Invader Removal Research Articles

Not All Roads Lead to Rome: A Meta-analysis of Invasive Plant Impact Methodology

The negative effects of invasive plant species on native ecosystems, which can be large and long-lasting, are the primary justifications for their research and management. Tremendous effort is focused on quantifying the ecological impacts of invasive plants, though two different methods are primarily used: observational (compare invaded and uninvaded) and removals (compare invaded and invader removal). However, it is unknown whether these methods, which differ in their assumptions and execution, yield similar results, which could affect our ability to draw broad conclusions within and across studies. Therefore, we performed a meta-analysis on 174 studies that described 547 impacts of 72 invasive plants to test the effect of study method, invader cover, and removal period on the direction and magnitude of impact. Overall, by only considering impact magnitude and not direction, both observational and removal methods yielded similar results—invasive plants are changing most aspects of the ecosystem—and the variation among species and study systems was dramatically reduced compared with traditional, directionally focused studies. This is contrary to a similar analysis that did not control for directionality of impacts, which found overall differences in impact depended on methodology. However, even when the effects of study ecosystem, invader life-form, and impact type were accounted for, significant differences occurred between removal and observational studies. Particularly vulnerable systems appear to be those that would be more greatly disturbed by the removal of the target species, such as tree species or invasive plants in riparian areas. Additionally, impact magnitude increased with invader cover and removal time. We confirm that invasive plants impact the systems they invade in a nonuniform manner; however, we suggest some study conditions may be more sensitive to study methodology. Careful consideration should be given as to which methodology is used in the context of the study system.

Mycorrhizae, invasions, and the temporal dynamics of mutualism disruption

Summary An increasingly recognized impact of plant invaders is the disruption of positive interactions between native plants and their belowground mutualistic mycorrhizal fungi. We reviewed 112 studies from 61 publications that report invader impacts on mycorrhizal fungi. We describe emerging patterns on the frequency of negative, neutral and positive invader effects on mycorrhizal fungal abundance, richness and community composition, and we evaluate how these outcomes vary with invasion age. We also describe the evidence for three mechanisms of disruption: (1) plant‐plant competition that alters host quantity or quality (2) changes in soil properties such as nutrient availability and (3) allelopathy. Invaders can disrupt native mutualisms if they are non‐mycorrhizal, associate with a different type of mycorrhiza, or associate with different taxa of the same type of mycorrhiza as the native plants. Invaders that enrich soil nutrients can cause declines in mycorrhizal abundance, shifts in fungal community composition, and cause native plants to reduce their dependence on mycorrhizas. Invaders that produce allelopathic compounds cause mycorrhizal abundance to decrease and alter community composition because of variation in the sensitivity of different fungi to toxins. While the evidence for disruption of the mycorrhizal mutualism with invasion is strong, temporal patterns have not yet emerged from the literature. Invasion age was not clearly associated with extent of disruption, and the timing of recovery following invader removal was highly variable. Differences in the biology of native and invasive plants, their interactions with mycorrhizal fungi and the surrounding landscape contributed to this variation. Synthesis. Our systematic review suggests that invaders frequently decrease mycorrhizal abundance and alter fungal community composition while only occasionally affecting fungal species richness. The development of invader impacts, as well as the restoration of native communities after invader removal, are influenced by a suite of plant, fungal and environmental traits that change over time. However, few studies have examined the temporal dynamics of mycorrhizal disruption, and results are variable. Future research should focus on the temporal scale of mutualism disruption while considering plant phylogenetics and demography, fungal functional traits such as spore longevity and dispersal, and soil chemistry.

Invasive herb Impatiens glandulifera has minimal impact on multiple components of temperate forest ecosystem function

Forests understories in Europe are known to generally resist invasion, though some alien plants do invade woodland communities. Here we focused on the impact of the widespread invasive annual Impatiens glandulifera, common along watercourses, but recently spreading in forests up to timberline. We investigated its impact on plant–soil feedback and ecosystem functioning. We recorded >40 variables focusing on: soil characteristics, including micro- and macro-nutrients; characteristics of litter layer and enzyme activity in litter; and richness and species composition of the forest understory. Three treatments were followed for 3 years: plots invaded by I. glandulifera; adjacent invader removal plots within the invaded area; and spatially separated uninvaded plots outside the invaded area. The effect of year-to-year variation was generally greater than that of the treatments, especially in soil and litter characteristics. Copper and boron were higher in invaded than invader removal and uninvaded plots, though in quantities that are unlikely to harm other plants. We found no effect of I. glandulifera on litter characteristics or enzyme activity. Despite almost 80% cover of I. glandulifera, we did not detect any difference in species richness and total vegetation cover between invaded and uninvaded plots. The floristic composition differed among the uninvaded, invader removal and invaded plots across 3 years. Our results indicate that the effect of I. glandulifera on the forest community studied was minor, and largely resulted from its increased shading to other plant species. In conclusion, we show how misleading the evaluation of impacts can be if based on a single season.

Invasion Shadows: The Accumulation and Loss of Ecological Impacts from an Invasive Plant

Ecological impacts from invasive plants that have been identified include reductions in biodiversity, changes in resource cycling, and disruptions of ecosystem function. To mitigate these negative ecological impacts, managers work to remove invasive plants. However, removal does not necessarily immediately lead to a return to the uninvaded ecological state. Similarly, the accumulation rate of ecological impacts following invader establishment is almost entirely unknown for most species, hindering identification of optimal management times. The accumulation and loss (so-called legacy effects) of impacts following invader establishment and removal represent an “invasion shadow.” To begin to understand invasion shadows, we measured the changes in biotic and abiotic ecological impacts during establishment and following removal of the forest understory invader Japanese stiltgrass. We found that when the abiotic metrics were considered, seeded areas became more functionally similar to the invaded landscape and removed areas became more similar to the uninvaded landscape. However, while the plant community did not change in a 3-yr period during a new invasion, following invader removal, it became less similar to both the invaded and uninvaded landscape altogether, suggesting legacies. Surprisingly, all changes occurred almost immediately and persisted following invader establishment and removal. Our results show, at least in a 3-yr period, that ecosystems can respond to changes in invader abundance, and in some cases simply removing the invader could result in long-term changes to the resident plant community.

Invasive N-fixer Impacts on Litter Decomposition Driven by Changes to Soil Properties Not Litter Quality

Invasive nitrogen (N)-fixing plants often fundamentally change key ecosystem functions, particularly N-cycling. However, the consequences of this for litter decomposition, and the mechanisms that underpin ecosystem responses, remain poorly understood. Moreover, few studies have determined how nutrient pools and fluxes shift as invader density increases and whether these effects persist following invader removal, despite the importance of this for understanding the timing and magnitude of invader impacts in ecosystems. We tested how the decomposition rates of four co-occurring grass species were influenced by changes in the density of the globally invasive N-fixing shrub Cytisus scoparius L. (Scotch broom) and whether these effects persisted following invader removal. We used a series of laboratory decomposition assays to disentangle the roles of changes in both litter quality and soil properties associated with increases in broom density. Broom invasion created a soil environment, such as higher rates of net N-mineralisation, which retarded litter decomposition. Litter C/N ratios of co-occurring species decreased as broom density increased, yet this had no effect on decomposition rates. Most relationships between broom density and impacts were nonlinear; this could explain some of the reported variation in invasive species impacts across previous studies that do not account for invader density. Ecosystem properties only partially recovered following invader removal, as broom left a legacy of increased N-availability in both soils and litter. Our findings suggest that invasive N-fixer impacts on soil properties, such as N-availability, were more important than changes in litter quality in altering decomposition rates of co-occurring species.

Invasive Impatiens parviflora has negative impact on native vegetation in oak-hornbeam forests

Impatiens parviflora (Balsaminaceae) is one of the most widespread invasive plant species in Central Europe. Nevertheless, both mechanisms and consequences of its invasion are still poorly understood. In this study we attempt to understand the impact of this species on native vegetation.The impact of I. parviflora on native vegetation was studied using removal experiment on permanent plots in oak-hornbeam forests in central Bohemia, Czech Republic. Nine pairs of plots were established in invaded vegetation, one plot in each pair stayed invaded and the other served as removal plot and all I. parviflora individuals were repeatedly removed from the plot. Species composition in 4 subsequent years was recorded in the plots. Species response to I. parviflora removal was correlated to plant traits to reveal trait characteristics of species suppressed by the invasion.Significant increase in both numbers and cover of native species was observed in removal plots in comparison with invaded plots during the experiment, with the greatest change in first two years after invader removal. Species composition also significantly differed between invaded and removal plots. Species with high affinity to removal plots, i.e. species that are most restricted by I. parviflora invasion, were mostly species with small releasing height and early start of flowering.Our results indicate that I. parviflora has negative impact on native vegetation but that the vegetation can recover within few years after the invader removal. Due to its extensive distribution and high turnover, removal of the species from larger plots is, however, not realistic in practice.

System‐level changes following invasion caused by disruption of functional relationships among plant and soil properties

The ecological impacts of invasive plants have served to justify the cost of their management, which is estimated to exceed $1 billion annually in the US alone. However, our understanding of the ecological impacts of most invasive plants is extremely limited, and when known, interpretation is confounded with varied measurements and methods. While this can provide important information about specific components of ecosystem function, it limits our understanding of the broader scope of impacts any one species may have. Using Japanese stiltgrass (Microstegium vimineum) as a study system, our objectives were to (1) survey a broad suite of 29 important ecological impact metrics (EIMs), (2) identify invader cover‐EIM relationships, and (3) test if the comparative reference (uninvaded or invader removal) influences interpretation. Japanese stiltgrass had the strongest effect on the plant community, followed by soil properties, soil nutrients, and other abiotic/biotic factors. Many EIM values differed among reference types, and plant community EIMs were reduced with increasing Japanese stiltgrass abundance. For example, plant biodiversity was lower in the invasion when compared to both removed and uninvaded sites; however, soil organic matter was higher only in the uninvaded site when compared to the invasion. The integrative ecosystem metric E(c) also showed that the system overall was impacted by the Japanese stiltgrass invasion, and this varied among sites. Interestingly, relationships among EIMs were also changed by the presence of Japanese stiltgrass. For example, a strong correlation between pH and soil organic matter disappeared when Japanese stiltgrass was present. Together this suggests that this invaded ecosystem functions in a different way through both individual and correlated alterations to ecosystem properties.

After an invasion: understanding variation in grassland community recovery following removal of a high-impact invader

A pervasive problem in invasion ecology is the limited recovery of native communities following removal of invaders. Little evidence exists on the causes of variation in post-invasion recovery. In a 4-year experiment using 65 sets of matched plots, we imposed an invader removal treatment (with control) on heterogeneous grassland plots invaded or uninvaded by an aggressive recent arrival, Aegilops triuncialis (barb goatgrass). We tested the validity of plot matching using transplants and soil analyses. We analyzed the community-level correlates of invader impacts, removal treatment side effects, and community recovery, each defined in two ways: by compositional similarity to uninvaded plots, and by relative native species richness. Recovery of native species richness in invaded and treated plots was high (approaching 100 %) although recovery of composition was not high (median 71 % Bray–Curtis dissimilarity to uninvaded untreated plots). We measured resilience as the residuals of community recovery in models that controlled for invader impacts and removal treatment side effects. Compositional resilience was highest where the uninvaded communities had the least cover by other invaders in the same functional group as the focal invader. Richness resilience was highest where the uninvaded communities had the lowest native species richness. Our study suggests that the recovery of native species per se may be a more relevant goal than the recovery of the exact pre-invasion species composition of particular sites, particularly in cases where pre-invasion species composition included exotic species other than the focal invader.

Global Invader Impact Network (GIIN): toward standardized evaluation of the ecological impacts of invasive plants

Terrestrial invasive plants are a global problem and are becoming ubiquitous components of most ecosystems. They are implicated in altering disturbance regimes, reducing biodiversity, and changing ecosystem function, sometimes in profound and irreversible ways. However, the ecological impacts of most invasive plants have not been studied experimentally, and most research to date focuses on few types of impacts, which can vary greatly among studies. Thus, our knowledge of existing ecological impacts ascribed to invasive plants is surprisingly limited in both breadth and depth. Our aim was to propose a standard methodology for quantifying baseline ecological impact that, in theory, is scalable to any terrestrial plant invader (e.g., annual grasses to trees) and any invaded system (e.g., grassland to forest). The Global Invader Impact Network (GIIN) is a coordinated distributed experiment composed of an observational and manipulative methodology. The protocol consists of a series of plots located in (1) an invaded area; (2) an adjacent removal treatment within the invaded area; and (3) a spatially separate uninvaded area thought to be similar to pre-invasion conditions of the invaded area. A standardized and inexpensive suite of community, soil, and ecosystem metrics are collected allowing broad comparisons among measurements, populations, and species. The method allows for one-time comparisons and for long-term monitoring enabling one to derive information about change due to invasion over time. Invader removal plots will also allow for quantification of legacy effects and their return rates, which will be monitored for several years. GIIN uses a nested hierarchical scale approach encompassing multiple sites, regions, and continents. Currently, GIIN has network members in six countries, with new members encouraged. To date, study species include representatives of annual and perennial grasses; annual and perennial forbs; shrubs; and trees. The goal of the GIIN framework is to create a standard yet flexible platform for understanding the ecological impacts of invasive plants, allowing both individual and synthetic analyses across a range of taxa and ecosystems. If broadly adopted, this standard approach will offer unique insight into the ecological impacts of invasive plants at local, regional, and global scales.

A long-term experimental case study of the ecological effectiveness and cost effectiveness of invasive plant management in achieving conservation goals: bitou bush control in booderee national park in eastern australia.

Invasive plant management is often justified in terms of conservation goals, yet progress is rarely assessed against these broader goals, instead focussing on short-term reductions of the invader as a measure of success. Key questions commonly remain unanswered including whether invader removal reverses invader impacts and whether management itself has negative ecosystem impacts. We addressed these knowledge gaps using a seven year experimental investigation of Bitou Bush, Chrysanthemoides monilifera subsp. rotundata. Our case study took advantage of the realities of applied management interventions for Bitou Bush to assess whether it is a driver or passenger of environmental change, and quantified conservation benefits relative to management costs of different treatment regimes. Among treatments examined, spraying with herbicide followed by burning and subsequent re-spraying (spray-fire-spray) proved the most effective for reducing the number of individuals and cover of Bitou Bush. Other treatment regimes (e.g. fire followed by spraying, or two fires in succession) were less effective or even exacerbated Bitou Bush invasion. The spray-fire-spray regime did not increase susceptibility of treated areas to re-invasion by Bitou Bush or other exotic species. This regime significantly reduced plant species richness and cover, but these effects were short-lived. The spray-fire-spray regime was the most cost-effective approach to controlling a highly invasive species and facilitating restoration of native plant species richness to levels characteristic of uninvaded sites. We provide a decision tree to guide management, where recommended actions depend on the outcome of post-treatment monitoring and performance against objectives. Critical to success is avoiding partial treatments and treatment sequences that may exacerbate invasive species impacts. We also show the value of taking advantage of unplanned events, such as wildfires, to achieve management objectives at reduced cost.

Mechanisms and reversibility of the effects of hybrid cattail on a Great Lakes marsh

Invasive plants compete with natives, and their litter can suppress natives and alter ecosystem function. Understanding the mechanisms of effect of invasive plants and testing whether invader removal reverses these effects is important to guide restoration and management. We addressed these questions in a Great Lakes marsh invaded by Typha x glauca (hybrid cattail), which produces monodominant stands with considerable litter accumulation. We teased apart effects of live T. x glauca versus its litter on the marsh ecosystem with a live/litter transplant experiment, and tested whether effects were reversible by a live/litter removal experiment in a Typha stand. Over two to four years, in both native marsh and Typha stand, the presence of litter decreased light and increased inorganic nitrogen. Live T. x glauca addition to the native marsh increased inorganic nitrogen and soil organic matter, but removal did not reverse these effects. Litter addition decreased native richness and abundance, and litter removal reversed these effects; however, characteristic native marsh sedges and rushes were not restored, but rather terrestrial grasses and forbs recruited. This suggests that litter removal is essential in cattail restoration projects to increase diversity, but that further active management will likely be necessary to restore marsh communities per se. Because many wetland invaders share similar traits with T. x glauca, including litter accumulation, conclusions are broadly applicable to a variety of wetland invasions.

Timing of Favorable Conditions, Competition and Fertility Interact to Govern Recruitment of Invasive Chinese Tallow Tree in Stressful Environments

The rate of new exotic recruitment following removal of adult invaders (reinvasion pressure) influences restoration outcomes and costs but is highly variable and poorly understood. We hypothesize that broad variation in average reinvasion pressure of Triadica sebifera (Chinese tallow tree, a major invader) arises from differences among habitats in spatiotemporal availability of realized recruitment windows. These windows are periods of variable duration long enough to permit establishment given local environmental conditions. We tested this hypothesis via a greenhouse mesocosm experiment that quantified how the duration of favorable moisture conditions prior to flood or drought stress (window duration), competition and nutrient availability influenced Triadica success in high stress environments. Window duration influenced pre-stress seedling abundance and size, growth during stress and final abundance; it interacted with other factors to affect final biomass and germination during stress. Stress type and competition impacted final size and biomass, plus germination, mortality and changes in size during stress. Final abundance also depended on competition and the interaction of window duration, stress type and competition. Fertilization interacted with competition and stress to influence biomass and changes in height, respectively, but did not affect Triadica abundance. Overall, longer window durations promoted Triadica establishment, competition and drought (relative to flood) suppressed establishment, and fertilization had weak effects. Interactions among factors frequently produced different effects in specific contexts. Results support our ‘outgrow the stress’ hypothesis and show that temporal availability of abiotic windows and factors that influence growth rates govern Triadica recruitment in stressful environments. These findings suggest that native seed addition can effectively suppress superior competitors in stressful environments. We also describe environmental scenarios where specific management methods may be more or less effective. Our results enable better niche-based estimates of local reinvasion pressure, which can improve restoration efficacy and efficiency by informing site selection and optimal management.

Responses to invasion and invader removal differ between native and exotic plant groups in a coastal dune

The spread of exotic, invasive species is a global phenomenon that is recognized as a major source of environmental change. Although many studies have addressed the effects of exotic plants on the communities they invade, few have quantified the effects of invader removal on plant communities, or considered the degree to which different plant groups vary in response to invasion and invader removal. We evaluated the effects of an exotic succulent, iceplant (Carpobrotus edulis), on a coastal dune plant community in northern California, as well as the community responses to its removal. To assess possible mechanisms by which iceplant affects other plants, we also evaluated its above- and belowground influences on the germination and growth of a dominant exotic annual grass, Bromus diandrus. We found that iceplant invasion was associated with reduced native plant cover as well as increased cover and density of some exotic plants-especially exotic annual grasses. However, iceplant removal did not necessarily lead to a reversal of these effects: removal increased the cover and density of both native and exotic species. We also found that B. diandrus grown in iceplant patches, or in soil where iceplant had been removed, had poorer germination and growth than B. diandrus grown in soil not influenced by iceplant. This suggests that the influence of iceplant on this dune plant community occurs, at least in part, due to belowground effects, and that these effects remain after iceplant has been removed. Our study demonstrates the importance of considering how exotic invasive plants affect not only native species, but also co-occurring exotic taxa. It also shows that combining observational studies with removal experiments can lead to important insights into the influence of invaders and the mechanisms of their effects.

Using economic instruments to develop effective management of invasive species: insights from a bioeconomic model

Economic growth is recognized as an important factor associated with species invasions. Consequently, there is increasing need to develop solutions that combine economics and ecology to inform invasive species management. We developed a model combining economic, ecological, and sociological factors to assess the degree to which economic policies can be used to control invasive plants. Because invasive plants often spread across numerous properties, we explored whether property owners should manage invaders cooperatively as a group by incorporating the negative effects of invader spread in management decisions (collective management) or independently, whereby the negative effects of invasive plant spread are ignored (independent management). Our modeling approach used a dynamic optimization framework, and we applied the model to invader spread using Linaria vulgaris. Model simulations allowed us to determine the optimal management strategy based on net benefits for a range of invader densities. We found that optimal management strategies varied as a function of initial plant densities. At low densities, net benefits were high for both collective and independent management to eradicate the invader, suggesting the importance of early detection and eradication. At moderate densities, collective management led to faster and more frequent invader eradication compared to independent management. When we used a financial penalty to ensure that independent properties were managed collectively, we found that the penalty would be most feasible when levied on a property's perimeter boundary to control spread among properties. At the highest densities, the optimal management strategy was "do nothing" because the economic costs of removal were too high relative to the benefits of removal. Spatial variation in L. vulgaris densities resulted in different optimal management strategies for neighboring properties, making a formal economic policy to encourage invasive species removal critical. To accomplish the management and enforcement of these economic policies, we discuss modification of existing agencies and infrastructure. Finally, a sensitivity analysis revealed that lowering the economic cost of invader removal would strongly increase the probability of invader eradication. Taken together, our results provide quantitative insight into management decisions and economic policy instruments that can encourage invasive species removal across a social landscape.

Effect of invader removal: pollinators stay but some native plants miss their new friend

Removal of invasive species often benefits biological diversity allowing ecosystems’ recovery. However, it is important to assess the functional roles that invaders may have established in their new areas to avoid unexpected results from species elimination. Invasive animal-pollinated plants may affect the plant–pollination interactions by changing pollinator availability and/or behaviour in the community. Thus, removal of an invasive plant may have important effects on pollinator community that may then be reflected positive or negatively on the reproductive success of native plants. The objective of this study was to assess the effect of removing Oxalis pes-caprae, an invasive weed widely spread in the Mediterranean basin, on plant–pollinator interactions and on the reproductive success of co-flowering native plants. For this, a disturbed area in central Portugal, where this species is highly abundant, was selected. Visitation rates, natural pollen loads, pollen tube growth and natural fruit set of native plants were compared in the presence of O. pes-caprae and after manual removal of their flowers. Our results showed a highly resilient pollination network but also revealed some facilitative effects of O. pes-caprae on the reproductive success of co-flowering native plants. Reproductive success of the native plants seems to depend not only on the number and diversity of floral visitors, but also on their efficiency as pollinators. The information provided on the effects of invasive species on the sexual reproductive success of natives is essential for adequate management of invaded areas.

Interpopulation variation in allelopathic traits informs restoration of invaded landscapes.

Invasive species can show substantial genetic variation in ecologically important traits, across ranges as well within the introduced range. If these traits affect competition with native species, then management may benefit from considering the genetic landscape of the invader. Across their introduced range, Alliaria petiolata populations vary in their investment in allelopathic traits according to invasion history, which could lead to gradients of impact on native species. Red oak (Quercus rubra) seedlings were transplanted into eight A. petiolata-invaded sites that varied in their invasion history and allelochemical concentrations. At each site, an invader removal treatment was crossed with experimental inoculations of native soil biota, to test whether the benefits of these restoration actions differed across invader populations. Q. rubra seedlings grew faster in invader populations with a longer invasion history and lower allelochemical concentrations. Invader removal and soil inoculation interacted to determine seedling growth, with the benefits of soil inoculation increasing in younger and more highly allelopathic invader populations. A greenhouse experiment using soils collected from experimentally inoculated field plots found similar patterns. These results suggest that the impact of this invader varies across landscapes and that knowledge of this variation could improve the efficacy and efficiency of restoration activities.

Invasion Age and Invader Removal Alter Species Cover and Composition at the Suisun Tidal Marsh, California, USA

Wetland ecosystems are vulnerable to plant species invasions, which can greatly alter species composition and ecosystem functioning. The response of these communities to restoration can vary following invader removal, but few studies have evaluated how recent and long-term invasions can affect the plant community’s restoration potential. Perennial pepperweed (Lepidium latifolium) has invaded thousands of hectares of marshland in the San Francisco Estuary, California, United States of America, while the effects of invasion and removal of this weed remain poorly studied. In this study, perennial pepperweed was removed along a gradient of invasion age in brackish tidal marshes of Suisun Marsh, within the Estuary. In removal plots, resident plant cover significantly increased during the 2-year study period, particularly in the densest and oldest parts of the perennial pepperweed colonies, while species richness did not change significantly. In bare areas created by removal of perennial pepperweed, recolonization was dominated by three-square bulrush (Schoenoplectus americanus). Ultimately, removal of invasive perennial pepperweed led to reinvasion of the resident plant community within two years. This study illustrates that it is important to consider invasion age, along with exotic species removal, when developing a restoration strategy in wetland ecosystems.

Pollinator Visits to Threatened Species Are Restored Following Invasive Plant Removal

An indirect consequence of plant invasions is the disruption of native plant-pollinator interactions. We examined effects of invasive baby’s breath (Gypsophila paniculata) and spotted knapweed (Centaurea maculosa) on floral visitors to federally threatened Pitcher’s thistle (Cirsium pitcheri) in Lake Michigan dunes. In sweep net surveys, abundances of pollinator taxa were five times higher in invaded than in naturally invader-free sites. However, plot-level G. paniculata removal treatments increased pollinator visits to C. pitcheri relative to invaded plots and restored visitation to levels found in naturally uninvaded plots. Invader removal also increased native plant species richness, which was positively correlated with pollinator visitation to C. pitcheri, suggesting an indirect effect on pollinators mediated through invader-altered plant composition. In temporary floral arrays, the rate of pollinator visitation to C. pitcheri was not affected by neighbor plant species identity. However, compared with native Monarda punctata, invasive C. maculosa attracted more total pollinators to the array but reduced the proportion of total visits that were to C. pitcheri and increased pollinator movements between plant species. While both G. paniculata and C. maculosa appear to act as magnet species by attracting more pollinators at the plot level, these invaders have the potential to reduce reproduction of C. pitcheri by decreasing pollinator visits and increasing interspecific pollen transfer.

Impacts of exotic mangrove forests and mangrove deforestation on carbon remineralization and ecosystem functioning in marine sediments

Abstract. To evaluate how mangrove invasion and removal can modify short-term benthic carbon cycling and ecosystem functioning, we used stable-isotopically labeled algae as a deliberate tracer to quantify benthic respiration and C-flow over 48 h through macrofauna and bacteria in sediments collected from (1) an invasive mangrove forest, (2) deforested mangrove sites 2 and 6 years after removal of above-sediment mangrove biomass, and (3) two mangrove-free control sites in the Hawaiian coastal zone. Sediment oxygen consumption (SOC) rates averaged over each 48 h investigation were significantly greater in the mangrove and mangrove removal site experiments than in controls and were significantly correlated with total benthic (macrofauna and bacteria) biomass and sedimentary mangrove biomass (SMB). Bacteria dominated short-term C-processing of added microalgal-C and benthic biomass in sediments from the invasive mangrove forest habitat and in the 6-yr removal site. In contrast, macrofauna were the most important agents in the short-term processing of microalgal-C in sediments from the 2-yr mangrove removal site and control sites. However, mean faunal abundance and C-uptake rates in sediments from both removal sites were significantly higher than in control cores, which collectively suggest that community structure and short-term C-cycling dynamics of sediments in habitats where mangroves have been cleared can remain fundamentally different from un-invaded mudflat sediments for at least 6-yrs following above-sediment mangrove removal. In summary, invasion by mangroves can lead to dramatic shifts in benthic ecosystem function, with sediment metabolism, benthic community structure and short-term C-remineralization dynamics being affected for years following invader removal.

Minimizing invader impacts: Striking the right balance between removal and restoration

Invasive species can cause severe damage in their introduced range; this damage often persists even after removal of the invader. In order to efficiently allocate a limited budget between invader removal and restoration of habitat from which the invader has been removed, it is vital to quantify the impacts of the invasion within an economic context. Here we develop optimal management strategies for biological invasions, which minimize both the direct economic costs of removal and restoration, and the ecological costs of present and future damage caused by the invasion. We demonstrate how this can be formulated as a linear programming problem, enabling the fast and efficient computation of optimal solutions. Using a simple example, we outline some general principles for the optimal control of an invader that damages its environment. Notably, we show that the most effective strategies often switch the priority of removal and restoration over time, and outline the conditions under which restoration is prioritized over removal. The proportion of total funds allocated to restoration will depend on the annual budget, the persistence of damage, and the relative costs of damage, removal and restoration.