Recruitment limitation of native species in invaded coastal dune communities

Recruitment limitation may limit the ability of sites to regenerate after disturbances such as weed invasion and weed management. We investigated seed bank constraints and dispersal limitation in coastal dune communities on the east coast of Australia. The ability of sites to regenerate naturally following weed removal was assessed in coastal dune communities invaded by the invasive alien, bitou bush (Chrysanthemoides monilifera subsp. rotundata). To investigate recruitment limitation, seed banks and vegetation of invaded, native, intensively managed (selective application of herbicide and some re-vegetation) and extensively managed (large-scale, non-selective herbicide application) sites were compared. We investigated the dispersal mechanisms of species in the seed bank and vegetation to determine if communities might be dispersal-limited, i.e. contain significant numbers of species with only short-distance dispersal capabilities. Species richness and composition of soil seed banks differed from the vegetation in foredunes and hinddunes. Invasion depleted seed banks further. About half of the species had short-distance dispersal mechanisms indicating the potential for dispersal limitation. Secondary weed invasion following management was evident although alien species occurred in both seed banks and vegetation. Our results indicated that coastal dune communities suffer recruitment limitation. Native, managed and invaded dune communities appear to be both seed bank and dispersal-limited although management and invasion exacerbates recruitment. Regeneration of coastal dune communities will require active reintroduction of species, particularly those with short-distance dispersal mechanisms.     

Phenotypes of Two Generations of Sporobolus airoides Seedlings Derived from Acroptilon repens‐invaded and Non‐invaded Grass Populations

Although the ecological impacts of invasive species are well known, the evolutionary impacts on recipient native grass communities are not. We suggest that remnant native plants may provide desirable seed sources for restoration and native plant production. Native populations exposed to the selective pressures associated with exotic invasion may retain traits that increase their ability to coexist with invasive species. Two generations of Sporobolus airoides Torr. (Alkali sacaton) plants derived from lineages collected from within long‐term invaded areas of Acroptilon repens (L.) DC (Russian knapweed) and from adjacent non‐invaded areas were propagated in a greenhouse to evaluate generational changes in phenotypic traits from the production environment. Given the difference in invasion history of the two populations, we hypothesized that invaded and non‐invaded subpopulations would differ phenotypically. Phenotypic measurements revealed that invaded subpopulations had greater vegetative growth, whereas non‐invaded subpopulations had increased sexual reproduction. Phenotypic expression changed from the first to the second generation, predominantly in the invaded subpopulation. Generational phenotypic shifts are disadvantageous for native seed production which requires a standard product to sell commercially. However, phenotypic variation may improve field seed survival. This research demonstrates the potential value of targeting post‐invasion remnant grass populations for restoration.     

Removal of invasive shrubs alters light but not leaf litter inputs in a deciduous forest understory

The establishment and spread of non‐native, invasive shrubs in forests poses an important obstacle to natural resource conservation and management. This study assesses the impacts of the physical removal of a complex of woody invasive shrub species on deciduous forest understory resources. We compared leaf litter quantity and quality and understory light transmittance in five pairs of invaded and removal plots in an oak‐dominated suburban mature forest. Removal plots were cleared of all non‐native invasive shrubs. The invasive shrubs were abundant (143,456 stems/ha) and diverse, dominated by species in the genera Ligustrum, Viburnum, Lonicera, and Euonymus. Annual leaf litter biomass and carbon inputs of invaded plots were not different from removal plots due to low leaf litter biomass of invasive shrubs. Invasive shrub litter had higher nitrogen (N) concentrations than native species; however, low biomass of invasive litter led to low N inputs by litter of invasive species compared to native. Light transmittance at the forest floor and at 2 m was lower in invaded plots than in removal plots. We conclude that the removal of the abundant invasive shrubs from a native deciduous forest understory did not alter litter quantity or N inputs, one measure of litter quality, and increased forest understory light availability. More light in the forest understory could facilitate the restoration of forest understory dynamics.     

Restoring a Mediterranean‐climate shrub community with perennial species reduces future invasion

Successful restoration of an invaded landscape to a diverse, invasion‐resistant native plant community requires determining the optimal native species mix to add to the landscape. We manipulated native seed mix (annuals, perennials, or a combination of the two), while controlling the growth of non‐native species to test the hypothesis that altering native species composition can influence native establishment and subsequent non‐native invasion. Initial survival of native annuals and perennials was higher when seeded in separate mixes than when combined, and competition between the native perennials and annuals led to lower perennial cover in year 2 of mixed‐seeded plots. The plots with the highest perennial cover had the highest resistance to invasion by Brassica nigra. To clarify interactions among different functional groups of natives and B. nigra, we measured competitive interactions in pots. We grew one native annual, one native perennial, and B. nigra alone or with different competitors and measured biomass after 12 weeks. Brassica nigra was the strongest competitor, limiting the growth of all native species, and was not impacted by competition with native annuals or perennial seedlings. Results from the potted plant experiment demonstrated the strong negative influence of B. nigra on native seedlings. Older native perennials were the strongest competitors against invasive species in the field, yet perennial seedling survival was limited by competition with native annuals and B. nigra. Management action that maximizes perennial growth in early years may lead to a relatively more successful restoration and the establishment of an invasion‐resistant community.     

Disruption of ant-seed dispersal mutualisms by the invasive Asian needle ant (<Emphasis Type="Italic">Pachycondyla chinensis</Emphasis>)

By disrupting the structure of native ant assemblages, invasive ants can have effects across trophic levels. Most studies to date, however, have focused on the impacts just two species (Linepithema humile and Solenopsis invicta). The impacts of many other invasive ant species on ecological processes in their introduced range are unknown. In this study we tested the hypothesis that the invasive ant Pachycondyla chinensis disrupts ant-seed dispersal mutualisms by displacing native ant species, especially the keystone mutualist Aphaenogaster rudis, while failing to disperse seeds itself. In a paired design we measured the impact of P. chinensis on the native ant-plant seed dispersal mutualism. The number of A. rudis workers was 96% lower in invaded than in intact plots, and the number of seeds removed was 70% lower in these plots. Finally, in invaded plots the abundance of Hexastylis arifolia, a locally abundant myrmecochorous plant, was 50% lower than in plots where P. chinensis was absent. A parsimonious interpretation of our results is that P. chinensis causes precipitous declines in the abundance of A. rudis within invaded communities, thereby disrupting the ant-plant seed dispersal mutualisms and reducing abundances of ant-dispersed plants. In sum, the magnitude of the effects of P. chinensis on seed dispersal is quantitatively similar to that documented for the intensively studied invasive Argentine ant. We suggest that more studies on the impacts of less-studied invasive ant species on seed dispersal mutualisms may increase our knowledge of the effects of these invaders on ecosystem function.     

Limits and effects of invasion by the nonindigenous wetland plant <Emphasis Type="Italic">Lythrum salicaria</Emphasis> (purple loosestrife): a seed bank analysis

We used seed bank analyses to investigate the role of dispersal in limiting invasion by Eurasian Lythrum salicaria within and among North American wetlands, and the changes in seed bank diversity associated with this invader. We compared the number and species composition of seedlings emerging from soil sampled in 11 uninvaded wetlands and paired uninvaded and invaded sites within 10 invaded wetlands under both seedling competition and noncompetitive conditions. Almost no L. salicaria emerged in samples from uninvaded wetlands, indicating dispersal limitation despite prodigious seed production in nearby wetlands. However L. salicaria emerged in all samples from uninvaded sites in invaded wetlands, suggesting environmental limits on establishment within invaded wetlands. Conditions that provided opportunities for seedlings to compete reduced survival of Typha spp. but not L. salicaria seedlings. However, this was due to species-specific differences in post-emergence mortality rather than response to competition. Competition did reduce seedling mass, but this effect did not differ among species. Species richness of emerging seedlings was lower for invaded than uninvaded wetlands. Lower seed bank richness may be a cause or consequence of L. salicaria invasion. Efforts to reduce seed dispersal to uninvaded wetlands would likely slow the spread of this invader.     

Plant community response following the removal of the invasive Lupinus arboreus in a coastal dune system

The removal of invasive species is common in restoration projects, yet the long‐term effects of pest management programs are seldom assessed. We present results of a long‐term program to remove the invasive species Lupinus arboreus (lupin) from sand dunes in New Zealand. We evaluate the response of plant communities to lupin removal, by comparing total plant cover, the cover of non‐native and native plant species, and species richness between sand dune sites where lupin removal has occurred, not occurred, and where lupin has never been present. Neither lupin presence nor removal had a significant impact on the foredune environment. Following removal, total and other non‐native plant cover remained higher, and the cover of several native sand dune species remained lower compared with uninvaded sites in the deflation and backdune environments. These changes can be attributed to persistent effects associated with the invasion of lupin, but have also developed in response to lupin removal. The results of this study have implications for restoration projects in sand dunes. Pest management alone is unlikely to be sufficient to restore plant communities. Given the difficulties in restoring plant communities once an invasive species has established, managers should prioritize actions to prevent the spread of invasive species into uninvaded areas of sand dunes. Finally, the response to lupin invasion and removal differed between dune habitats. This highlights the importance of tailoring a pest management program to restoration goals by, for example, prioritizing areas in which the impacts of the invading species are greatest.     

Impact of grasshoppers and an invasive grass on establishment and initial growth of restoration plant species

Exotic plant invasion can have dramatic impacts on native plants making restoration of native vegetation at invaded sites challenging. Though invasives may be superior competitors, it is possible their dominance could be enhanced by insect herbivores if native plants are preferred food sources. Insect herbivory can regulate plant populations, but little is known of its effects in restoration settings. There is a need to better understand relationships between insect herbivores and invasive plants with regard to their combined potential for impacting native plant establishment and restoration success. The objective of this study was to assess impacts of grasshopper herbivory and the invasive grass Bromus tectorum (cheatgrass) on mortality and growth of 17 native plant species used in restoration of critical sagebrush steppe ecosystems. Field and greenhouse experiments were conducted using moderate densities of a common, generalist pest grasshopper (Melanoplus bivittatus). Grasshoppers had stronger and more consistent impacts on native restoration plants in field and greenhouse studies than cheatgrass. After 6 weeks in the greenhouse, grasshoppers were associated with 36% mortality over all native restoration species compared to 2% when grasshoppers were absent. Herbivory was also associated with an approximately 50% decrease in native plant biomass. However, effects varied among species. Artemisia tridentata, Chrysothamnus viscidiflorus, and Coreopsis tinctoria were among the most negatively impacted, while Oenothera pallida, Pascopyrum smithii, and Leymus cinerus were unaffected. These findings suggest restoration species could be selected to more effectively establish and persist within cheatgrass infestations, particularly when grasshopper populations are forecasted to be high.     

Shading by invasive shrub reduces seed production and pollinator services in a native herb

Plant invasions disrupt native plant reproduction directly via competition for light and other resources and indirectly via competition for pollination. Furthermore, shading by an invasive plant may reduce pollinator visitation and therefore reproduction in native plants. Our study quantifies and identifies mechanisms of these direct and indirect effects of an invasive shrub on pollination and reproductive success of a native herb. We measured pollinator visitation rate, pollen deposition, and female reproductive success in potted arrays of native Geranium maculatum in deciduous forest plots invaded by the non-native shrub Lonicera maackii and in two removal treatments: removal of aboveground L. maackii biomass and removal of flowers. We compared fruit and seed production between open-pollinated and pollen-supplemented plants to test for pollen and light limitation of reproduction. Plots with L. maackii had significantly lower light, pollinator visitation rate, and conspecific pollen deposition to G. maculatum than biomass removal plots. Lonicera maackii flower removal did not increase pollinator visitation or pollen deposition compared to unmanipulated invaded plots, refuting the hypothesis of competition for pollinators. Thus, pollinator-mediated impacts of invasive plants are not limited to periods of co-flowering or pollinator sharing between potential competitors. Geranium maculatum plants produced significantly fewer seeds in plots containing L. maackii than in plant removal plots. Seed set was similar between pollen-supplemented and open-pollinated plants, but pollen-supplemented plants exhibited higher seed set in plant removal plots compared to invaded plots. Therefore, we conclude that the mechanism of impact of L. maackii on G. maculatum reproduction was increased understory shade.     

Forest edges and fire ants alter the seed shadow of an ant-dispersed plant

Exotic species invade fragmented, edge-rich habitats readily, yet the distinct impacts of habitat edges and invaders on native biota are rarely distinguished. Both appear detrimental to ant-dispersed plants such as bloodroot, Sanguinaria canadensis. Working in northeastern Georgia (USA), an area characterized by a rich ant-dispersed flora, fragmented forests, and invasions by the red imported fire ant, Solenopsis invicta , I monitored the interactions between ants and S. canadensis seeds in uninvaded forest interiors, uninvaded forest edges, invaded forest interiors, and invaded forest edges. I observed 95% of the seed dispersal events that occurred within the 60-min observation intervals. Seed collection rates were similar among all four (habitat × invasion) groups. The presence of invasive ants had a strong effect on seed dispersal distance: S. invicta collected most seeds in invaded sites, but was a poorer disperser than four of five native ant taxa. Habitat type (interior versus edge) had no effect on seed dispersal distance, but it had a strong effect on seed dispersal direction. Dispersal towards the edge was disproportionately rare in uninvaded forest edges, and ants in those habitats moved the average dispersed seed approximately 70 cm away from that edge. Dispersal direction was also skewed away from the edge in uninvaded forest interiors and invaded forest edges, albeit non-significantly. This biased dispersal may help explain the rarity of myrmecochorous plants in younger forests and edges, and their poor ability to disperse between fragments. This is the first demonstration that forest edges and S. invicta invasion influence seed dispersal destination and distance, respectively. These forces act independently.     

Non-native grass invasion alters native plant composition in experimental communities

Invasions of non-native species are considered to have significant impacts on native species, but few studies have quantified the direct effects of invasions on native community structure and composition. Many studies on the effects of invasions fail to distinguish between (1) differential responses of native and non-native species to environmental conditions, and (2) direct impacts of invasions on native communities. In particular, invasions may alter community assembly following disturbance and prevent recolonization of native species. To determine if invasions directly impact native communities, we established 32 experimental plots (27.5 m²) and seeded them with 12 native species. Then, we added seed of a non-native invasive grass (Microstegium vimineum) to half of the plots and compared native plant community responses between control and invaded plots. Invasion reduced native biomass by 46, 64, and 58%, respectively, over three growing seasons. After the second year of the experiment, invaded plots had 43% lower species richness and 38% lower diversity as calculated from the Shannon index. Nonmetric multidimensional scaling ordination showed a significant divergence in composition between invaded and control plots. Further, there was a strong negative relationship between invader and native plant biomass, signifying that native plants are more strongly suppressed in densely invaded areas. Our results show that a non-native invasive plant inhibits native species establishment and growth following disturbance and that native species do not gain competitive dominance after multiple growing seasons. Thus, plant invaders can alter the structure of native plant communities and reduce the success of restoration efforts.     

The ecological barriers to the recovery of bridal creeper (Asparagus asparagoides (L.) Druce) infested sites: Impacts on vegetation and the potential increase in other exotic species

Abstract To protect native biodiversity from environmental weeds, the impacts that these weeds cause need to be known before weed control commences. Asparagus asparagoides (L.) Druce (bridal creeper) (Asparagaceae) is a serious environmental weed and has been selected for biological control in Australia. To predict the responses of plant communities to the control of bridal creeper, a prerelease baseline of the impacts of bridal creeper on native plant communities was undertaken. Plant assemblages in areas invaded by bridal creeper were compared with reference areas that contained little or no bridal creeper. Areas invaded by bridal creeper contained 52% fewer native plant species when compared with nearby reference areas. However, there was no difference in the number of other exotic plant species between areas. Similar trends were found for the germinable seed bank. Although a greater number of exotic species were present in the seed bank compared with the vegetation surveys, there was still no difference between areas with and without bridal creeper. In a glasshouse trial, exotic species germinated more frequently compared with native species. This could indicate that as bridal creeper density decreases following control, exotic species have an advantage over native species when colonizing areas left vacant by bridal creeper. Second, as bridal creeper areas contained reduced native species richness and cover, they may be susceptible to further weed invasion after bridal creeper is removed. Therefore, simply reducing the presence of bridal creeper may not guarantee successful restoration of invaded areas and additional restoration efforts will be needed to ensure the ultimate goal of protecting native biodiversity is reached.     

Invasion,alien control and restoration: Legacy effects linked to folivorous insects and phylopathogenic fungi

Invasive alien trees increase native tree stress and may increase attack by herbivores and pathogenic fungi. Alien tree removal should ameliorate such impacts. Here we compared the levels of damage by phylopathogenic fungi and folivorous insects on Brabejum stellatifolium and Metrosideros angustifolia (native trees) and Acacia mearnsii (invasive tree species) among near‐pristine, invaded and restored sites. Generally, foliar damage levels were higher at invaded than at near‐pristine sites. Damage levels at restored sites were similar, or even higher than those at invaded sites. Decreased native tree species richness did not explain these patterns, as restored sites had native tree species richness levels similar to those of near‐pristine sites. Increased host abundance and leaf nitrogen content did not significantly correlate with increased damage in most cases. Therefore, plant species richness recovers following restoration, but native trees still experience increased pressure from folivores and phylopathogenic fungi, which may even exceed levels experienced at invaded sites, thus impacting recovery trajectories.     

Invasion by Solidago species has limited impacts on soil seed bank communities

Increasing attention in invasion biology is being paid to measuring and understanding the impacts of invasive species. For plant invasions, however, the impact of invasion on soil seed bank communities has been under-studied. At six sites in southern Germany, we investigated whether areas invaded by Solidago gigantea and Solidago canadensis experienced a reduction in seed bank species richness, size and diversity, and a change in species composition compared to adjacent uninvaded areas. We found no overall effect of invasion on seed bank size, or on species richness and diversity. Seed bank size significantly decreased from 0–5 cm to 5–10 cm depth in both invaded and uninvaded areas. A significant amount of variation in species composition was explained by invasion, but it was only one-tenth of that explained solely by site effects. Our study suggests that invasion by Solidago species may not have the same impacts on the soil seed banks of native species as other invasive perennial forbs that have so far been studied.     

Implications of newly-formed seed-dispersal mutualisms between birds and introduced plants in northern California,USA

I examined the role of bird dispersal in invasiveness of three non-native plant species in California, USA: Triadica sebifera, Ligustrum lucidum, and Olea europaea. I selected these species because their invasiveness in California is uncertain, but a survey of ornithologists highlighted them as likely bird-dispersed. I quantified bird frugivory of these plants, compared them with a native species (Heteromeles arbutifolia), and explored the management implications of dispersal mutualisms for these and other incipient invasive plants. Fruit removal by birds was sufficient to permit spread for all study species. Seed dispersers (rather than seed predators) and pulse feeders (flocking species with potential for long distance dispersal) performed most fruit removal for the non-native species, a pattern indicative of an effective dispersal regime. The number of fruiting trees per stand was a significant predictor of bird visitation. Founding population size may thus be important in management of invasive, bird-dispersed plants. Disperser-defined niches were relatively narrow because a few disperser species performed the majority of fruit removal from study trees, but each fruit species was consumed by a variety of potential dispersers. This results in strong pairwise niche overlap between some plant species. Ordinated by bird use, study site-species combinations clustered more by geographic location than by plant species, emphasizing the opportunistic nature of bird foraging. None of the non-native focal plant species appears dispersal limited, and all have formed novel mutualisms in California. It is possible that these plants are now in lag phases preceding bird-mediated invasion. Consideration of bird dispersal when evaluating invasiveness is therefore an imperative.     

Impacts of Myriophyllum aquaticum invasion in a Mediterranean wetland on plant and macro-arthropod communities

The invasion by alien macrophytes in aquatic ecosystems may produce a strong alteration of the native aquatic vegetation leading to heavy impacts for both plant and faunal native diversity. Myriophyllum aquaticum is an aquatic plant native of Southern America, invasive in several part of the world. We studied the effects of M. aquaticum invasion on plant and macro-arthropod communities in the canals around a protected wetland in the Mediterranean basin. We sampled plant and macro-arthropod communities in 10 transects in invaded and non-invaded tracts of the canals. We assessed the differences in plant and macro-arthropod species richness, diversity, taxonomic diversity and species composition between invaded and non-invaded habitats by means of univariate and multivariate analyses. Our study shows a significant loss of plant diversity between non-invaded to invaded sites, leading to communities numerically and taxonomically impoverished and highly divergent in the species composition. We also detected significant differences in arthropod species composition between invaded and non-invaded transects. Some taxa such as mosquitoes and malacostraca were more frequent in the M. aquaticum-dominated stands. Furthermore, the study shows a positive relation between invaded habitats and juvenile individuals of the invasive alien crayfish Procambarus clarkii.     

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.     

Seed removal decrease by invasive Argentine ants in a high Nature Value farmland

Seed dispersal by ants is an important ecological process that maintains the structure anddiversity of natural communities, however, it is vulnerable to biological invasions. Argentine ants are one of the worst invasive ant species and cause severe changes in ecosystem processes and native ant biodiversity declines in invaded sites. Here, we studied seed removal by ants combining observations and a cafeteria experiment with seeds of four myrmecochorous plant species (Centaurea sphaerocephala, Rosmarinus officinalis, Silybum marianum, and Ulex australis) in two sites (invaded and uninvaded) located in the Mediterranean Montado ecosystem and classified as High Nature Value farmland (HNV). Significant differences in daily seed removal rates were found between the two study sites. In uninvaded sites, several native ant species were attracted to the seeds, resulting in all seeds being removed rapidly. The majority of seed removal events were carried out by two key seed disperses Pheidole pallidula (71%) and Aphaenogaster iberica (26%) with a clear preference for diaspored with larger and heavier elaiosome (i.e., C. sphaerocephala, S. marianum). By contrast, while the Argentine ant showed some interest (68% of seeds were interacted with), no seed removal events were observed. The extirpation of the local ant fauna by the Argentine ant and its inability to ensure seed dispersal services may lead to the interference and eventually to the collapse of seed dispersal of the four studied myrmecochorous plants in the invaded site in the future. We argue that these discrete but severe consequences of an invasive species on a key ecological process may strongly affect the functioning of the Montado ecosystem.     

Positive plant-soil feedback may drive dominance of a woodland invader, <Emphasis Type="Italic">Euonymus fortunei</Emphasis>

Positive feedback between invasive and native plants may contribute to invasive species dominance, although this pattern may not be general for all invasions and has not been well explored in woodland systems. We examined the pairwise feedback relationship between Euonymus fortunei, an emerging invader of North American deciduous forests, and Asarum canadense, a co-occuring native groundcover, to determine whether positive feedback might contribute to Euonymus dominance. In a greenhouse in Bloomington, Indiana, USA, we conditioned live woodland soil via growth with either Euonymus or Asarum, then used conditioned soils to inoculate monocultures and pairwise mixtures of each species. After eight weeks, we harvested plants and measured percent growth. We found evidence for positive plant-soil feedback between Euonymus and Asarum. Euonymus grew better in soil conditioned by conspecifics than in soil conditioned by Asarum, whereas total Asarum growth was not different in soil conditioned by conspecifics vs. Euonymus. These results held whether Euonymus and Asarum grew in monoculture or pairwise competition. This study supports a role for positive plant-soil feedback as a driving factor behind the invasion of Euonymus fortunei. Future work should examine the role of abiotic vs. biotic factors in mediating feedback between Euonymus and Asarum. Furthermore, researchers should examine pairwise feedback between Euonymus and other native species in order to better understand the role of positive feedback in Euonymus invasion. By evaluating the importance of this and other possible invasion mechanisms, we can improve our understanding of invasion ecology while facilitating management of harmful invasive species.     

Dispersal and recruitment limitation in native versus exotic tree species: life‐history strategies and Janzen‐Connell effects

Life‐history traits of invasive exotic plants are typically considered to be exceptional vis‐à‐vis native species. In particular, hyper‐fecundity and long range dispersal are regarded as invasive traits, but direct comparisons with native species are needed to identify the life‐history stages behind invasiveness. Until recently, this task was particularly problematic in forests as tree fecundity and dispersal were difficult to characterize in closed stands. We used inverse modelling to parameterize fecundity, seed dispersal and seedling dispersion functions for two exotic and eight native tree species in closed‐canopy forests in Connecticut, USA. Interannual variation in seed production was dramatic for all species, with complete seed crop failures in at least one year for six native species. However, the average per capita seed production of the exotic Ailanthus altissima was extraordinary: > 40 times higher than the next highest species. Seed production of the shade tolerant exotic Acer platanoides was average, but much higher than the native shade tolerant species, and the density of its established seedlings (≥ 3 years) was higher than any other species. Overall, the data supported a model in which adults of native and exotic species must reach a minimum size before seed production occurred. Once reached, the relationship between tree diameter and seed production was fairly flat for seven species, including both exotics. Seed dispersal was highly localized and usually showed a steep decline with increasing distance from parent trees: only Ailanthus altissima and Fraxinus americana had mean dispersal distances > 10 m. Janzen‐Connell patterns were clearly evident for both native and exotic species, as the mode and mean dispersion distance of seedlings were further from potential parent trees than seeds. The comparable intensity of Janzen‐Connell effects between native and exotic species suggests that the enemy escape hypothesis alone cannot explain the invasiveness of these exotics. Our study confirms the general importance of colonization processes in invasions, yet demonstrates how invasiveness can occur via divergent colonization strategies. Dispersal limitation of Acer platanoides and recruitment limitation of Ailanthus altissima will likely constitute some limit on their invasiveness in closed‐canopy forests.