Can Carbon Addition Increase Competitiveness of Native Grasses? A Case Study from California

There is growing interest in the addition of carbon (C) as sucrose or sawdust to the soil as a tool to reduce plant‐available nitrogen (N) and alter competitive interactions among species. The hypothesis that C addition changes N availability and thereby changes competitive dynamics between natives and exotics was tested in a California grassland that had experienced N enrichment. Sawdust (1.2 kg/m) was added to plots containing various combinations of three native perennial bunchgrasses, exotic perennial grasses, and exotic annual grasses. Sawdust addition resulted in higher microbial biomass N, lower rates of net N mineralization and net nitrification, and higher concentrations of extractable soil ammonium in the soil. In the first year sawdust addition decreased the degree to which exotic annuals competitively suppressed the seedlings of Nassella pulchra and, to a lesser extent, Festuca rubra, both native grasses. However there was no evidence of reduced growth of exotic grasses in sawdust‐amended plots. Sawdust addition did not influence interactions between the natives and exotic perennial grasses. In the second year, however, sawdust addition did not affect the interactions between the natives and either group of exotic grasses. In fact, the native perennial grasses that survived the first year of competition with annual grasses significantly reduced the aboveground productivity of annual grasses even without sawdust addition. These results suggest that the addition of sawdust as a tool in the restoration of native species in our system provided no significant benefit to natives over a 2‐year period.     

Plant–soil feedbacks of exotic plant species across life forms: a meta-analysis

Invasive exotic plant species effects on soil biota and processes in their new range can promote or counteract invasions via changed plant–soil feedback interactions to themselves or to native plant species. Recent meta-analyses reveale that soil influenced by native and exotic plant species is affecting growth and performance of natives more strongly than exotics. However, the question is how uniform these responses are across contrasting life forms. Here, we test the hypothesis that life form matters for effects on soil and plant–soil feedback. In a meta-analysis we show that exotics enhanced C cycling, numbers of meso-invertebrates and nematodes, while having variable effects on other soil biota and processes. Plant effects on soil biota and processes were not dependent on life form, but patterns in feedback effects of natives and exotics were dependent on life form. Native grasses and forbs caused changes in soil that subsequently negatively affected their biomass, whereas native trees caused changes in soil that subsequently positively affected their biomass. Most exotics had neutral feedback effects, although exotic forbs had positive feedback effects. Effects of exotics on natives differed among plant life forms. Native trees were inhibited in soils conditioned by exotics, whereas native grasses were positively influenced in soil conditioned by exotics. We conclude that plant life form matters when comparing plant–soil feedback effects both within and between natives and exotics. We propose that impact analyses of exotic plant species on the performance of native plant species can be improved by comparing responses within plant life form.     

Intraspecific and interspecific pair-wise seedling competition between exotic annual grasses and native perennials: plant–soil relationships

Few studies have examined plant–soil relationships in competitive arenas between exotic and native plants in the western United States. A pair-wise competitive design was used to evaluate plant–soil relationships between seedlings of the exotic annual grasses Bromus tectorum and Taeniatherium caput-medusae and the native perennial grasses Elymus elymoides and Pseudoroegneria spicata. Two soils were tested: an arid soil (argid) occupied by E. elymoides and presently invaded by B. tectorum and a high elevation, high organic matter, soil (aquept) where none of the tested species would typically occur. Plant growth proceeded for 85 days at which time above-ground biomass and tissue nutrient concentrations were quantified. Soil also was collected from the rooting zone beneath each species and analyzed for various nutrient pools. The exotic species had significantly greater above-ground biomass than the natives and grew far better in the aquept soil than the argid soil. Growth of B. tectorum, and to some degree, T. caput-medusae was suppressed in intraspecific competition and enhanced, especially in the aquept soil, when competing with the natives. Although not significant, biomass of natives strongly trended downward when competing with the exotic grasses. Overall, concentrations of tissue nutrients were minimally affected by competition, but natives tended to be more negatively affected by competition with exotics. Except for phosphorus (P), all species had significantly greater nutrient concentrations when growing in the aquept soil compared to the argid soil. In both soils, exotics had significant greater tissue concentrations of manganese (Mn), magnesium (Mg), and iron (Fe), while natives had significantly greater nitrogen (N). Species affects on soil nutrient pools occurred mostly in the aquept soil with exotic species significantly decreasing pools of available N, potentially available N, and soil-solution pools of calcium (Ca²⁺), potassium (K⁺), and magnesium (Mg²⁺) relative to natives. Overall, the data suggest that, in the seedling state, B. tectorum is a superior competitor. Moreover, when the natives compete intra- or interspecifically, particularly in the aquept soil, availability of N and other nutrients in their rooting zone is consistently greater than when they compete interspecifically with the exotic grasses. These data suggest the exotics are able to co-opt nutrients in the rooting zone of the natives and perhaps gain a competitive advantage.     

Within‐Year Soil Legacies Contribute to Strong Priority Effects of Exotics on Native California Grassland Communities

Understanding priority effects, in which one species in a habitat decreases the success of later species, may be essential for restoring native communities. Priority effects can operate in two ways: size‐asymmetric competition and creation of “soil legacies,” effects on soil that may last long after the competitive effect. We examined how these two types of priority effects, competition and soil legacies, drive interactions between seedlings of native and exotic California grassland plants. We established native and exotic communities in a mesocosm experiment. After 5 weeks, we removed the plants from half the treatments (soil legacy treatment) and retained the plants in the other half (priority effect treatment, which we interpret to include both competition and soil legacies). We then added native or exotic seed as the colonizing community. After 2 months, we measured the biomass of the colonizing community. When germinating first, both natives and exotics established priority effects, reducing colonist biomass by 86 and 92%, respectively. These priority effects were predominantly due to size‐asymmetric competition. Only exotics created soil legacies, and these legacies only affected native colonizers, reducing biomass by 74%. These results imply that exotic species priority effects can affect native grassland restorations. Although most restorations focus on removing exotic seedlings, amending soil to address soil legacies may also be critical. Additionally, because native species can exclude exotics if given a head start, ensuring that natives germinate first may be a cost‐effective restoration technique.     

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.     

Native versus exotic community patterns across three scales: Roles of competition,environment and incomplete invasion

Three fundamental, interrelated questions in invasion ecology are: (1) to what extent do exotic species outcompete natives; (2) are native and exotic communities functionally similar or different; and (3) are differences in biogeographic patterns in native and exotic communities due to incomplete invasions among exotics? These questions are analogous to general questions in community ecology regarding the relative roles of competition, environmental response and dispersal limitation in community assembly. We addressed each of these questions for plant communities in discrete meadow patches, using analyses at three scales ranging from the landscape to microsites. A weak positive relationship between native and exotic species richness in microsites, and a predominance of positive correlations in abundance among native and exotic species pairs suggest that competition has been less important than other factors in determining native versus exotic abundance and community composition. In contrast, models of species richness and community compositional change across scales suggest native versus exotic community patterns are largely determined by a mix of scale-dependent concordant (shared positive or negative) and discordant relationships with environmental variables. In addition, detailed analyses of species-area and species-abundance relationships suggest ongoing expansion of exotic species populations, indicating that the assembly of the exotic community is in its early stages. Thus, while competition does not appear to strongly affect native versus exotic abundances and compositions at present, it may intensify in the future. Our results indicate that synoptic patterns in native versus exotic richness that have been previously attributed to a single cause may in fact be due to a complex mix of concordant and discordant responses to environmental factors across scales. They also suggest that conservation efforts aimed at promoting natives and reducing exotics should focus on the factors and scales for which such a response (i.e., promotion of high native and low exotic richness) can be expected.     

Arbuscular Mycorrhizal Assemblages in Native Plant Roots Change in the Presence of Invasive Exotic Grasses

Plant invasions have the potential to significantly alter soil microbial communities, given their often considerable aboveground effects. We examined how plant invasions altered the arbuscular mycorrhizal fungi of native plant roots in a grassland site in California and one in Utah. In the California site, we used experimentally created plant communities composed of exotic (Avena barbata, Bromus hordeaceus) and native (Nassella pulchra, Lupinus bicolor) monocultures and mixtures. In the Utah semi-arid grassland, we took advantage of invasion by Bromus tectorum into long-term plots dominated by either of two native grasses, Hilaria jamesii or Stipa hymenoides. Arbuscular mycorrhizal fungi colonizing roots were characterized with PCR amplification of the ITS region, cloning, and sequencing. We saw a significant effect of the presence of exotic grasses on the diversity of mycorrhizal fungi colonizing native plant roots. In the three native grasses, richness of mycorrhizal fungi decreased; in the native forb at the California site, the number of fungal RFLP patterns increased in the presence of exotics. The exotic grasses also caused the composition of the mycorrhizal community in native roots to shift dramatically both in California, with turnover of Glomus spp., and Utah, with replacement of Glomus spp. by apparently non-mycorrhizal fungi. Invading plants may be able to influence the network of mycorrhizal fungi in soil that is available to natives through either earlier root activity or differential carbon provision compared to natives. Alteration of the soil microbial community by plant invasion can provide a mechanism for both successful invasion and the resulting effects of invaders on the ecosystem.     

The roles of exotic grasses and forbs when restoring native species to highly invaded southern California annual grassland

Many semi-arid shrublands in the western US have experienced invasion by a suite of exotic grasses and forbs that have altered community structure and function. The effect of the exotic grasses in this area has been studied, but little is known about how exotic forbs influence the plant community. A 3-year experiment in southern California coastal sage scrub (CSS) now dominated by exotic grasses was done to investigate the influence of both exotic grasses (mainly Bromus spp.) and exotic forbs (mainly Erodium spp.) on a restoration seeding (9 species, including grasses, forbs, and shrubs). Experimental plots were weeded to remove one, both, or neither group of exotic species and seeded at a high rate with a mix of native species. Abundance of all species varied with precipitation levels, but seeded species established best when both groups of exotic species were removed. The removal of exotic grasses resulted in an increase in exotic and native forb cover, while removal of exotic forbs led to an increase in exotic grass cover and, at least in one year, a decrease in native forb cover. In former CSS now converted to exotic annual grassland, a competitive hierarchy between exotic grasses and forbs may prevent native forbs from more fully occupying the habitat when either group of exotics is removed. This apparent competitive hierarchy may interact with yearly variation in precipitation levels to limit restoration seedings of CSS/exotic grassland communities. Therefore, management of CSS and exotic grassland in southern California and similar areas must consider control of both exotic grasses and forbs when restoration is attempted.     

The effects of phylogenetic relatedness on invasion success and impact: deconstructing Darwin's naturalisation conundrum

Darwin's naturalisation conundrum describes the paradox that the relatedness of exotic species to native residents could either promote or hinder their success through opposing mechanisms: niche pre‐adaptation or competitive interactions. Previous studies focusing on single snapshots of invasion patterns have provided support to both sides of the conundrum. Here, by examining invasion dynamics of 480 plots over 40 years, we show that exotic species more closely related to native species were more likely to enter, establish and dominate the resident communities, and that native residents more closely related to these successful exotics were more likely to go locally extinct. Therefore, non‐random displacement of natives during invasion could weaken or even reverse the negative effects of exotic–native phylogenetic distances on invasion success. The scenario that exotics more closely related to native residents are more successful, but tend to eliminate their closely related natives, may help to reconcile the 150‐year‐old conundrum.     

基于现代物种共存理论的入侵生态学概念框架

在入侵生态学60多年的发展历程中, 生态学家提出了多种多样的假说来解释生物入侵的机制。这些纷繁复杂的假说在丰富我们对生物入侵认知的同时, 也给入侵生态学概念的整合带来了困难。其中, 外来种和土著种是否存在生态学差异, 以及这种差异如何影响生物入侵, 是入侵生态学研究和争论的焦点问题。现代物种共存理论通过将外来种和土著种的生态学差异划分为生态位差异和适合度差异, 为入侵生态学概念的整合提供了新的视角。依据该理论, 外来种可以通过两种策略实现成功入侵: 一是扩大与土著种的生态位差异, 二是提高自身相较于土著种的适合度优势。因此, 外来种-土著种的生态位差异和适合度差异共同决定了入侵的成败与危害程度。通过对经典入侵假说进行梳理, 我们发现大部分假说都可以在该理论框架下进行解读, 不同假说的主要差别在于强调不同生态学过程对生态位和适合度差异的影响。同时, 这一理论框架很好地解释了为什么外来种-土著种的亲缘关系和性状差异会对生物入侵产生复杂的影响, 为达尔文归化谜团的和解以及外来种-土著种功能性状的比较研究提供了新的思路。目前, 现代物种共存理论还处于快速发展的阶段, 依旧存在很多不足, 但将其运用到生物入侵的实证研究中将是入侵生态学今后一个重要的发展方向。     

Mechanisms underlying the impacts of exotic annual grasses in a coastal California meadow

Biological invasions can impact the abundance and diversity of native species, but the specific mechanisms remain poorly discerned. In California grasslands, invasion by European annual grasses has severely reduced the quality of habitat for native forb species. To understand how introduced grasses suppress native and exotic forbs, we examined the response of a Southern California grassland community to factorial removals of live grass and the litter produced in previous seasons. To examine the role that belowground competition for water plays in mediating the impact of grasses, we crossed grass and litter removal treatments with water addition. Our results show that forbs were almost equally suppressed by both competition from live grass and direct interference by litter. Water addition did not ameliorate the effect of grass competition, suggesting that water was not the resource for which plants compete. This evidence is consistent with the susceptibility of forbs to light limitation, especially considering that litter does not consume water or nutrients. Interestingly, despite different histories of co-occurrence with annual grass dominants, native and exotic forbs were comparably suppressed by exotic grasses. Our results indicate that suppression by both live and dead stems underlie the influence of exotic grasses on forb competitors.     

Can a breakdown in competition–colonization tradeoffs help explain the success of exotic species in the California flora?

Determining combinations of functional traits that allow a species to colonize new habitats has been central in the development of invasion ecology. Species able to establish in new communities harbor abilities or traits that allow them to use resources or tolerate stress in ways that native species cannot. Tradeoffs among species functional traits along the competition–colonization (CC) continuum, where competitive ability is a decreasing function of dispersal capacity, may allow invasive species to establish themselves in new habitats. The California flora offers a well‐characterized model system to examine whether native and exotic species differ in the distribution of functional traits and to examine whether a breakdown of the CC tradeoff is present. We used a random subset of 1000 plants and examined seed traits and life form characteristics along with their seed size and adult height using the Jepson Manual of the plants of California. To test the hypothesis that active dispersal strategies aid in the success of exotic species, we classified species into four seed types according to the presence/absence of mechanisms associated with efficient dispersal. In addition, for each species we compiled data on seed size and adult plant height. We conducted all comparisons between native and exotic species within the four most speciose families to control for potential taxonomic non‐independence. Exotic species had smaller seed size but greater plant height than natives of the same families. On the other hand, exotic species also displayed significantly greater proportions of functional traits that enhanced dispersal ability. Additionally, certain sets of functional traits were significantly associated with exotic species, such as annual life histories with small seeds and high dispersal capacity. In the random subset of the California flora examined, exotics of the most speciose plant families show functional trait combinations that appear to violate the tradeoff structures observed in their California counterparts. Our results suggest that taxonomically controlled comparisons of the CC tradeoff structure between natives and exotic species may shed light of the capacity of those exotic species invasive ability to colonize new habitats.     

Properties of native plant communities do not determine exotic success during early forest succession

Considerable research has been devoted to understanding how plant invasions are influenced by properties of the native community and to the traits of exotic species that contribute to successful invasion. Studies of invasibility are common in successionally stable grasslands, but rare in recently disturbed or seral forests. We used 16 yr of species richness and abundance data from 1 m² plots in a clearcut and burned forest in the Cascade Range of western Oregon to address the following questions: 1) is invasion success correlated with properties of the native community? Are correlations stronger among pools of functionally similar taxa (i.e. exotic and native annuals)? Do these relationships change over successional time? 2) Does exotic abundance increase with removal of potentially dominant native species? 3) Do the population dynamics of exotic and native species differ, suggesting that exotics are more successful colonists? Exotics were primarily annual and biennial species. Regardless of the measure of success (richness, cover, biomass, or density) or successional stage, most correlations between exotics and natives were non‐significant. Exotic and native annuals showed positive correlations during mid‐succession, but these were attributed to shared associations with bare ground rather than to direct biotic interactions. At peak abundance, neither cover nor density of exotics differed between controls and plots from which native, mid‐successional dominants were removed. Tests comparing nine measures of population performance (representing the pace, magnitude, and duration of population growth) revealed no significant differences between native and exotic species. In this early successional system, local richness and abundance of exotics are not explained by properties of the native community, by the presence of dominant native species, or by superior colonizing ability among exotics species. Instead natives and exotics exhibit individualistic patterns of increase and decline suggesting similar sets of life‐history traits leading to similar successional roles.     

Applying competition theory to invasion: resource impacts indicate invasion mechanisms in California shrublands

Despite widespread work documenting invasion, it remains a challenge to determine invasion mechanisms and incorporate them into invasive species management. Competition theory presents a strong model for evaluating the role of resource reduction and requirements in invasion. Additionally, alternative models suggest fluctuations in resources, niche differences, or non-resource priority effects are key factors determining invasion success. We propose a comparative framework that incorporates resource impacts of native and invasive species, performance in controlled invasion trials, and long-term natural invasion patterns to elucidate relative importance of these invasion mechanisms. To demonstrate this framework, we established monocultures of two representative native and two invasive plant species in Southern California’s coastal sage scrub (CSS), measured resource impacts (i.e., R*), and conducted invasion trials to test whether resource impacts predicted invasion success. We then related experimental results to field invasion patterns. Compared to exotic herbaceous species, native shrubs were associated with greater resource depletion of key resources: light, soil water (at multiple depths), and soil inorganic nitrogen (particularly at depth). In invasion trials, natives resisted invasion by the exotics, as resource depletion measures would predict. However, these results did not follow long-term natural invasion patterns indicating that these exotic species invade areas once dominated by native shrubland. Applying our results to the invasion framework, we conclude that disturbance, or a similar mechanism causing resources to fluctuate, is needed for exotics to invade CSS habitats. This resource-based comparative analysis of invasion mechanisms can point out important processes and help suggest effective management actions.     

Responses of vegetation to a changing regime of disturbance: effects of feral pigs in a Californian coastal prairie

Disturbance can eliminate sensitive native species and facilitate invasions by exotics, but disturbance is also important in the maintenance of many native-dominated ecosystems Because of this dual role, disturbance can have complex implications for biodiversity I have investigated the effects of an introduced agent of disturbance, the feral pig Sus scrofa L, in meadows in northern California Pigs were the principal agent of soil disturbance at this site, annually overturning an average of 74% of the total surface area Grubbed areas re vegetated rapidly, but grubbing bad significant effects on the composition of the affected vegetation Species richness was reduced in grubbed plots in the first year following disturbance, but rose thereafter, often exceeding the richness of undisturbed controls Disturbance did not exclusively benefit either native or exotic species Changes in richness primarily reflected the early colonization of disturbed plots by natives, particularly annuals, although alien annual grasses also increased in disturbed sites Consequently, though non-natives did respond positively to disturbance, at least in the short-term they did not simply replace natives Pigs' effects may typify the complicated events to be expected when an ecosystem's regime of disturbance is significantly altered, either by direct human intervention or as a consequence of a biological invasion     

Darwin’s naturalization hypothesis up-close: Intermountain grassland invaders differ morphologically and phenologically from native community dominants

Darwin’s naturalization hypothesis predicts that successful invaders will tend to differ taxonomically from native species in recipient communities because less related species exhibit lower niche overlap and experience reduced biotic resistance. This hypothesis has garnered substantial support at coarse scales. However, at finer scales, the influence of traits and niche use on invasibility and invader impacts is poorly understood. Within grasslands of western Montana, USA, we compared morphological and phenological traits for five top exotic invasive forbs and five dominant native forbs using multivariate techniques to examine niche separation between exotics and natives. Exotic forbs differed from native forbs in multivariate space. Phenologically, native forbs synchronized vegetative growth with bolting and flowering early in spring. In contrast, exotics initiated vegetative growth concurrent with natives but bolted and flowered later. Morphologically, vegetative growth of exotics was three times shorter and narrower, but flowering stem growth was 35% taller and 65% wider than the natives. Collectively, these patterns suggest different strategies of resource uptake and allocation. Additionally, following wildfire, survival was four times higher for exotics compared to natives, and three times more of the surviving exotics flowered. The exotics we examined appeared to be exploiting an empty community-level niche. The resulting pattern of trait differences between exotics and natives suggests a predictable pattern of invasion and a predictable trajectory of community change. Our results illustrate how quantifying trait differences between invading exotics and natives at the within-community scale can improve understandings of community invasibility and invader impacts.     

Exotic plant invasion alters chaparral ecosystem resistance and resilience pre- and post-wildfire

Perturbations such as wildfire and exotic plant invasion have significant impacts on soils, and the extent to which invaded soils are resistant or resilient to these disturbances varies by ecosystem type. Replacement of shrublands by herbaceous exotics pre- and post-wildfire may drastically alter soil chemical and biological properties for an unknown duration. We assessed above and belowground resistance and resilience to exotic plant invasion both before and after a chaparral wildfire. We hypothesized that exotic plant species would change chemical characteristics of chaparral soils by altering litter and microbial inputs, and that controlling exotics and seeding native species would restore chemical characteristics to pre-invaded conditions. We additionally hypothesized that exotic plant species would slow succession above- and belowground, as well as recovery of post-wildfire chaparral structure and function. Plant species composition and soil nutrient pools and cycling rates were evaluated in mature and invaded chaparral pre- and post-wildfire. Exotic plant species were weeded and native species were seeded to assess impacts of exotic competition on native species recovery. Invasion did not impact all soil characteristics before fire, but increased soil C/N ratio, pH, and N cycling rates, and reduced NO₃-N availability. After fire, invasives slowed succession above- and belowground. Removal of exotics and seeding natives facilitated succession and resulted in plant composition similar to uninvaded, post-wildfire chaparral. The chaparral ecosystem was not resistant to impacts of invasion as indicated by altered soil chemistry and C and N cycling rates; however, short-term restoration led to recovery of extractable nitrogen availability indicating resilience of chaparral soils. This suggests that the permanence of exotic plant species, once established, represents a greater ecological challenge than exotic plant impacts on soils.     

Enemy release or invasional meltdown? Deer preference for exotic and native trees on Isla Victoria,Argentina

Abstract How interactions between exotic species affect invasion impact is a fundamental issue on both theoretical and applied grounds. Exotics can facilitate establishment and invasion of other exotics (invasional meltdown) or they can restrict them by re‐establishing natural population control (as predicted by the enemy‐release hypothesis). We studied forest invasion on an Argentinean island where 43 species of Pinaceae, including 60% of the world's recorded invasive Pinaceae, were introduced c. 1920 but where few species are colonizing pristine areas. In this area two species of Palearctic deer, natural enemies of most Pinaceae, were introduced 80 years ago. Expecting deer to help to control the exotics, we conducted a cafeteria experiment to assess deer preferences among the two dominant native species (a conifer, Austrocedrus chilensis, and a broadleaf, Nothofagus dombeyi) and two widely introduced exotic tree species (Pseudotsuga menziesii and Pinus ponderosa). Deer browsed much more intensively on native species than on exotic conifers, in terms of number of individuals attacked and degree of browsing. Deer preference for natives could potentially facilitate invasion by exotic pines. However, we hypothesize that the low rates of invasion currently observed can result at least partly from high densities of exotic deer, which, despite their preference for natives, can prevent establishment of both native and exotic trees. Other factors, not mutually exclusive, could produce the observed pattern. Our results underscore the difficulty of predicting how one introduced species will effect impact of another one.     

Stronger effect of gastropods than rodents on seedling establishment,irrespective of exotic or native plant species origin

Experimental evidence about how generalist consumers affect exotic plant invasions is equivocal, but most tests have been limited to few plant species, single herbivore guilds, and single locations. Using a seed‐addition experiment, we studied effects of gastropods and rodents on recruitment success of 37 exotic and 37 native plant species affiliated to three different functional groups (i.e. grasses, legumes and non‐legume herbs). We replicated our seed addition x herbivore exclusion experiment at multiple grassland sites, located within a few km of each other in two regions, coastal central California (USA) and southern Saxony–Anhalt (Germany). The two study regions differed in climate, land‐use, invasion history and species pools which allowed us to disentangle general from context‐specific effects. In both regions, herbivory by gastropods had a stronger impact on the proportion of recruited seedlings and the proportion of recruited species than rodent herbivory, but this effect was much more pronounced in California than in Germany. Especially, seedling recruitment of non‐legume herbs and legumes suffered from gastropod herbivory. Contrastingly, the effect of rodents was negative at the German sites and positive at the Californian sites, likely driven by context‐specific differences in the rodent assemblages. Across both study regions, exotics had higher seedling recruitment than natives, indicating that higher recruitment success constitutes an inherent feature of exotic species. After two years, more exotic than native species established at grassland sites in California while the opposite was true for the German grassland sites. Consistently across regions, native and exotic species did, however, not differ in their response to herbivory, suggesting that generalist consumers suppress recruitment and colonization of plant species irrespective of their origin. Our results demonstrate the importance of a multi‐species, multi‐site approach to separate general responses of exotic and native plants to generalist herbivory from local, regional or species‐specific peculiarities.     

Plant invasions differentially affected by diversity and dominant species in native‐ and exotic‐dominated grasslands

Plant invasions are an increasingly serious global concern, especially as the climate changes. Here, we explored how plant invasions differed between native‐ and novel exotic‐dominated grasslands with experimental addition of summer precipitation in Texas in 2009. Exotic species greened up earlier than natives by an average of 18 days. This was associated with a lower invasion rate early in the growing season compared to native communities. However, invasion rate did not differ significantly between native and exotic communities across all sampling times. The predictors of invasion rate differed between native and exotic communities, with invasion being negatively influenced by species richness in natives and by dominant species in exotics. Interestingly, plant invasions matched the bimodal pattern of precipitation in Temple, Texas, and did not respond to the pulse of precipitation during the summer. Our results suggest that we will need to take different approaches in understanding of invasion between native and exotic grasslands. Moreover, with anticipated increasing variability in precipitation under global climate change, plant invasions may be constrained in their response if the precipitation pulses fall outside the normal growing period of invaders.