Invader Dominance Research Articles

Suppression of reed canarygrass by assisted succession: A sixteen‐year restoration experiment

Abstract Assisted succession could enable long‐term restoration where successional trajectories stall due to competition from invasive plants. Many invasives are shade‐intolerant; therefore, interventions reducing light availability should suppress invasion and re‐establish successional processes. However, given how ubiquitous nonlinearities are in ecology, restoration success also depends on identifying critical system thresholds, for example invader abundances below which regeneration of desired species is possible. We report the successful use of assisted succession to restore a swamp forest invaded by Phalaris arundinacea (reed canarygrass; hereafter Phalaris), initiated by a high‐density planting of woody species to outcompete Phalaris by reducing light availability. We established five pre‐planting treatments in a Phalaris near‐monoculture in Wisconsin, USA: herbicide‐only, herbicide+plough, herbicide+burn, mow+herbicide and control. In 2003 we planted 22 tree and shrub species at high densities, then in 2019 we censused the site to: (1) screen for long‐term differences among treatments, (2) evaluate long‐term effects of our interventions on community composition and (3) characterize the critical thresholds that enable invader suppression and restoration success. Vegetation responses and light availability across our four pre‐planting invader removal treatments did not differ. Late fall glyphosate application suppressed Phalaris long enough that a dense canopy of native woody species could establish and eventually out‐shade it. Overstory tree and shrub densities of 0.071/m2 suppressed Phalaris to 50% cover, but, due to nonlinearities, much higher densities were needed to reduce light availability and thus Phalaris cover enough to shift the system from being invader‐dominated. Compositional similarities between juvenile woody species and the overstory suggest a long‐term restoration success. Synthesis and applications. Invasive species management and the restoration of target plant communities can be aided by assisting successional trajectories that have stalled. We document a restoration strategy for forests invaded by shade‐intolerant invaders that is both effective and economical, as only a simple site preparation and single planting effort is required. Establishing a dense canopy of woody species in this way can break the feedbacks maintaining invader dominance and re‐introduce feedbacks enabling long‐term ecosystem recovery. We also illustrate the value of identifying critical thresholds influencing the abundance and impact of key invasive species.

Self‐help or self‐sabotage? A common invader's soil legacy does not impede, and may facilitate, potential competitors

AbstractIntroduced plants create lasting abiotic and biotic changes to soil that are increasingly implicated as major drivers of invasion success, yet models for investigating patterns in the impacts of these soil legacies are lacking. Here, we used Bohemian knotweed (Reynoutria × bohemica) to present a novel approach for testing whether accounting for diverse evolutionary drivers can clarify how an invader's soil legacy mediates its success. For most tested species, R. × bohemica's soil legacy did not influence seedling performance irrespective of its evolutionary history or the evolutionary novelty of species that share its soil. However, we found the relative importance of legacy effects versus other mechanisms of invader dominance depends on the life stage of the plants growing in soil conditioned by the invader. While the effects of R. × bohemica's soil legacy on seedling performance were context dependent, emergence was, on average, higher in R. × bohemica‐conditioned soils than in unconditioned soils. Thus, contrary to theory, R. × bohemica's soil legacy not only fails to impede potential competitors but may also facilitate them. Our results are particularly powerful: we detected novel insights in the face of biological complexity, even for R. × bohemica, which likely does not depend on heterospecific plant–soil feedbacks for its competitive dominance in its introduced range. Designing rigorous investigations that explore how plant species' evolutionary histories mediate an invader's soil legacy across diverse abiotic environments, as shown here, is key for producing insights in other systems.

Scale-dependent changes in species richness caused by invader competition

Ecologists have produced substantial literature about the impact of biological invasions on species diversity. However, due to scale-dependent factors, no consistent conclusion has been reached regarding whether biological invasion leads to a loss of species diversity. In this project, we incorporate an invasion scenario into community assembly theories through spatially explicit individual-based models. We explore how invader dominance shifts community richness at local scales and how local-scale extinctions translate to broader scales. To do so, native community dynamics are simulated according to the niche-neutral continuum theory and constructed on a heterogeneous landscape with a limited environmental gradient. We synthetically assume that the establishment of invaders is wholly affected by their niche or fitness difference from native populations. By analyzing richness changes across all sample sizes, we obtained the following results: (1) for randomly distributed habitat resources, the magnitude of richness loss linearly decreases with sample size when strong niche overlap occurs, but the magnitude is more nonlinearly shaped in communities assembled by niche departures; (2) spatially structured native populations could have greater resistance to invasion competition, and if on clustered habitats and the species suffering more niche overlap, then larger extinction events would occur at broader scales; and (3) these invasion modification trends could be linked with native population spatial structures and abundance evenness. The above findings are solely based on simulation experiments and implemented under several synthetic assumptions. This suggests theoretical linkages between invasion scale-dependent influences and mechanistic issues, such as the properties of the invader and invaded communities, which can inform invasive species management and restoration.

Invasive grass fuel loads suppress native species by increasing fire intensity and soil heating

Abstract Non‐native invasive grasses are driving intense fires across the globe but the impacts of native versus invader‐fuelled fires on community assemblages are poorly understood. By increasing fire intensity, grass invasions might increase below‐ground mortality of heat‐sensitive seeds and buds, thereby shifting community composition. We compared fuel loads in native and non‐native invasive (cogongrass, Imperata cylindrica) plant‐dominated areas of pine savannas in Florida. Then, we conducted a field experiment to examine how fuel loads and native and invasive fuel types affected soil heating and seedling emergence or resprouting of native and invasive plant species. Average fuel loads in invaded communities were 152% greater than that in native communities. Soil heating, including heating duration >60°C, maximum temperature and heat flux >60°C, increased, and seedling emergence and resprouting decreased with greater fuel loads; these relationships were similar across the overlapping range of native and invasive fuel loads. However, longer durations of soil heating at the higher average fuel loads of invaded communities resulted in 23% lower predicted probability of seedling emergence compared to average fuel loads of native communities. Invasive cogongrass resprouting was not affected by fuel loads, indicating that cogongrass tolerates the intense fires it generates. In contrast, seedling emergence and resprouting of most other species was reduced by greater fuel loads. Synthesis and applications. By increasing fuel loading and soil heating, grass invasions may alter post‐fire community assemblages and facilitate invasive grass dominance at the expense of native species via an invasion‐fire cycle. Fuel loads can be used to predict soil heating duration and depth, and these data, combined with information on species tolerances to heating, can be used to forecast the impacts of invasions on post‐fire community composition. To maintain fire regimes that promote native communities and resist invader dominance, it is critical to manage invasive species that increase fuel loads.

Plant and microbial impacts of an invasive species vary across an environmental gradient

Abstract Invasive plants often successfully occupy large areas encompassing broad environmental gradients in their invaded range, yet how invader dominance and effects on ecological communities vary across the landscape has rarely been explored. Furthermore, while the impacts of invasion on plant communities are well studied, it is not well understood whether responses of above‐ground (plant) and below‐ground (microbial) communities are coupled. Here we test patterns in Phragmites australis (common reed) invasion in a field survey of eight sites situated across a salinity gradient, ranging from freshwater to saline marsh, in Southeast Louisiana. At each site, we surveyed plant composition and used metabarcoding methods to assess soil fungal and bacterial composition in plots within the dense Phragmites stand, in a transition zone of ~50:50 Phragmites:native plants, and in native‐only areas. We hypothesized that Phragmites' abundance and impact on above‐ and below‐ground communities would vary across the salinity gradient and that the responses of above‐ and below‐ground communities to invasion would be coupled. We found weak evidence that invasion varied across the gradient: Phragmites stem densities increased slightly with salinity, and Phragmites increased above‐ground litter accumulation more in fresh and saline areas compared to brackish. We found stronger evidence that plant and microbial responses to invasion varied with salinity. Phragmites strongly reduced native plant density across the gradient, with slightly greater reductions in fresh and saline areas. Plant species richness displayed consistent decreases with invasion across the salinity gradient; however, fungal and bacterial richness increased sharply with invasion only in brackish sites. Furthermore, the effect of Phragmites on plant and microbial community composition became stronger as salinity increased. Plants and microbes exhibited coupled responses to invasion in the magnitude of compositional shifts brought on by Phragmites, but Phragmites' effects on richness were not coupled. Synthesis. Overall, the variability in Phragmites impacts across the gradient, particularly soil microbial impacts, suggests that it may be difficult to generalize invader effects from single‐site or single‐ecosystem studies. However, above‐ and below‐ground communities showed some coupled responses to Phragmites; thus understanding plant community responses to invasion gives insight into impacts occurring below‐ground.

Constraining invader dominance: Effects of repeated herbicidal management and environmental factors on curlyleaf pondweed dynamics in 50 Minnesota lakes

Abstract Curlyleaf pondweed (Potamogeton crispus) is one of the most widespread and widely managed aquatic invasive plants in North America. Despite decades of management, the efficacy of long‐term management strategies and the effects of environmental drivers on curlyleaf pondweed populations remain uncertain. To evaluate the effects of management and environmental factors on within‐lake distribution and local density of curlyleaf pondweed, we collated monitoring data from point–intercept surveys collected by a variety of lake managers across Minnesota, U.S.A. Using this dataset, comprising 177 lake‐years of plant data, we examined the influence of herbicide treatment, water clarity, snow depth, and ice cover duration on curlyleaf pondweed distribution and density between 2006 and 2015. We evaluated the effects of herbicides on curlyleaf pondweed at three time points relative to treatment: within year, carryover effects the following year, and cumulative effects over multiple years of treatment. All three temporal measures were associated with significant reductions of curlyleaf pondweed. Additionally, herbicidal management reduced both the density and distribution of curlyleaf pondweed. Given that herbicide management led to reductions that carried over into future years, managers may be able to design multi‐year treatments to reduce total management effort over time. We also found strong effects of environmental conditions on curlyleaf pondweed. Elevated lake productivity and decreased winter snow cover were associated with increased springtime distributions of curlyleaf pondweed, whereas duration of winter ice cover had no influence. The influence of productivity suggests that reductions of this invasive species may be an ancillary benefit of water‐quality improvements that lower lakes’ trophic status. Our results also show that decreased winter snow cover, as predicted under climate change, could exacerbate problematic growth of curlyleaf pondweed. Harnessing monitoring data from multiple projects, as this study does, allows for robust inference about environmental and management constraints on macrophytes. Because of environmental and management variability, we suggest that treatment regimens follow an adaptive management cycle, with outcomes of management monitored and evaluated, and strategies updated accordingly. It is also vital to continue monitoring both managed and unmanaged lakes to enable stronger inferences about treatment effectiveness.

Populations of a widespread invader and co-occurring native species vary in phenotypic plasticity.

Phenotypic plasticity can promote plant invasions and enhance impacts on native species, but little is known about variation in plasticity among invader populations compared with native species. Variation in plasticity among invader populations could inform more precise predictions of invader spread and impacts across heterogeneous resource environments. We used a common garden experiment with sun and shade treatments to test for variation in plasticity among 12 populations of an invasive grass (Imperata cylindrica), and to determine whether the invader exhibited greater plasticity than six native species that co-occur in the Southeast USA. Principal component analysis revealed that invader populations from different native ranges consistently varied from each other and native species in traits linked to more favorable phenotypes under resource limitation. Overall, the invader exhibited greater plasticity than native species did, as demonstrated by higher plasticity index values for traits such as plant height, leaf mass ratio, and root:shoot ratio. Variation in phenotypic plasticity among invader populations suggests the potential for evolution of plasticity, and greater plasticity of invader populations than native species may underlie invader dominance. Differences in plasticity among populations appears to play an important role in predictions of the spread and potentially the impacts of invasive species.

Facilitating natural succession in a heavily invaded ecosystem

Invasive plants have major impacts on forest ecosystems. Where invasive species alter successional pathways, a site may reach an alternate stable state in which compositional change is unlikely without management intervention. We conducted a restoration experiment designed to alter the successional factors of site availability, species availability, and species performance to disrupt the stable state of invader dominance. In a field dominated by invasive reed canarygrass (Phalaris arundinacea L.), we compared two plot preparation methods (late-season site mulching plus Oust® application, vs. late-season Rodeo® application) to increase site availability and reduce Phalaris performance, and planted native trees and shrubs as seeds and as bare root seedlings at three densities to improve native species availability. We monitored Phalaris growth, other herbaceous species establishment, tree survival, and deer browsing (another influence on species performance) for two growing seasons. Although the mulching plus Oust treatment was initially more successful in terms of its ability to increase site availability (as measured by litter percent cover and depth), this pattern reversed in the second year of the experiment. Throughout the experiment, the Rodeo treatment was superior in terms of its ability to decrease Phalaris performance (as measured by cover and height), and in the second year it also improved the performance of herbaceous plants naturally establishing in the plots (as measured by cover). Volunteer herbaceous plant cover averaged 60% in Rodeo-treated plots by the end of the second growing season. The performance of planted, seeded, and naturally-occurring woody plants was similar across the two herbicide treatments. Overall survival of bare root seedlings was 41%; survival was greater than 70% for both swamp white oak and Dutch elm disease-resistant American elm. Deer preferentially browsed on silver maple seedlings, while demonstrating avoidance of American elm, cottonwood, sandbar willow and swamp white oak. Our results suggest that to promote succession in Phalaris-dominated sites, decreasing Phalaris performance is more important than immediately removing thatch. In terms of species availability, we conclude that propagule limitation was occurring for woody plants but not for herbaceous species; natural tree recruitment was very low, so our alteration of tree species availability was critical in terms of promoting succession to floodplain forest. The restoration approach of stimulating natural succession can be applied to other ecosystems, providing a link between successional theory and management practice.

Effects of a non‐native grass invasion decline over time

Summary Most research on dynamics and impacts of plant invasions has evaluated patterns and effects over brief time periods (i.e. <4 years). As such, little is known about the persistence of invasions and their long‐term impacts on native species. To experimentally evaluate longer‐term effects of invasions, we established field plots with native tree and herbaceous species and then invaded half of the plots with the most widespread invasive grass in the eastern United States (Microstegium vimineum). Over 8 years, we quantified invader and native plant biomass, native plant diversity, and tree density and size. Microstegium was dominant during the first 4 years of the experiment, constituting more than 60% of herbaceous plant biomass in the plots, and native herbaceous biomass was 57% lower and diversity was 44% lower in invaded vs. control plots. However, both Microstegium and herbaceous native species declined in later years. By the end of the experiment, Microstegium was only 2% of community biomass, and there was no difference in native herbaceous biomass in invaded and control plots. We applied a fire treatment in years 6 and 7 to test if repeated disturbance is required to maintain invader dominance and to evaluate how this common management practice in eastern US forests affected invasive and native plants. Tree density was 65% lower and tree diameters were 28–51% smaller on average in fire‐treated compared with control quadrants, resulting in significantly greater light availability in fire‐treated areas. Consequently, Microstegium and native herbaceous species biomass increased significantly where fire was applied. However, only native species were persistent, and after 8 years, Microstegium was nearly absent, regardless of the fire treatment. Synthesis. The invasive grass was initially abundant and suppressed native species, but invader decline over time corresponded with succession to native herbaceous species dominance when fire was applied, and to native tree dominance without fire such that after 8 years, the initial effects of invasion were no longer apparent. Thus, our data provide some of the first experimental evidence that while the initial effects of plant invasions can be dramatic, invaders and their impacts may decline over time.

Acclimation by invasive mussels: spatiotemporal variation in phenotypic response to turbidity

Increasing rates of invasion have led to shifts in dominance, and even replacement, of nonindigenous species. Shifts in invader dominance from the Zebra Mussel (Dreissena polymorpha) to a functionally similar and closely related species, the Quagga Mussel (Dreissena rostriformis bugensis), have occurred in several European and North American water bodies. We tested whether patterns of local dominance of these mussels in the St Lawrence River, a large heterogeneous system, could be explained by interspecific differences in morphological plasticity. We conducted field experiments across sites of contrasting turbidity (an important stressor for sessile filter feeders) to assess whether Zebra and Quagga Mussels grow at different rates, and whether such differences are related to variation in the morphology of their feeding organs (the palp-to-gill area ratio [PGR]) under different turbidity conditions. Turbidity had a small negative influence on the shell growth of both species, but Zebra Mussels were affected more severely. Across all sites, Zebra Mussels sustained reduced growth and greater mortality than Quagga Mussels. Spatial and temporal variation in PGR was observed for both species, but was more pronounced for Zebra Mussels. Reciprocal transplants suggested that these patterns reflect local acclimation rather than selection or genetic differentiation among local populations. Zebra Mussels exhibited significant shifts in PGR both across the growing season and after being transplanted from low- to high-turbidity conditions, whereas Quagga Mussels altered their PGR only while residing at, or after being transplanted to, a low-turbidity site. We propose that interspecific differences in morphological acclimation to low food conditions result in a competitive advantage for Quagga Mussels.

Invasive plants accelerate nitrogen cycling: evidence from experimental woody monocultures

Summary Although it is widely believed that non‐native invasive species threaten the functional integrity of forest ecosystems, their impact on important ecosystem processes such as nitrogen (N) cycling is not well understood. To examine how invasive species alter ecosystem N dynamics, we established monocultures of five phylogenetic pairs of native and non‐native invasive understory woody species common to Eastern U.S. forests. After 3 years, we found invaders increased N cycling by enhancing the flow of N to the soil through greater litter N production and litter N content, and increased the uptake of available soil N, via greater fine root production and specific root length. Synthesis. Our results highlight the importance of linking above‐ and below‐ground processes to better understand invader impacts on ecosystem nutrient processes. The rapid shifts in soil N processes as a result of invader dominance observed in our study suggest that invaders may be an important driver of forest ecosystem functioning.

Landscape‐scale GPP and carbon density inform patterns and impacts of an invasive tree across wet forests of Hawaii

Plant invasion typically occurs within a landscape-scale framework of abiotic and biotic conditions, often resulting in emergent feedbacks among environment, ecosystem functions, and the dominance of invasive species. Understanding the mechanisms underlying successful invasions is an important component of conservation and management efforts, but this has been poorly investigated in a spatially explicit manner. Knowing where and why invasion patterns change throughout the landscape enables managers to use context-specific controls on the spread of invasive species. Using high-resolution airborne imaging spectroscopy, we studied plant performance in growth within and across landscapes to examine the dominance and spatial distribution of an invasive tree, Psidium cattleianum (strawberry guava), in heterogeneous environmental conditions of a submontane Hawaiian tropical forest. We assessed invader performance using the GPP ratio index, which is the relative difference in remotely sensed estimates of gross primary productivity between canopies of guava and canopies of the invaded plant community. In addition, we used airborne LiDAR data to evaluate the impacts of guava invasion on the forest aboveground carbon density in different environments. Structural equation modeling revealed that substrate type and elevation above sea level interact and amplify landscape-scale differences in productivity between the invasive species and the host plant community (GPP ratio); differences that ultimately control levels of dominance of guava. We found shifts in patterns of forest carbon storage based on both gradual increase of invader dominance and changes in environmental conditions. Overall, our results demonstrate that the remotely sensed index defined as the GPP ratio provided an innovative spatially explicit approach to track and predict the success of invasive plants based in their canopy productivity, particularly within a landscape-scale framework of varying environmental factors such as soils and elevation. This approach may help managers accurately predict where invaders of forests, scrublands, or grasslands are likely to exhibit high levels of dominance before the environment is fully invaded.

Interactions Among Invasive Plants: Lessons from Hawai‘i

Most ecosystems have multiple-plant invaders rather than single-plant invaders, yet ecological studies and management actions focus largely on single invader species. There is a need for general principles regarding invader interactions across varying environmental conditions, so that secondary invasions can be anticipated and managers can allocate resources toward pretreatment or postremoval actions. By reviewing removal experiments conducted in three Hawaiian ecosystems (a dry tropical forest, a seasonally dry mesic forest, and a lowland wet forest), we evaluate the roles environmental harshness, priority effects, productivity potential, and species interactions have in influencing secondary invasions, defined here as invasions that are influenced either positively (facilitation) or negatively (inhibition/priority effects) by existing invaders. We generate a conceptual model with a surprise index to describe whether long-term plant invader composition and dominance is predictable or stochastic after a system perturbation such as a removal experiment. Under extremely low resource availability, the surprise index is low, whereas under intermediate-level resource environments, invader dominance is more stochastic and the surprise index is high. At high resource levels, the surprise index is intermediate: Invaders are likely abundant in the environment but their response to a perturbation is more predictable than at intermediate resource levels. We suggest further testing across environmental gradients to determine key variables that dictate the predictability of postremoval invader composition.

Extended leaf phenology may drive plant invasion through direct and apparent competition

Invasive plants can inflict great harm, yet drivers of successful invasion remain unclear. Many invaders of North American deciduous forests exhibit extended leaf phenology (ELP), or longer growing season relative to natives. ELP may grant invaders competitive advantages, but we argue that ELP more potently drives invasion in the presence of herbivores. ELP invaders can support herbivores by lessening starvation during winter; consequently, native plants may suffer when attacked later through apparent competition. As modeled here, even short ELP can promote competitive success of invaders, and apparent competition sharply enhances ELP invader dominance. In ‘partial enemy escape’ scenarios, a less palatable ELP invader nearly excludes a preferred native where an invader without ELP could not. Together, ELP and apparent competition enhance invasion even when biotic resistance should suppress it, i.e. when the invader competes weakly or provides preferred forage. Thus, ELP‐apparent competition interactions grant invaders considerable success while challenging core tenets of invasion ecology.

Euonymus fortunei dominance over native species may be facilitated by plant–soil feedback

Positive plant–soil feedback (PSF) may be a mechanism of invader dominance, whereas PSF is often negative for native species. Previous work in Eastern deciduous forests of North America has shown that the invasive liana Euonymus fortunei participates in a net positive PSF with native groundcover Asarum canadense, indicating that PSF may contribute to invader dominance. However, to identify PSF as a general invasion driver for Euonymus, we must consider the average net pairwise feedback for multiple native–invasive species pairs, and compare this to the average net pairwise feedback amongst native–native pairs. Here, we test E. fortunei in net pairwise feedback against five native species, comparing native–invader feedback to feedback amongst natives over a gradient of light availability. PSF was on average neutral for invader–native pairs and on average negative for native–native pairs, indicating that Euonymus does not face the same constraints that limit the growth of native species. Because even neutral feedback can facilitate invasion, results indicate that PSF may facilitate invader dominance over a broad range of native functional groups and light conditions in Eastern deciduous forest.

Climate warming alters competition between two highly invasive alien plant species and dominant native competitors

SummaryBiological invasions and climate warming are two major threats to the world's biodiversity. To date, their impacts have largely been considered independently, despite indications that climate warming may increase the success of many invasive alien species. We therefore set up an experiment to study competition between native and invasive alien plant species and the effects of climate warming thereon. Two highly invasive alien plant species in Belgium, each with a native competitor that dominated in invaded sites, were grown either together (1:1 mixtures) or in isolation (monocultures) at ambient and elevated temperature (+3°C) in climate‐controlled chambers. To detect possible mechanisms underlying the effects of competition and elevated temperature, we conducted a combined growth and nitrogen uptake analysis. In one pair, Senecio inaequidens – Plantago lanceolata (alien invasive – native), the alien species dominated in mixture, while in the other pair, Solidago gigantea – Epilobium hirsutum, the native species dominated. Climate warming reduced the invader dominance in the first species pair, but stimulated the suppressed invader in the latter. Many of the biomass responses for these two species pairs could be traced to the root‐specific nitrogen uptake capacity. The responses of the native‐invasive interaction to warming could not always be extrapolated from the monoculture responses, pointing to the necessity for experiments that take into account species interactions.

Eco-evolutionary litter feedback as a driver of exotic plant invasion

Many studies have examined positive feedbacks between invasive plant traits and nutrient cycling, but few have investigated whether feedbacks arise from introduction of pre-adapted species or from eco-evolutionary feedback that develops after introduction. Eco-evolutionary feedback could occur between an invader's leaf tissue C:N ratio and its response to litter accumulation. Previous modeling predicts that occurrence of this feedback would be reflected by: (1) field data showing higher litter:biomass ratios in the invasive range; (2) high C:N genotypes benefiting more from experimental litter additions than low C:N genotypes; (3) this beneficial effect on high C:N genotypes inducing a critical transition toward invader dominance when a critical amount of litter is added to a native species-dominated community experiencing low nutrient conditions. Here, we empirically tested these predictions for the invasive grass Phalaris arundinacea, which has undergone post-introduction evolutionary change toward attaining higher C:N ratios under high nutrient conditions. We performed a biogeographical comparison of litter:biomass ratios in the native (Europe) and invasive (USA) range, and an experiment with mesocosms from the invasive range under low nutrient conditions. Low and high C:N Phalaris genotypes were introduced into native-dominated and bare mesocosms, to which varying litter amounts were added. The biogeographical comparison revealed that litter:biomass ratios were higher in the invasive range. The mesocosm experiment showed that when grown in isolation, only high C:N genotypes responded positively to litter. This effect, however, was not strong enough to stimulate Phalaris when exposed to competition with native species. Our results suggest that eco-evolutionary feedback between Phalaris’ C:N ratio and litter accumulation could occur, but only under high nutrient conditions. Our experiments suggest that eco-evolutionary feedback may select for specialist rather than superior genotypes. Hence, genotypic variation induced by post-introduction admixture may be subject to context-dependent selection due to eco-evolutionary feedback, increasing trait variation within invasive populations.

Are competitive effects of native species on an invader mediated by water availability?

AbstractQuestionClimate change processes could influence the dynamics of biotic interactions such as plant competition, especially in response to disturbance phenomena such as invasional processes. Are competitive effects of native species on an invader mediated by water availability?LocationGlasshouse facility, New South Wales, Australia.MethodsWe constructed competitive hierarchies for a representative suite of species from coastal dune communities that have been invaded by the Asteraceae shrub, bitou (Chrysanthemoides monilifera subsp. rotundata). We used a comparative phytometer approach, where the invader species was grown with or without a suite of native species in glasshouse trials. This was used to construct competition hierarchies under two water stress conditions: non‐droughted and droughted. The treatments were designed to simulate current and potential future water availability respectively.ResultsWe found that the invader experienced fewer competitive effects from some native species under water stress, particularly with regard to below‐ground biomass effects. Native species were often poor competitors with the invader, despite their adaptation to periodic water stress in native coastal environments. Of the native species with significant competitive effects on the invader, functionally similar shrub species were the most effective competitors, as expressed in below‐ground biomass. The relative position of species in the hierarchy was consistent across water treatments based on below‐ground bitou biomass, but was contingent on water treatment when based on above‐ground bitou biomass.ConclusionsThe competitive effects of native species on an invader are affected by water stress. While the direction of response to water stress is species‐specific, many species have small competitive effects on the invader under droughted conditions. This could allow an increase in invader dominance with climate change.

Invasion by mobile aquatic consumers enhances secondary production and increases top-down control of lower trophic levels

Increased biological diversity due to invasion by non-indigenous species (NIS) is a global phenomenon with potential effects on trophic interactions and ecosystem processes in the invaded habitat. We assessed the effects of resource availability and invasion of three non-indigenous invertebrate grazers (two crustaceans and a snail) on secondary production, relative dominance of NIS grazers and resource depletion in experimental freshwater mesocosms. The relative dominance of NIS grazers increased with increasing initial resource availability, although the effect was largest for one of the three species. The effect was due to the fact that all the included non-indigenous grazers were able to expand their populations quickly in response to resource addition. For the most dominating species, the increased grazer diversity due to invasion in turn resulted in higher production of grazer biomass and a more efficient depletion of the periphyton resource. The effect was largest at high initial resource availability, where NIS dominance was most pronounced. Our results show that an invasion-induced increase in species diversity can increase resource depletion and consequently production, but that the effect depends on identity of the introduced species. The results also suggest that properties of the recipient system, such as resource availability, can modulate ecosystem effects of NIS by affecting invader success and dominance.

Disturbance governs dominance of an invasive forb in a temporary wetland

Dominance of invasive species is often assumed to be due to a superior ability to acquire resources. However, dominance in plant communities can arise through multiple interacting mechanisms, including disturbance. Inter-specific competition can be strongly affected by abiotic conditions, which can determine the outcome of competitive interactions. We evaluated competition and disturbance as mechanisms governing dominance of Phyla canescens (hereafter lippia), an invasive perennial forb from South America, in Paspalum distichum (perennial grass, hereafter water couch) meadows in floodplain wetlands of eastern Australia. Water couch meadows (in the study area) are listed under the Ramsar Convention due to their significance as habitat for migratory waterbirds. In the field, we monitored patterns of vegetation boundaries between the two species in response to flooding. Under controlled glasshouse conditions, we explored competitive interactions between the native water couch and lippia subject to different soil moisture/inundation regimes. We did this using a pairwise factorial glasshouse experiment that manipulated neighbor density (9 treatments) and soil moisture/inundation (4 treatments). In the field trial, inundation increased the cover of water couch. Under more controlled conditions, the invader had a competitive effect on the native species only under dry soil conditions, and was strongly inhibited by inundation. This suggests that dry conditions favor the growth of the invader and wetter (more historical) conditions favor the native grass. In this system, invader dominance is governed by altered disturbance regimes which give the invader a competitive advantage over the native species.