of which may facilitate cascading trophic disruptions further exacerbating global change consequences to ecosystem structure and function.     

Native granivores reduce the establishment of native grasses but not invasive <Emphasis Type="Italic">Bromus tectorum</Emphasis>

Seed predation can structure plant communities by imposing strong population controls on some species but not others. In this context, studies from various ecosystems report that native granivores selectively forage for seeds from native species over seeds from exotic invaders, which could disproportionately favor the establishment of invaders and facilitate their dominance in communities. However, few studies have connected selective foraging for native seeds to differential patterns of establishment among native and invasive species. Thus, the extent to which preferential foraging for native seeds favors the establishment of invasive plants is unclear. Here, we used experimental seed additions and exclosure treatments at five field sites distributed across?≈?80,000 km² of the Great Basin Desert, USA to compare the effects of rodent foraging on the establishment of less-preferred cheatgrass (Bromus tectorum—an annual species native to Eurasia that is exotic and highly invasive across the Great Basin) and four species of more-preferred native grasses that commonly co-occur with cheatgrass. Rodent foraging reduced the establishment of each native species by at least 80% but had no effect on the establishment of cheatgrass, and this finding was consistent across study sites. Our results suggest that selective foraging for native species may favor the establishment of cheatgrass over native grasses, potentially exacerbating one of the most extensive plant invasions in North America.     

Life history trait divergence among populations of a non‐palatable species reveals strong non‐trophic indirect effects of an abundant herbivore

When large herbivores exert selection on their prey plant species, co‐occurring, non‐prey species may experience selection through non‐trophic indirect effects. Such selection is likely common where herbivores are overabundant. Yet, empirical studies of non‐trophic indirect effects as drivers of non‐prey trait evolution are lacking. Here we test for adaptive shifts in life history traits in an unpalatable species, Arisaema triphyllum, a common forest perennial that is unique because it exhibits size‐dependent sex switching. We collected A. triphyllum from six sites that experience a gradient in abiotic stress caused by deer browse pressure on prey plant species that generate indirect effects. We grew A. triphyllum from these sites in a common garden for five years to evaluate life history predictions linking strong indirect effects and abiotic stress to changes in life history traits: flowering onset size threshold, female flowering size threshold, relative growth rate (RGR), biomass allocation, and asexual reproduction. Despite observed differences among phenotypes in the field, expression of flowering onset size threshold, biomass allocation, and asexual reproduction did not differ among the six populations in the garden, indicating common plastic responses. In contrast, A. triphyllum collected from sites experiencing the two highest deer impacts exhibited smaller female flowering size thresholds and the highest RGR. Responses in these traits support the predictions of adaptive divergence in response to indirect effects. Our results reinforce the idea that non‐trophic indirect effects of large herbivores can elicit evolutionary responses in some traits of non‐prey species. In general, life history traits of unpalatable species may be cryptically adapting to stressful indirect effects where large herbivores are overabundant.