Strengthening Invasion Filters to Reassemble Native Plant Communities: Soil Resources and Phenological Overlap

Preventing invasion by exotic species is one of the key goals of restoration, and community assembly theory provides testable predictions about native community attributes that will best resist invasion. For instance, resource availability and biotic interactions may represent “filters” that limit the success of potential invaders. Communities are predicted to resist invasion when they contain native species that are functionally similar to potential invaders; where phenology may be a key functional trait. Nutrient reduction is another common strategy for reducing invasion following native species restoration, because soil nitrogen (N) enrichment often facilitates invasion. Here, we focus on restoring the herbaceous community associated with coastal sage scrub vegetation in Southern California; these communities are often highly invaded, especially by exotic annual grasses that are notoriously challenging for restoration. We created experimental plant communities composed of the same 20 native species, but manipulated functional group abundance (according to growth form, phenology, and N‐fixation capacity) and soil N availability. We fertilized to increase N, and added carbon to reduce N via microbial N immobilization. We found that N reduction decreased exotic cover, and the most successful seed mix for reducing exotic abundance varied depending on the invader functional type. For instance, exotic annual grasses were least abundant when the native community was dominated by early active forbs, which matched the phenology of the exotic annual grasses. Our findings show that nutrient availability and the timing of biotic interactions are key filters that can be manipulated in restoration to prevent invasion and maximize native species recovery.     

Are Local Filters Blind to Provenance? Ant Seed Predation Suppresses Exotic Plants More than Natives

The question of whether species’ origins influence invasion outcomes has been a point of substantial debate in invasion ecology. Theoretically, colonization outcomes can be predicted based on how species’ traits interact with community filters, a process presumably blind to species’ origins. Yet, exotic plant introductions commonly result in monospecific plant densities not commonly seen in native assemblages, suggesting that exotic species may respond to community filters differently than natives. Here, we tested whether exotic and native species differed in their responses to a local community filter by examining how ant seed predation affected recruitment of eighteen native and exotic plant species in central Argentina. Ant seed predation proved to be an important local filter that strongly suppressed plant recruitment, but ants suppressed exotic recruitment far more than natives (89% of exotic species vs. 22% of natives). Seed size predicted ant impacts on recruitment independent of origins, with ant preference for smaller seeds resulting in smaller seeded plant species being heavily suppressed. The disproportionate effects of provenance arose because exotics had generally smaller seeds than natives. Exotics also exhibited greater emergence and earlier peak emergence than natives in the absence of ants. However, when ants had access to seeds, these potential advantages of exotics were negated due to the filtering bias against exotics. The differences in traits we observed between exotics and natives suggest that higher-order introduction filters or regional processes preselected for certain exotic traits that then interacted with the local seed predation filter. Our results suggest that the interactions between local filters and species traits can predict invasion outcomes, but understanding the role of provenance will require quantifying filtering processes at multiple hierarchical scales and evaluating interactions between filters.     

Different flowering phenology of alien invasive species in Spain: evidence for the use of an empty temporal niche?

Flowering phenology is an important and poorly understood plant trait that may possibly be related to the invasiveness potential of alien species. The present work evaluates whether flowering time of invasive alien species is a key trait to overcome the climatic filters operating in continental Mediterranean ecosystems of Spain (characterised by summer drought and low temperatures in winter). We conducted comparisons between the flowering phenology of the invasive species in their native range and in Spain, and between flowering phenology of 91 coexisting invasive–native species pairs. For the alien species, geographical change from the native to the invaded region did not result in shifts in the start and the length of the flowering period. Overall, climatic conditions in the native range of species selected for a flowering pattern is maintained after translocation of the species to another region. Flowering of tropical and temperate invasive alien species peaked in summer, which contrasts with the spring flowering of native and invasive alien species of Mediterranean climate origin. By exploiting this new temporal niche, these invasive alien species native to tropical and temperate regions benefit from reduced competition with natives for abiotic and biotic resources. We suggest that human-mediated actions have reduced the strength of the summer drought filter in particular microhabitats, permitting the invasion of many summer-flowering aliens.     

Disturbance of biological soil crust increases emergence of exotic vascular plants in California sage scrub

Biological soil crusts (BSCs) are comprised of soil particles, bacteria, cyanobacteria, green algae, microfungi, lichens, and bryophytes and confer many ecosystem services in arid and semiarid ecosystems worldwide, including the highly threatened California sage scrub (CSS). These services, which include stabilizing the soil surface, can be adversely affected when BSCs are disturbed. Using field and greenhouse experiments, we tested the hypothesis that mechanical disturbance of BSC increases emergence of exotic vascular plants in a coastal CSS ecosystem. At Whiting Ranch Wilderness Park in southern California, 22 plots were established and emergence of exotic and native plants was compared between disturbed and undisturbed subplots containing BSC. In a separate germination study, seed fate in disturbed BSC cores was compared to seed fate in undisturbed BSC cores for three exotic and three native species. In the field, disturbed BSCs had significantly (>3×) greater exotic plant emergence than in undisturbed BSC, particularly for annual grasses. Native species, however, showed no difference in emergence between disturbed and undisturbed BSC. Within the disturbed treatment, emergence of native plants was significantly, and three times less than that of exotic plants. In the germination study, seed fates for all species were significantly different between disturbed and undisturbed BSC cores. Exotic species had greater emergence in disturbed BSC, whereas native plants showed either no response or a positive response. This study demonstrates another critical ecosystem service of BSCs—the inhibition of exotic plant species—and underscores the importance of BSC conservation in this biodiversity hotspot and possibly in other aridland ecosystems.     

Exotic herbivores directly facilitate the exotic grasses they graze: mechanisms for an unexpected positive feedback between invaders

The ability of an exotic species to establish in a system may depend not only on the invasibility of the native community, but also on its interactions with other exotic species. Though examples of mutually beneficial interactions between exotic species are known, few studies have quantified these effects or identified specific mechanisms. We used the co-invasion of an endangered island ecosystem by exotic Canada geese (Branta canadensis) and nine exotic annual grasses to study the effects of an invading herbivore on the success of invading grasses. On our study islands in southwestern Canada, we found that geese fed selectively on the exotic grasses and avoided native forbs. Counter to current theory suggesting that the grasses should be limited by a selective enemy, however, the grasses increased in proportional abundance under grazing whereas forbs showed declining abundance. Testing potential mechanisms for the effects of grazing on grasses, we found that the grasses produced more stems per area when grazing reduced vegetation height and prevented litter accumulation. Forming dense mats of short stems appeared to be an efficient reproductive and competitive strategy that the Eurasian grasses have evolved in the presence of grazers, conferring a competitive advantage in a system where the native species pool has very few annual grasses and no grazers. Germination trials further demonstrated that selective herbivory by geese enables their dispersal of exotic grass seed between heavily invaded feeding areas and the small islands used for nesting. In summary, the exotic geese facilitated both the local increase and the spatial spread of exotic grasses, which in turn provided the majority of their diet. This unexpected case of positive feedback between exotic species suggests that invasion success may depend on the overall differences between the evolutionary histories of the invaders and the evolutionary history of the native community they enter. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Nutrient versus seed bank depletion approaches to controlling exotic annuals in threatened Box Gum woodlands

Control of exotic annuals is often a priority when restoring degraded grasslands or shrublands. This study evaluated combinations of nutrient‐depleting (carbon addition) and seed bank‐depleting approaches for controlling exotic annuals, and compared the seed bank depletion technique of spring burning with the more easily applied technique of pulse grazing. Treatments were applied in two Box Gum woodlands over 4 years. Consistent with earlier studies, carbon addition dramatically reduced exotic annuals and available nutrients in all 3 years at both sites. Exotic annual grass abundance was significantly reduced in burnt plots following the first year and in grazed plots following the second year of application. Spring burning or grazing did not reduce available nutrients or exotic annual broadleaf abundance at either site. The effect of carbon addition on exotic annuals and available nutrients was so powerful that no additional benefit of the combination treatments was found, although at one site burning and grazing slightly reduced the effectiveness of carbon addition in suppressing broadleaf exotic annuals. We conclude that nutrient‐depleting approaches are most effective because they control both exotic annual grasses and broadleaf exotics annuals, but given their expense seed bank‐depleting approaches may be more practical where exotic annual grasses dominate. In particular, pulse grazing is readily applied across large areas, offering a relatively simple tool that if appropriately implemented could enhance outcomes of restoration investments in vegetation communities invaded by exotic annual grasses.     

Does a giant tortoise taxon substitute enhance seed germination of exotic fleshy-fruited plants?

Aims The use of exotic species as taxon substitutes to restore lost ecological interactions is currently hotly debated. Aldabrachelys gigantea giant tortoises have recently been introduced to three islands in the Mascarene archipelago (Ile aux Aigrettes, Round Island and Rodrigues) to resurrect herbivory and seed dispersal functions once performed by extinct giant tortoises. However, potential unintended impacts by frugivore substitutes on native ecosystems, e.g. whether they will facilitate the germination of exotic plant species, are largely unknown. We investigated whether A. gigantea introduced to Rodrigues in 2006 could enhance the germination percentage of four widespread fleshy-fruited exotic species on the island. Using germination trials to forecast unintended impacts that could arise from the introduction of a frugivorous taxon substitute enables conservation managers to limit potential adverse negative interactions before they occur.Methods In germination trials that ran over 4 months, we investigated the effects of ingestion (gut passage and deposition in faeces) by sub-adult and adult A. gigantea on the germination percentage of four exotic fleshy-fruited plant species introduced to Rodrigues. We fed fruits of these plant species to sub-adult and adult A. gigantea to test how variation in age and size of the frugivore would affect seed germination. Feeding of distinctly coloured plastic pellets together with the fruits allowed us to test for individual tortoise effects on seed germination.Important findings Ingestion by A. gigantea increased the percentage of seeds germinating of Mimusops coriacea and Lantana camara, but not percentage of germination of Veitchia merrillii or Wikstroemia indica. Seeds were more likely to germinate following ingestion by sub-adult rather than adult tortoises, which may be a consequence of the shorter gut passage time observed for sub-adults. Our results demonstrate that introduced frugivorous taxon substitutes could facilitate germination of exotic and invasive plants and highlight the need for conservation managers to weigh the risk of taxon substitutes potentially facilitating the germination and recruitment of exotic fleshy-fruited plants against the benefit of restoring lost seed dispersal functions of threatened indigenous plants. Our findings also highlight the importance of considering age and size variation in frugivores, in particular in long-lived taxa such as giant tortoises, when studying ingestion effects on the germination performance of plants.     

Local interactions, dispersal, and native and exotic plant diversity along a California stream

Although the species pool, dispersal, and local interactions all influence species diversity, their relative importance is debated. I examined their importance in controlling the number of native and exotic plant species occupying tussocks formed by the sedge Carex nudata along a California stream. Of particular interest were the factors underlying a downstream increase in plant diversity and biological invasions. I conducted seed addition experiments and manipulated local diversity and cover to evaluate the degree to which tussocks saturate with species, and to examine the roles of local competitive processes, abiotic factors, and seed supply in controlling the system‐wide patterns.
Seeds of three native and three exotic plants sown onto experimentally assembled tussock communities less successfully established on tussocks with a greater richness of resident plants. Nonetheless, even the most diverse tussocks were somewhat colonized, suggesting that tussocks are not completely saturated with species. Similarly, in an experiment where I sowed seeds onto natural tussocks along the river, colonization increased two‐ to three‐fold when I removed the resident species. Even on intact tussocks, however, seed addition increased diversity, indicating that the tussock assemblages are seed limited. Colonization success on cleared and uncleared tussocks increased downstream from km 0 to km 3 of the study site, but showed no trends from km 3 to km 8. This suggests that while abiotic and biotic features of the tussocks may control the increase in diversity and invasions from km 0 to km 3, similar increases from km 3 to km 8 are more likely explained by potential downstream increases in seed supply. The effective water dispersal of seed mimics and prevailingly downstream winds indicated that dispersal most likely occurs in a downstream direction. These results suggest that resident species diversity, competitive interactions, and seed supply similarly influence the colonization of native and exotic species.     

Context dependency of the allelopathic effects of Lonicera maackii on seed germination

Allelopathic effects of invasive plants on native flora may be mitigated by the abiotic and biotic environment into which the allelochemicals are released. Lonicera maackii (Amur honeysuckle), an invasive plant of the eastern deciduous forest, suppresses seed germination in laboratory assays. We investigated how L. maackii leachate interacts with abiotic conditions and with the soil microbial community. First, we tested the effects of leaf extract from L. maackii on germination of the native woodland herb, Blephilia hirsuta, under different light and soil conditions. We found that germination of Blephilia hirsuta was reduced by L. maackii extract, but abiotic conditions did not interact with this effect. We also tested the effects of leaf extract on germination of five native woodland species and L. maackii placed in sterile or live soil. There was an overall suppressive effect of L. maackii extract on itself and the other five native species tested. However, L. maackii extract interacted with live soil in ways that differed with the species being tested and, in some cases, changed over time. Our results indicate that allelopathic potential of L. maackii shows context dependency with respect to soil microorganisms and native species identity but not to light conditions or soil type. Our results imply that restoration of invaded areas may require active reintroduction of species sensitive to allelopathy in live soil. Further, laboratory assays of allelopathy should consider the interaction of allelochemicals with biotic and abiotic conditions to more accurately predict the impacts of allelopathy on plant communities.     

Increasing native, but not exotic, biodiversity increases aboveground productivity in ungrazed and intensely grazed grasslands

Species-rich native grasslands are frequently converted to species-poor exotic grasslands or pastures; however, the consequences of these changes for ecosystem functioning remain unclear. Cattle grazing (ungrazed or intensely grazed once), plant species origin (native or exotic), and species richness (4-species mixture or monoculture) treatments were fully crossed and randomly assigned to plots of grassland plants. We tested whether (1) native and exotic plots exhibited different responses to grazing for six ecosystem functions (i.e., aboveground productivity, light interception, fine root biomass, tracer nitrogen uptake, biomass consumption, and aboveground biomass recovery), and (2) biodiversity-ecosystem functioning relationships depended on grazing or species origin. We found that native and exotic species exhibited different responses to grazing for three of the ecosystem functions we considered. Intense grazing decreased fine root biomass by 53% in exotic plots, but had no effect on fine root biomass in native plots. The proportion of standing biomass consumed by cattle was 16% less in exotic than in native grazed plots. Aboveground biomass recovery was 30% less in native than in exotic plots. Intense grazing decreased aboveground productivity by 25%, light interception by 14%, and tracer nitrogen uptake by 54%, and these effects were similar in native and exotic plots. Increasing species richness from one to four species increased aboveground productivity by 42%, and light interception by 44%, in both ungrazed and intensely grazed native plots. In contrast, increasing species richness did not influence biomass production or resource uptake in ungrazed or intensely grazed exotic plots. These results suggest that converting native grasslands to exotic grasslands or pastures changes ecosystem structure and processes, and the relationship between biodiversity and ecosystem functioning.     

Contrasting effects of grazing on the early stages of woody encroachment in a Neotropical savanna

Woody encroachment in savannas represents an ecological process of current global interest given its negative impact on ecosystem functioning, particularly on forage production. Traditional savanna models propose competition and niche differentiation as the main mechanisms allowing tree-grass coexistence. Demographic models, instead, propose abiotic and biotic factors as bottlenecks controlling vital rates and transitions from seeds to adult trees. The role played by domestic grazing on woody encroachment is yet controversial. Here, using a multistage tree life approach, we combine both models and evaluate the role of grazing and herbaceous vegetation on woody recruitment in a Neotropical savanna dominated by Vachellia caven, a successful and widely spread encroacher tree species. We performed three experiments to evaluate seed predation, seedling emergence and survival of V. caven by manipulating cattle grazing (grazed and ungrazed areas) and herbaceous vegetation presence (vegetated and unvegetated). Finally, we combined the results of the three experiments to estimate the probability of plant recruitment across these experimental factors. Grazing decreased seed predation by half, did not modify seedling emergence and decreased seedling survival. Herbaceous vegetation did not affect seed predation nor seedling emergence rate, but increased seedling survival. Overall, the net effect of grazing on V. caven recruitment was neutral since the increase in seed availability due to the reduction in seed predation rate was compensated by the negative effect of grazing on seedling survival. Our analysis revealed that cattle grazing and herbaceous vegetation had contrasting effects on the seed and seedling life stages. We propose that in order to restrain the early stages of encroachment, cattle grazing pressure could be managed following the seasonality of demographic tree transitions. Through rotational grazing amongst paddocks, stocking rates could be relaxed during the primary dispersal stage to maximize granivory, and then increased to enhance the chance of seedling consumption and trampling.     

The invasive grass,Melinis minutiflora,inhibits tree regeneration in a Neotropical savanna

Abstract Exotic grasses are becoming increasingly abundant in Neotropical savannas, with Melinis minutiflora Beauv. being particularly invasive. To better understand the consequences for the native flora, we performed a field study to test the effect of this species on the establishment, survival and growth of seedlings of seven tree species native to the savannas and forests of the Cerrado region of Brazil. Seeds of the tree species were sown in 40 study plots, of which 20 were sites dominated by M. minutiflora, and 20 were dominated by native grasses. The exotic grass had no discernable effect on initial seedling emergence, as defined by the number of seedlings present at the end of the first growing season. Subsequent seedling survival in plots dominated by M. minutiflora was less than half that of plots dominated by native species. Consequently, at the end of the third growing season, invaded plots had only 44% as many seedlings as plots with native grasses. Above‐ground grass biomass of invaded plots was more than twice that of uninvaded plots, while seedling survival was negatively correlated with grass biomass, suggesting that competition for light may explain the low seedling survival where M. minutiflora is dominant. Soils of invaded plots had higher mean Ca, Mg and Zn, but these variables did not account for the higher grass biomass or the lower seedling survival in invaded plots. The results indicate that this exotic grass is having substantial effects on the dynamics of the tree community, with likely consequences for ecosystem structure and function.     

Radicle emergence with increased temperatures following summer dispersal in Trillium camschatcense: A species with deep simple double morphophysiological dormancy in seeds

Seeds with deep simple double morphophysiological dormancy (MPD) need cold stratification during the first winter after dispersal for radicle emergence, followed by the summer for root and bud development and finally a second winter for shoot emergence. In a previous study, we demonstrated that Trillium camschatcense seeds have this type of dormancy with radicles emerging from most seeds after the first winter. However, radicles also emerged from a few seeds in autumn during the same year as dispersal. We thought that temperatures after seed dispersal played a role in radicle emergence before the first winter. To confirm our idea, we investigated germination phenology outdoors, relationships between temperatures after seed dispersal and radicle emergence in the first year outdoors, radicle emergence in the first winter under varied temperatures using incubators, and shoot emergence from seeds with an emerged radicle in the first year outdoors. Our phenology study confirmed that T. camschatcense seeds have deep simple double MPD. Over 7 years, 0.2–7.5% of radicles emerged in the first year before winter and these percentages were moderately positively correlated with temperatures, especially minimum temperatures. Increasing August and September temperatures increased radicle emergence in the laboratory. Shoots emerged from seeds with an emerged radicle in the first year after the first winter. With increased autumn temperatures in warmer regions or with global warming, we predict that germination phenology may shift: increased radicle emergence in the first year and shoot emergence following the first (and not second) winter.     

Differentiation between native and exotic plant species in a dry grassland: realized responses to perturbation,and comparison with fundamental responses

Abstract An area of dry grassland in New Zealand, comprising an equal mixture of native and exotic species, was subject to perturbations of irrigation, fertilization and cessation of grazing. The vegetation response was recorded for 3 years. Total cover, and the contribution of native species to that cover, fluctuated between years even in the control plots. Irrigation increased total cover, but decreased the cover of native species. Fertilization produced the same effects, only less strongly, and also reduced species richness, the loss being in native species. In spite of overall effects of treatments on native and exotic cover, when individual species’ responses to irrigation, fertilization or exclosure were calculated, there was no significant difference between the native and exotic plant guilds. Species differed in their responses, but the native and exotic guilds overlapped. When grouped by morphology, the only significant difference between the responses to perturbation was that forbs and graminoids responded more positively to irrigation than woody and cryptogamic species. The realized responses of the species to the perturbations described here showed little correlation with their physiological responses as determined in previous greenhouse experiments. It is suggested that the realized responses are strongly, and currently unpredictably, influenced by competition from the other species present. Soil nutrients and soil water were both important controls on the community. The relative similarity in the nature of the response to these two factors – nutrients and water – suggests that they affect species in similar ways, possibly because the greater growth rate of the exotic species mediates the short‐term response to both. Grazing has less effect on the current community than either nutrients or water, although it may have been historically important in shaping the species pool. From the poor predictability of field responses from morphological guilds or from ecophysiological responses, it is suggested that the ‘functional types’ approach, although conceptually attractive, lacks experimental support in these grasslands. It is concluded that the exotic species have invaded by being pre‐adapted to the environment with the same environmental responses as the natives, but with the advantage of generally higher growth rates.     

An invasive plant provides refuge to native plant species in an intensely grazed ecosystem

Invasion by exotic plant species and herbivory can individually alter native plant species diversity, but their interactive effects in structuring native plant communities remain little studied. Many exotic plant species escape from their co-evolved specialized herbivores in their native range (in accordance with the enemy release hypothesis). When these invasive plants are relatively unpalatable, they may act as nurse plants by reducing herbivore damage on co-occurring native plants, thereby structuring native plant communities. However, the potential for unpalatable invasive plants to structure native plant communities has been little investigated. Here, we tested whether presence of an unpalatable exotic invader Opuntia ficus-indica was associated with the structure of native plant communities in an ecosystem with a long history of grazing by ungulate herbivores. Along 17 transects (each 1000 m long), we conducted a native vegetation survey in paired invaded and uninvaded plots. Plots that harboured O. ficus-indica had higher native plant species richness and Shannon–Wiener diversity H′ than uninvaded plots. However, mean species evenness J was similar between invaded and uninvaded plots. There was no significant correlation between native plant diversity and percentage plot cover by O. ficus-indica. Presence of O. ficus-indica was associated with a compositional change in native community assemblages between paired invaded and uninvaded plots. Although these results are only correlative, they suggest that unpalatable exotic plants may play an important ecological role as refugia for maintenance of native plant diversity in intensely grazed ecosystems.     

Long‐term effects of prairie restoration on plant community structure and native population dynamics

The key to restoring degraded grassland habitats is identifying feasible and effective techniques to reduce the negative impacts of exotic species and promote self‐sustaining native populations. It is often difficult to extend monitoring of restoration efforts to evaluate long‐term success, but doing so is essential to understanding how initial outcomes change over time. To assess how initial treatment effects persist, we revisited degraded patches of Pacific Northwest prairie habitat 6 years after experimental restoration efforts ceased. We evaluated plant community composition to determine the lasting effects of supplemental native seeding and disturbance treatments (burning, mowing, and herbicide to reduce exotic species). We tracked the persistence of seeded species and measured spread of their populations to evaluate suitability of species for restoration and the ability of the habitat to support native plant populations. We found that plots that received supplemental seeding continued to exhibit higher richness of native species than those left unseeded, and that both seeding and disturbance treatments could positively influence native species abundance over the long term. The initially observed effects of disturbance treatments on reducing exotic grass abundance had diminished, highlighting the importance of long‐term monitoring and ongoing control of exotic species. Nevertheless, these treatments significantly influenced the population trajectories of 4 out of 8 seeded native species. There was evidence of spatial advance of most seeded species. Results from extended monitoring confirm that dispersal limitation of native species and difficulties maintaining the reduction of exotic grasses continue to be major barriers to success in restoration of invaded grasslands.     

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.     

When do herbivores affect plant invasion? Evidence for the natural enemies and biotic resistance hypotheses

Two venerable hypotheses, widely cited as explanations for either the success or failure of introduced species in recipient communities, are the natural enemies hypothesis and the biotic resistance hypothesis. The natural enemies hypothesis posits that introduced organisms spread rapidly because they are liberated from their co‐evolved predators, pathogens and herbivores. The biotic resistance hypothesis asserts that introduced species often fail to invade communities because strong biotic interactions with native species hinder their establishment and spread. We reviewed the evidence for both of these hypotheses as they relate to the importance of non‐domesticated herbivores in affecting the success or failure of plant invasion.
To evaluate the natural enemies hypothesis, one must determine how commonly native herbivores have population‐level impacts on native plants. If native herbivores seldom limit native plant abundance, then there is little reason to think that introduced plants benefit from escape from these enemies. Studies of native herbivore‐native plant interactions reveal that plant life‐history greatly mediates the strength with which specialist herbivores suppress plant abundance. Relatively short‐lived plants that rely on current seed production for regeneration are most vulnerable to herbivory that reduces seed production. As such, these plants may gain the greatest advantage from escaping their specialist enemies in recipient communities. In contrast, native plants that are long lived or that possess long‐lived seedbanks may not be kept “in check” by native herbivores. For these species, escape from native enemies may have little to do with their success as exotics; they are abundant both where they are native and introduced.
Evidence for native herbivores providing biotic resistance to invasion by exotics is conflicting. Our review reveals that: 1) introduced plants can attract a diverse assemblage of native herbivores and that 2) native herbivores can reduce introduced plant growth, seed set and survival. However, the generality of these impacts is unclear, and evidence that herbivory actually limits or reduces introduced plant spread is scarce. The degree to which native herbivores provide biotic resistance to either exotic plant establishment or spread may be greatly determined by their functional and numerical responses to exotic plants, which we know little about. Generalist herbivores, through their direct effects on seed dispersal and their indirect effects in altering the outcome of native–non‐native plant competitive interactions, may have more of a facilitative than negative effect on exotic plant abundance.     

Historical legacies and ecological determinants of grass naturalizations worldwide

The global distribution of exotic species is the result of abiotic, biotic and dispersal filtering processes that shape the movement and success of species outside their native range. In this study we aim to understand how these filtering processes drive the fluxes of grass species among regions, the factors that influence which species establish outside of their native range, and where they do so. We used national and subnational checklists of native and introduced grass species to determine the extent to which each region was a source or recipient of exotic grass species. We asked how species traits may distinguish those grass species that have naturalized outside their native range from those that have not, and how environmental conditions are related to the distribution of exotic grass species. We found that exotic grass establishment is shaped by an array of factors including characteristics of regions, traits of species and their interactions. Regions with a longer history of human occupation and larger numbers of native grass species were generally the most important sources of exotic species. Global flows of species were mostly driven by a climate match between the native and exotic ranges, but were also highly asymmetric, with regions with recent human arrival being the major hosts of exotic grass species. Tall, annual and C4 grass species exhibited particularly high probabilities of establishment outside their native range. Despite the idiosyncrasy and stochasticity characteristic of exotic species establishment, this biogeographical analysis revealed important generalities across this large plant group. Our results suggest that grass species that have co-occurred with humans for a longer time may be better adapted to living in anthropogenic landscapes, explaining the global asymmetry in species introductions.     

Mechanisms of resistance to invasion in a California grassland: the roles of competitor identity, resource availability, and environmental gradients

Resistance to the invasion of exotic plants may sometimes result from the strong effects of a relatively small number of resident species. Understanding the mechanisms by which such species resist invasion could provide important insights for the management of invaded ecosystems. Furthermore, the individualistic responses of community members to resource availability and environmental gradients could drive spatial variation in resistance at the local to landscape scales. We tested the resistance of monoculture plots of three native perennial grasses from the California coastal prairie to the invasion of the European perennial grass Holcus lanatus. We also used a watering treatment that increased early summer water availability and a natural elevational gradient in resource availability and soil texture to evaluate how resident identity interacted with abiotic resistance to affect Holcus establishment. Two native species, Festuca rubra and Calamagrostis nutkaensis , exhibited strong resistance, correlated with their negative effects on light availability. A third native grass, Bromus carinatus var. maritimus , had either no effect or a weakly facultative effect on Holcus performance relative to bare plots. Water addition did not alter the resistance of these species, but the elevation gradient did. Holcus invasion increased with improving abiotic conditions towards the slope bottom in bare and Bromus plots, but invasion decreased towards the bottom in Calamagrostis plots, where better conditions favored competitive residents. These results support the idea that resistance to invasion can sometimes be due to a subset of native species, and that the resistance provided by even a single species is likely to vary across the landscape. Identifying the mechanisms by which species resist invasion could facilitate the selection of management strategies that at best increase, or at worst do not decrease, natural resistance.