Uninvaded Communities Research Articles

No consistencies in abundance–impact relationships across herbaceous invasive species and ecological impact metrics

Abstract The general shape (from linear to complex curve), direction (negative and positive) and strength (steepness of the slope) of abundance–impact relationships for different impact metrics are poorly known, despite their importance in understanding and predicting ecological repercussions of invasive species. It is also an open question how the functional traits of invasive species may influence the abundance–impact relationship. We studied 11 widespread herbaceous invasive alien species of East‐Central Europe and their 16 impact metrics (resident plant communities' ecological characteristics, trait composition, functional diversity and soil parameters) by sampling invaded and similar, uninvaded sites (space‐for‐time substitution method). Our aim was to (1) investigate the detailed ecological impacts of invasive plants on native plant communities; (2) explore the type of cover–impact relationships across impact metrics and their consistency across species and (3) study whether the cover–impact relationship depends on functional traits of invasive species. When considering all invasive species together, we found that invaded plant communities were less species rich and less functionally even but showed higher values of Rao's Q diversity index, and higher nitrogen, phosphorus and organic carbon soil nutrient content compared to uninvaded communities. However, the species‐wise analyses revealed strikingly different impacts among the 11 invasive species and also among impact metrics. Regarding the type of cover–impact relationships, we found no consistencies across invasive species and impact metrics. Still, nonlinear relationships prevailed when species were analysed together and linear relationships when species were studied individually. The functional traits of the invasive species explained only a small part of this response heterogeneity; mostly, the small‐seeded perennial invasive species affected the cover–species richness relationship. Synthesis: Herbaceous invasive plant species have a cover‐dependent impact on resident plant communities, but there are no consistent patterns across impact metrics and invasive species. Specific traits or trait syndrome of invasive species may affect the heterogeneity of cover–impact relationships, but that would need further study. We highlighted the importance of impact assessments involving invasive species' abundance to unmask cryptic impacts for species that show contrasting effects along an abundance gradient.

Expansive shrubs: Expansion factors and ecological impacts in northern Ethiopia

Expansive native species have the same ecological and economic consequences as invasive alien species through the reduction of ecosystem goods and services. We assessed human disturbance and environmental factors that are contributing to the expansion of two expansive shrubs, Cadia purpurea and Tarchonanthus camphoratus in Desa’a dry Afromontane forest, northern Ethiopia. We also evaluated their expansion effect on plant species composition, diversity, structure, and regeneration, and herbaceous cover. The forest was stratified into “invaded” and “uninvaded” based on percentage coverage of the two shrubs. Vegetation data were collected from each stratum using 75 nested plots per strata of 20m × 20m, 5m × 5m, and 1m × 1m for adult tree/shrub, sapling, and seedling, respectively. Moreover, data on environmental and human disturbance variables were collected from the main plots. We found that higher cover of the expansive shrubs was related to both human disturbance and environmental variables while some were species-specific. Species richness and diversity of plant species decreased significantly with an increase in abundance of the expansive shrubs. However, evenness did not change significantly with an increase in the abundance of the expansive shrubs. The density, mean diameter, mean height, and mean basal area of the plants decreased significantly with an increase with the cover of the shrubs. The expansive shrubs had a significant negative impact on regeneration and herbaceous cover. This study shows a distinctive variation in floristic composition, diversity, and regeneration status between the invaded and uninvaded communities, which suggests the need to be counteracted by management measures. Converting the underutilized expansive species into use as part of the management option should be given urge attention before it loses its ecological resilience.

Using stable isotopes to analyse extinction risks and reintroduction opportunities of native species in invaded ecosystems

Invasive non-native species have pervasive impacts on native biodiversity, including population extirpations and species extinctions. Identifying reasons why a population of a native species is extirpated following an invasion often relies on literature-based results of anecdotal observations. The well-established schemes of existing risk assessments for invasive species assume that a species’ information (e.g. impacts or behavioural and biological traits) can be projected from one area to another to estimate the potential impact of a species in another environment. We used stable isotope data (δ13C, δ15N) from both invaded and uninvaded communities to predict such invasion impacts by reconstructing trophic relationships. This approach was tested on a community within a protected lake in Northern Spain where, following the introductions of non-native species, the last resident native species (the common tench Tinca tinca, the European eel Anguilla anguilla, and the whirligig beetle Gyrinus sp.) had been extirpated. Through the application of this novel approach, we found evidence that native species’ declines were related to direct predation by and resource overlap with non-native species, which occurred in conjunction with habitat modification. Using this approach, we outlined the mechanisms involved in the extirpation of native species in the post-invasion period. To compensate for losses of native species induced by invasions of non-native species, native species reintroductions might be an appropriate tool. For this, we further suggested and discussed a novel approach that predicts the outcome of arising interactions by superimposing stable isotope data from alternative sources to better estimate the success of native species´ reintroductions.

Effects of the Invasive Little Fire Ant (Wasmannia auropunctata) on Ant Community Composition on UF Campus

Invasive species are a serious threat to Florida’s native ecosystems and can have significant economic impacts. The invasive little fire ant (Wasmannia auropunctata) was first observed on the University of Florida’s campus in the Field and Fork Gardens in Summer 2018 and a targeted eradication of this population began in Fall 2018. This project aimed to compare ant community composition in invaded and uninvaded areas to better understand how ant invasions affect native ant communities. For comparison with Field and Fork Gardens invaded sites, four additional sites on UF’s campus were surveyed for ants through leaf litter sampling. Samples were sorted and all ants identified to the genus level; samples from invaded sites were identified to species. In samples from invaded sites, the community had lower species richness and a lower relative abundance of ant species compared to uninvaded communities. Determining how the invasion of the little fire ant affects ant communities is important as invasive ants do not fill the same ecosystem roles as native ants, such as seed dispersal and mutualist interactions. Data on community composition in uninvaded areas could be useful in efforts to restore a site after eradication has been achieved.

Arbuscular mycorrhizal fungi help explain invasion success of Solidago canadensis

The importance of soil microbes as a whole has long been recognized in plant invasions, yet relatively few studies address the relative importance of different soil microbial guilds. To this end, we collected soils that were conditioned by plants in 18 pairs of invaded and uninvaded communities and conducted an assemblage experiment (invasive Solidago canadensis and five native plants) via four inoculations. Arbuscular mycorrhizal fungi (AMF) increased the growth of S. canadensis and its competitive suppression (the potential of S. canadensis to suppress its neighboring native plants). The positive feedback effect of AMF on S. canadensis was stronger than their negative feedback effect on five native plants. Solidago canadensis grew larger and had lower competitive suppression in conspecific soils than heterospecific soils. These findings suggest that AMF play a crucial role in driving S. canadensis invasions and also highlight that conspecific and heterospecific soils contribute to the success of S. canadensis through different pathways.

Expanding the invasion footprint: Ventenata dubia and relationships to wildfire, environment, and plant communities in the Blue Mountains of the Inland Northwest, USA

AbstractQuestionsA recently introduced non‐native annual grass, Ventenata dubia, is challenging previous conceptions of community resistance in forest mosaic communities in the Inland Northwest. However, little is known of the drivers and potential ecological impacts of this rapidly expanding species. Here we (1) identify abiotic and biotic habitat characteristics associated with the V. dubia invasion and examine how these differ between V. dubia and other problematic non‐native annual grasses, Bromus tectorum and Taeniatherum caput‐medusae; and (2) determine how burning influences relationships between V. dubia and plant community composition and structure to address potential impacts on Inland Northwest forest mosaic communities.LocationBlue Mountains of the Inland Northwest, USA.MethodsWe measured environmental and plant community characteristics in 110 recently burned and nearby unburned plots. Plots were stratified to capture a range of V. dubia cover, elevations, biophysical classes, and fire severities. We investigated relationships between V. dubia, wildfire, environmental, and plant community characteristics using non‐metric multidimensional scaling and linear regressions.ResultsVentenata dubia was most abundant in sparsely vegetated, basalt‐derived rocky scablands interspersed throughout the forested landscape. Plant communities most heavily invaded by V. dubia were largely uninvaded by other non‐native annual grasses. Ventenata dubia was abundant in both unburned and burned areas, but negative relationships between V. dubia cover and community diversity were stronger in burned plots, where keystone sagebrush species were largely absent after fire.ConclusionsVentenata dubia is expanding the overall invasion footprint into previously uninvaded communities. Burning may exacerbate negative relationships between V. dubia and species richness, evenness, and functional diversity, including in communities that historically rarely burned. Understanding the drivers and impacts of the V. dubia invasion and recognizing how these differ from other annual grass invasions may provide insight into mechanisms of community invasibility, grass‐fire feedbacks, and aid the development of species‐specific management plans.

Inter‐ and intraspecific trait variation shape multidimensional trait overlap between two plant invaders and the invaded communities

Invader success and ecosystem impact are both expected to be largely driven by the functional trait distinctiveness of the resident species relative to the invaded communities. To understand the importance of trait distinctiveness for plant invasions, and the native community's trait response to the invasion, it is key to measure multiple traits simultaneously, and to incorporate intraspecific trait variation. Here we explored multidimensional patterns of inter‐ and intraspecific trait variation during the invasion of two functionally contrasting species. We constructed multidimensional trait hypervolumes for the invaders, their invaded communities, and uninvaded reference communities. The functional distinctiveness hypothesis predicts that invaders will occupy a mostly unique part of the trait hypervolume and that invasion will shift the trait composition of the native community to minimize trait overlap with the invader.Impatiens glanduliferawas characterized by acquisitive traits, whileRosa rugosahad more resource conservative traits, reflecting their respective invaded habitats. Both invaders showed relatively little hypervolume overlap with the uninvaded communities, although this overlap was higher forR. rugosa(31.9%) than forI. glandulifera(14.3%). Both invaders affected community traits, mainly due to intraspecific trait changes of the resident species. As expected, invasion byR. rugosareduced trait overlap between invader and community to 18.3%. Invasion byI. glandulifera, however, resulted in an increased trait overlap to 26.7%. In both cases the community traits shifted towards a more resource acquisitive strategy following invasion, irrespective of the invader's trait composition. This suggests that invasion‐driven community‐level intraspecific trait shifts are likely caused by increased competition for light under the invader, rather than by niche overlap. These, at first sight contradictory, results demonstrate the need for better trait‐based invasion and community ecology theory. Our results furthermore show that invaders can shift the trait occupancy of resident communities.

The cryptic impacts of invasion: functional homogenization of tropical ant communities by invasive fire ants

The diversity and distribution of traits in an ecological community shapes its responses to change and the ecosystem processes it modulates. This ‘functional diversity’, however, is not necessarily a direct outcome of taxonomic diversity. Invasions by exotic insects occur in ecosystems worldwide, but there is limited understanding of how they impact functional diversity. We present the first comprehensive trait‐based investigation of the impacts of an ant invasion, and the first incorporating intraspecific polymorphisms in species‐level functional diversity. The fire ant Solenopsis invicta is an invasive species with a global distribution. Focusing on invaded and uninvaded plots in tropical grasslands of Hong Kong, we investigated how the presence of S. invicta affects the diversity and distribution of ant species and traits within and across communities, the functional identities of communities, and functionally unique species. Using trait probability density functions, we built trait spaces for 29 different species, and scaled up these components to calculate functional diversity at community and landscape levels. We found that invasion had limited effects on species and functional richness but pronounced effects on functional composition. Specifically, invaded communities had fewer functionally‐unique individuals, and were characterized by species with narrower heads and bodies and shorter mandibles. Moreover, invaded communities showed substantially higher levels of functional redundancy (+56%) due to a clustering of trait values. Consequently, across the landscape, invaded communities displayed 23% less functional turnover than uninvaded communities despite showing comparable levels of taxonomic turnover – a result confirming theoretical predictions of the effects of high local functional redundancy. In sum, the presence of S. invicta alters the functional properties of multiple local communities selectively, resulting in functional homogenization across the landscape. The disparities between taxonomic and functional impacts of invasion highlight the need to consider how trait diversity across ecological scales shapes biodiversity and its responses to change.

Community- and trophic-level responses of soil nematodes to removal of a non-native tree at different stages of invasion.

Success of invasive non-native plant species management is usually measured as changes in the abundance of the invasive plant species or native plant species following invader management, but more complex trophic responses to invader removal are often ignored or assumed. Moreover, the effects of invader removal at different stages of the invasion process is rarely evaluated, despite a growing recognition that invader impacts are density or stage-dependent. Therefore, the effectiveness of invasive species management for restoring community structure and function across trophic levels remains poorly understood. We determined how soil nematode diversity and community composition respond to removal of the globally invasive tree species Pinus contorta at different stages of invasion by reanalysing and expanding an earlier study including uninvaded vegetation (seedlings removed continuously), early invader removal (saplings removed), late removal (trees removed), and no removal (invaded). These treatments allowed us to evaluate the stage-dependent belowground trophic responses to biological invasion and removal. We found that invaded plots had half the nematode taxa richness compared to uninvaded plots, and that tree invasion altered the overall composition of the nematode community. Differences in nematode community composition between uninvaded nematode communities and those under the tree removal strategy tended to dilute higher up the food chain, whereas the composition of uninvaded vs. sapling removal strategies did not differ significantly. Conversely, the composition of invaded compared to uninvaded nematode communities differed across all trophic levels, altering the community structure and function. Specifically, invaded communities were structurally simplified compared to uninvaded communities, and had a higher proportion of short life cycle nematodes, characteristic of disturbed environments. We demonstrate that a shift in management strategies for a globally invasive tree species from removing trees to earlier removal of saplings is needed for maintaining the composition and structure of soil nematode communities to resemble uninvaded conditions.

Functional Diversity and Invasive Species Influence Soil Fertility in Experimental Grasslands.

Ecosystem properties can be positively affected by plant functional diversity and compromised by invasive alien plants. We performed a community assembly study in mesocosms manipulating different functional diversity levels for native grassland plants (communities composed by 1, 2 or 3 functional groups) to test if functional dispersion could constrain the impacts of an invasive alien plant (Solidago gigantea) on soil fertility and plant community biomass via complementarity. Response variables were soil nutrients, soil water nutrients and aboveground biomass. We applied linear mixed-effects models to assess the effects of functional diversity and S. gigantea on plant biomass, soil and soil water nutrients. A structural equation model was used to evaluate if functional diversity and invasive plants affect soil fertility directly or indirectly via plant biomass and soil pH. Invaded communities had greater total biomass but less native plant biomass than uninvaded ones. While functional diversity increased nutrient availability in the soil solution of uninvaded communities, invasive plants reduced nutrient concentration in invaded soils. Functional diversity indirectly affected soil water but not soil nutrients via plant biomass, whereas the invader reduced native plant biomass and disrupted the effects of diversity on nutrients. Moreover, invasive plants reduced soil pH and compromised phosphate uptake by plants, which can contribute to higher phosphate availability and its possible accumulation in invaded soils. We found little evidence for functional diversity to constrain invasion impacts on nutrients and plant biomass. Restoration of such systems should consider other plant community features than plant trait diversity to reduce establishment of invasive plants.

Fungal communities do not recover after removing invasive Alliaria petiolata (garlic mustard)

The negative impacts of non-native invasive plants on native plants has prompted intensive eradication efforts, but whether eradication can restore soil microbial communities that are also sensitive to invasion is generally not considered. Some invasive plants, like Alliaria petiolata (garlic mustard), specifically alter soils in ways that promote the invasion process. Garlic mustard disrupts mycorrhizas, increases fungal pathogen loads, and elevates soil nutrient availability and soil pH; thus, the fungal community and soil property responses to garlic mustard eradication may be key to restoring ecosystem function in invaded forests. We conducted a garlic mustard eradication experiment at eight temperate, deciduous forests. 1 and 3 years after initiating annual garlic mustard removal (hand pulling), we collected soil samples and characterized fungal community structure using DNA metabarcoding alongside a suite of edaphic properties. We found that fungal richness, the number of shared fungal species, fungal biomass, and the relative abundance of fungal guilds became less similar to invaded plots by year three of eradication and more similar to uninvaded reference plots. However, fungal community composition did not resemble uninvaded communities by the third year of eradication and remained comparable to invaded communities. Soil chemical and physical properties also remained similar to invaded conditions. Overall soil abiotic–biotic restoration was not observed after 3 years of garlic mustard removal. Garlic mustard eradications may therefore not achieve management goals until soil physical, chemical, and biological properties become more similar to uninvaded forested areas or at least more dissimilar to invaded conditions that can promote invasion.

Invasive Bromus tectorum alters natural selection in arid systems

Summary While much research has documented the impact of invaders on native communities and ecosystem services, there has been less work quantifying how invasion affects the genetic composition of native populations. That is, when invaders dominate a community, can they shift selection regimes and impact the evolutionary trajectory of native populations? The invasion of the annual grass Bromus tectorum in the Intermountain West provides an opportunity to quantify the effects of invasion on natural selection in wild populations. The shift from a perennial‐dominated native community to one dominated by a highly competitive annual species alters the timing and intensity of competitive pressure, which has the potential to strongly shift selection regimes for native plants. To quantify traits under selection in contrasting environments, we planted seeds of two native perennial grasses, Elymus multisetus and Poa secunda, into three invaded, invaded but weeded and relatively uninvaded sagebrush systems. We quantified phenotypic traits of seedlings from separate maternal plants, describing differences in phenotypes among individuals. We then asked which traits were associated with survival and plant size in adjacent invaded and uninvaded sagebrush systems, following individual seed performance for 3 years. We found evidence for divergent selection between invaded and uninvaded sagebrush systems, with contrasting phenotypic traits associated with greater survival or plant size in these different growing conditions at all three field sites. For example, at one field site, P. secunda families with higher root tip production were more likely to survive in invaded and weeded environments, but this was not the case in uninvaded environments. Similarly, for E. multisetus, root mass fraction, seed mass and allocation to coarse or fine roots affected survival and plant size, again with contrasting relationships across invaded, weeded or uninvaded environments. Synthesis. Impacts of invasive species extend beyond ecosystem and community composition changes and can affect the evolutionary trajectory of native populations. By quantifying natural selection in invaded landscapes, we identified phenotypic traits that are potentially adaptive in these invaded systems. Importantly, these traits differed from traits associated with success in uninvaded communities. This insight into adaptive, contemporary evolution in native species can guide restoration and conservation efforts.

A role for the sampling effect in invaded ecosystems

Species loss and invasion of exotic species are two components of global biodiversity change that are expected to influence ecosystem functioning. Yet how they interact in natural settings remains unclear. Experiments have revealed two major mechanisms for the observed increase in primary productivity with plant species richness. Plant productivity may rise with species richness due to the increased amount of resources used by more diverse communities (niche complementarity) or through the increased probability of including a highly productive, dominant species in the community (sampling effect). Current evidence suggests that niche complementarity is the most relevant mechanism, whereas the sampling effect would only play a minor and transient role in natural systems. In turn, exotic species can invade by using untapped resources or because they possess a fitness advantage over resident species allowing them to dominate the community. We argue that the sampling effect can be a significant biodiversity mechanism in ecosystems invaded by dominant exotic species, and that the effect can be persistent even after decades of succession. We illustrate this idea by analyzing tree species richness–productivity relationships in a subtropical montane forest (NW Argentina) heavily invaded by Ligustrum lucidum, an evergreen tree from Asia. We found that the forest biomass increased along a natural gradient of tree species richness whether invaded by L. lucidum or not. Consistent with the sampling effect, L. lucidum invasion tripled total tree biomass irrespective of species richness, and monocultures of L. lucidum were more productive than any of the most species‐rich, uninvaded communities. Hence, the sampling effect may not be restricted to randomly assembled, synthetic communities. We emphasize that studying invaded ecosystems may provide novel insights on the mechanisms underlying the effect of biodiversity on ecosystem function.

Pinus contorta invasions increase wildfire fuel loads and may create a positive feedback with fire.

Invasive plant species that have the potential to alter fire regimes have significant impacts on native ecosystems. Concern that pine invasions in the Southern Hemisphere will increase fire activity and severity and subsequently promote further pine invasion prompted us to examine the potential for feedbacks between Pinus contorta invasions and fire in Patagonia and New Zealand. We determined how fuel loads and fire effects were altered by P.contorta invasion. We also examined post-fire plant communities across invasion gradients at a subset of sites to assess how invasion alters the post-fire vegetation trajectory. We found that fuel loads and soil heating during simulated fire increase with increasing P.contorta invasion age or density at all sites. However, P.contorta density did not always increase post-fire. In the largest fire, P.contorta density only increased significantly post-fire where the pre-fire P.contorta density was above an invasion threshold. Below this threshold, P.contorta did not dominate after fire and plant communities responded to fire in a similar manner as uninvaded communities. The positive feedback observed at high densities is caused by the accumulation of fuel that in turn results in greater soil heating during fires and high P.contorta density post-fire. Therefore, a positive feedback may form between P.contorta invasions and fire, but only above an invasion density threshold. These results suggest that management of pine invasions before they reach the invasion density threshold is important for reducing fire risk and preventing a transition to an alternate ecosystem state dominated by pines and novel understory plant communities.

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.

Dispersal acts as both bane and balm for invaded zooplankton communities

Dispersal of native species from the regional pool can recover invaded communities to a pre-invaded state by supplementing declining populations or providing resistant species. However, dispersal may also exacerbate the negative effects of an invader. Introduced species can open or create new niche space, which could facilitate the establishment of competitors or predators that previously could not succeed in the uninvaded local community. To investigate the interaction between dispersal and invasion by a non-native consumer, we conducted a field mesocosm experiment that introduced zebra mussels into native zooplankton communities. Regional zooplankton were collected and added to both invaded and uninvaded communities. In uninvaded communities, zooplankton dispersal reduced cladoceran diversity by 40%, rotifer abundance by 65% and copepod nauplii abundance by 80%. In invaded communities, dispersal increased cladoceran diversity by 60%, but also further exacerbated the negative effects of zebra mussels on rotifer abundance. This experiment illustrates the potential for dispersal to both positively and negatively affect local communities, and how these effects may change with disturbance and the taxa or community metric of study.

Competitive dominance in the organisation of Mediterranean ant communities

1. Competition by dominant species is thought to be key to structuring ant communities. However, recent findings suggest that the effect of dominant species on communities is less pronounced than previously assumed.2. The aim of the present study was to identify the role of dominant ants in the organisation of Mediterranean communities, particularly the role of competition in invaded and uninvaded communities. The effects on ant assemblages of two dominant ants, the invasive Argentine ant and the native ant, Tapinoma nigerrimum Nylander, were assessed.3. The abundances of both dominant ants were significantly correlated with a decrease in native ant richness at traps. However, only the invasive ant was associated with a reduction in diversity and abundance of other ant species at site scale. In the presence of T. nigerrimum, species co‐occurrence patterns were segregated or random. Community structure in both the dominant‐free and the Argentine ant sites showed random patterns of species co‐occurrence.4. The present findings indicate that dominant ants regulate small‐scale diversity by competition. However, at the broader scale of the assemblage, T. nigerrimum may only affect species distribution, having no apparent effect on community composition. Moreover, we find no evidence that inter‐specific competition shapes species distribution in coastal Mediterranean communities free of dominant ants.5. These results show that dominant species may affect ant assemblages but that the nature and the intensity of such effects are species and scale dependent. This confirms the hypothesis that competitive dominance may be only one of a range of factors that structure ant communities.

No universal scale-dependent impacts of invasive species on native plant species richness

A growing number of studies seeking generalizations about the impact of plant invasions compare heavily invaded sites to uninvaded sites. But does this approach warrant any generalizations? Using two large datasets from forests, grasslands and desert ecosystems across the conterminous United States, we show that (i) a continuum of invasion impacts exists in many biomes and (ii) many possible species-area relationships may emerge reflecting a wide range of patterns of co-occurrence of native and alien plant species. Our results contradict a smaller recent study by Powell et al. 2013 (Science 339, 316-318. (doi:10.1126/science.1226817)), who compared heavily invaded and uninvaded sites in three biomes and concluded that plant communities invaded by non-native plant species generally have lower local richness (intercepts of log species richness-log area regression lines) but steeper species accumulation with increasing area (slopes of the regression lines) than do uninvaded communities. We conclude that the impacts of plant invasions on plant species richness are not universal.

Invasive Plants Have Scale-Dependent Effects on Diversity by Altering Species-Area Relationships

Although invasive plant species often reduce diversity, they rarely cause plant extinctions. We surveyed paired invaded and uninvaded plant communities from three biomes. We reconcile the discrepancy in diversity loss from invaders by showing that invaded communities have lower local richness but steeper species accumulation with area than that of uninvaded communities, leading to proportionately fewer species loss at broader spatial scales. We show that invaders drive scale-dependent biodiversity loss through strong neutral sampling effects on the number of individuals in a community. We also show that nonneutral species extirpations are due to a proportionately larger effect of invaders on common species, suggesting that rare species are buffered against extinction. Our study provides a synthetic perspective on the threat of invasions to biodiversity loss across spatial scales.

Invasion success and impact of an invasive fish, round goby, in Great Lakes tributaries

AbstractAimEnvironmental and biological characteristics interact in complex ways to determine ecosystem susceptibility to invasive species, and a greater understanding of their relative roles in invader spread and impact is needed. We evaluated relationships between environmental characteristics, biodiversity of indigenous species, and the abundance and ecological impact of an invasive fish, round goby (Neogobius melanostomus).LocationTributaries to Lake Michigan, WI, USA.MethodsWe assessed the distribution and abundance of round gobies and native fishes in tributaries via electrofishing. We compared fish community composition and diversity in streams with (n = 30) and without (n = 52) round gobies and examined how trends in round goby abundance from 2007 to 2010 correlated with changes in abundance of five native benthic fishes. We used redundancy analysis to determine how indigenous stream communities related to environmental characteristics and round goby abundance.ResultsRound goby abundance was best explained by environmental characteristics, with watershed area and temperature explaining 22.4% of the variation. Species richness and Shannon diversity only explained 6.9% of the variation in round goby abundance and only an additional 2.3% after considering environmental characteristics. Round goby abundance was not a significant predictor of fish community composition, which was best explained by seven environmental variables (30.3% of the variation). Invaded communities had significantly higher indigenous species richness than uninvaded communities (8.38 vs. 6.54). Round goby abundance was low compared with estimates from other studies, but showed an increasing trend in many tributaries (average 10.8‐fold increase from 2007 to 2010). Surprisingly, there were no temporal trends in native benthic fish abundance despite increases in round goby abundance.Main conclusionsEnvironmental characteristics favouring high fish diversity (e.g. resource availability and warm water temperature) were positively associated with round goby abundance. However, round goby density and impact are relatively low in most tributaries at present, emphasizing the importance of considering heterogeneity in an invader's abundance when assessing invasion success.