Poised to prosper? A cross‐system comparison of climate change effects on native and non‐native species performance

Climate change and biological invasions are primary threats to global biodiversity that may interact in the future. To date, the hypothesis that climate change will favour non‐native species has been examined exclusively through local comparisons of single or few species. Here, we take a meta‐analytical approach to broadly evaluate whether non‐native species are poised to respond more positively than native species to future climatic conditions. We compiled a database of studies in aquatic and terrestrial ecosystems that reported performance measures of non‐native (157 species) and co‐occurring native species (204 species) under different temperature, CO₂ and precipitation conditions. Our analyses revealed that in terrestrial (primarily plant) systems, native and non‐native species responded similarly to environmental changes. By contrast, in aquatic (primarily animal) systems, increases in temperature and CO₂ largely inhibited native species. There was a general trend towards stronger responses among non‐native species, including enhanced positive responses to more favourable conditions and stronger negative responses to less favourable conditions. As climate change proceeds, aquatic systems may be particularly vulnerable to invasion. Across systems, there could be a higher risk of invasion at sites becoming more climatically hospitable, whereas sites shifting towards harsher conditions may become more resistant to invasions.     

Can herbarium records be used to map alien species invasion and native species expansion over the past 100 years?

Aim To determine if the temporal and spatial pattern of alien plant invasion and native plant expansion can be observed using 100 years of herbarium data from Oklahoma, USA, and to eliminate herbarium collection biases in such analyses. Location Oklahoma, USA. Methods Using herbarium records from the Oklahoma Vascular Plants Database from 1903 to 2004, we reconstructed the spatial and temporal collection history of two alien invasive taxa (Lonicera japonica and Tamarix spp.) and three native expansive species (Ambrosia psilostachya, Amphiachyris dracunculoides and Juniperus virginiana). To compare the overall collecting trend, groups of native non‐expansive taxa were selected as counterparts. We recorded the year of the first collection in each township in Oklahoma for all taxa. The cumulative number of occupied townships was log‐transformed, plotted against time and modelled with linear regression. The slope of the linear regression represented collection trend over time for the non‐expansive counterpart group. However, for the invasive and expansive species, the regression slope represented the collection effort plus the invasion or expansion rate. We calculated the proportion of invasive and expansive species to non‐expansive species by dividing the cumulative number of townships for each invasive or expansive species by the cumulative number of townships occupied by the counterpart group (proportion curve). Results Maps of the collection records of invasive and expansive taxa illustrated no discernible spatial invasion or expansion pattern. The slopes of the linear regression for alien invasive taxa were significantly steeper than those of their associated native non‐expansive counterparts, indicating an increase in abundance. Juniperus virginiana, L. japonica and Tamarix spp. exhibited one or more periods during which they were collected at a disproportionately higher rate than their native non‐expansive counterparts. Main conclusions Patterns of species invasion and expansion in Oklahoma were detected using techniques developed for regions with longer collecting plant histories. The proportion curve analysis eliminated some biases inherent in herbarium data by reducing the effect of collecting effort. Both the regression model and proportion curve analyses illustrate the temporal invasion patterns of alien invasive species. The native species did not show a clear expansion pattern. The information found in recently established herbaria may not be sensitive enough to detect the increase in abundance of native species.     

Multiple anthropogenic changes cause biodiversity loss through plant invasion

Non‐native invasive plants are a widely acknowledged threat to global biodiversity. However, our understanding of the mechanisms underlying plant invasion, and the relative importance of multiple rather than single drivers, remains poor. Here, we provide a case study using time‐series data to reconstruct patterns of change, and field experiments to test for causality. We show how, over a 50‐year period, a series of unrelated human‐induced changes created highly favorable conditions for the non‐native tree mallow (Lavatera arborea) to turn invasive, causing loss of native vegetation and seabird breeding habitat. The combination of three drivers: human‐introduced disease, climate warming and a fisheries‐mediated increase in seabird populations, removed major constraints on plant population growth, (i.e. grazer control, climatic control, germination opportunity and nutrient limitation). Collectively, these changes created optimal conditions for the rapid expansion of tree mallow. The resulting dramatic impact on both the native vegetation and breeding seabirds, notably puffins (Fratercula arctica), exemplifies how non‐native invasive plant species can transform terrestrial ecosystems. While climate change is regarded as a key factor behind plant invasion, we highlight that multiple rather than single factors may be critical to biodiversity loss.     

Diversity and distribution of genetic variation in gammarids: Comparing patterns between invasive and non‐invasive species

Biological invasions are worldwide phenomena that have reached alarming levels among aquatic species. There are key challenges to understand the factors behind invasion propensity of non‐native populations in invasion biology. Interestingly, interpretations cannot be expanded to higher taxonomic levels due to the fact that in the same genus, there are species that are notorious invaders and those that never spread outside their native range. Such variation in invasion propensity offers the possibility to explore, at fine‐scale taxonomic level, the existence of specific characteristics that might predict the variability in invasion success. In this work, we explored this possibility from a molecular perspective. The objective was to provide a better understanding of the genetic diversity distribution in the native range of species that exhibit contrasting invasive propensities. For this purpose, we used a total of 784 sequences of the cytochrome c oxidase subunit I of mitochondrial DNA (mtDNA‐COI) collected from seven Gammaroidea, a superfamily of Amphipoda that includes species that are both successful invaders (Gammarus tigrinus, Pontogammarus maeoticus, and Obesogammarus crassus) and strictly restricted to their native regions (Gammarus locusta, Gammarus salinus, Gammarus zaddachi, and Gammarus oceanicus). Despite that genetic diversity did not differ between invasive and non‐invasive species, we observed that populations of non‐invasive species showed a higher degree of genetic differentiation. Furthermore, we found that both geographic and evolutionary distances might explain genetic differentiation in both non‐native and native ranges. This suggests that the lack of population genetic structure may facilitate the distribution of mutations that despite arising in the native range may be beneficial in invasive ranges. The fact that evolutionary distances explained genetic differentiation more often than geographic distances points toward that deep lineage divergence holds an important role in the distribution of neutral genetic diversity.     

Distinct seasonal migration patterns of Japanese native and non‐native genotypes of common carp estimated by environmental DNA

Understanding behavioral differences between intraspecific genotypes of aquatic animals is challenging because we cannot directly observe the animals underwater or visually distinguish morphologically similar counterparts. Here, we tested a new monitoring tool that uses environmental DNA (eDNA), an assemblage of DNA in environmental water, to specifically detect Japanese native and introduced non‐native genotypes of common carp (Cyprinus carpio) in Lake Biwa, Japan, and estimated differences between the two genotypes in the use of inland habitats. We monitored the ratios of native and non‐native single nucleotide polymorphism alleles of a mitochondrial locus of common carp in a lagoon connected to Lake Biwa for 3 years using eDNA. We observed seasonal dynamics in the allele frequency showing that the native genotype frequency peaked every spring, suggesting that native individuals migrated to the lagoon for spawning and then returned to the main lake, whereas non‐native individuals tended to stay in the lagoon. The estimated migration patterns corresponded with the estimates of a previous study, which were based on commercial fish catch data. Our findings suggest that eDNA‐based monitoring can be useful tool for addressing intraspecific behavioral differences underwater.     

Undaria pinnatifida: A case study to highlight challenges in marine invasion ecology and management

Marine invasion ecology and management have progressed significantly over the last 30 years although many knowledge gaps and challenges remain. The kelp Undaria pinnatifida, or “Wakame,” has a global non‐native range and is considered one of the world's “worst” invasive species. Since its first recorded introduction in 1971, numerous studies have been conducted on its ecology, invasive characteristics, and impacts, yet a general consensus on the best approach to its management has not yet been reached. Here, we synthesize current understanding of this highly invasive species and adopt Undaria as a case study to highlight challenges in wider marine invasion ecology and management. Invasive species such as Undaria are likely to continue to spread and become conspicuous, prominent components of coastal marine communities. While in many cases, marine invasive species have detectable deleterious impacts on recipient communities, in many others their influence is often limited and location specific. Although not yet conclusive, Undaria may cause some ecological impact, but it does not appear to drive ecosystem change in most invaded regions. Targeted management actions have also had minimal success. Further research is needed before well‐considered, evidence‐based management decisions can be made. However, if Undaria was to become officially unmanaged in parts of its non‐native range, the presence of a highly productive, habitat former with commercial value and a broad ecological niche, could have significant economic and even environmental benefit. How science and policy reacts to the continued invasion of Undaria may influence how similar marine invasive species are handled in the future.     

Invader presence disrupts the stabilizing effect of species richness in plant community recovery after drought

Higher biodiversity can stabilize the productivity and functioning of grassland communities when subjected to extreme climatic events. The positive biodiversity–stability relationship emerges via increased resistance and/or recovery to these events. However, invader presence might disrupt this diversity–stability relationship by altering biotic interactions. Investigating such disruptions is important given that invasion by non‐native species and extreme climatic events are expected to increase in the future due to anthropogenic pressure. Here we present one of the first multisite invader × biodiversity × drought manipulation experiment to examine combined effects of biodiversity and invasion on drought resistance and recovery at three semi‐natural grassland sites across Europe. The stability of biomass production to an extreme drought manipulation (100% rainfall reduction; BE: 88 days, BG: 85 days, DE: 76 days) was quantified in field mesocosms with a richness gradient of 1, 3, and 6 species and three invasion treatments (no invader, Lupinus polyphyllus, Senecio inaequidens). Our results suggest that biodiversity stabilized community productivity by increasing the ability of native species to recover from extreme drought events. However, invader presence turned the positive and stabilizing effects of diversity on native species recovery into a neutral relationship. This effect was independent of the two invader's own capacity to recover from an extreme drought event. In summary, we found that invader presence may disrupt how native community interactions lead to stability of ecosystems in response to extreme climatic events. Consequently, the interaction of three global change drivers, climate extremes, diversity decline, and invasive species, may exacerbate their effects on ecosystem functioning.     

Linking the pattern to the mechanism: How an introduced mammal facilitates plant invasions

Non‐native mammals that are disturbance agents can promote non‐native plant invasions, but to date there is scant evidence on the mechanisms behind this pattern. We used wild boar (Sus scrofa) as a model species to evaluate the role of non‐native mammals in promoting plant invasion by identifying the degree to which soil disturbance and endozoochorous seed dispersal drive plant invasions. To test if soil disturbance promotes plant invasion, we conducted an exclosure experiment in which we recorded emergence, establishment and biomass of seedlings of seven non‐native plant species planted in no‐rooting, boar‐rooting and artificial rooting patches in Patagonia, Argentina. To examine the role of boar in dispersing seeds we germinated viable seeds from 181 boar droppings and compared this collection to the soil seed bank by collecting a soil sample adjacent to each dropping. We found that both establishment and biomass of non‐native seedlings in boar‐rooting patches were double those in no‐rooting patches. Values in artificial rooting patches were intermediate between those in boar‐rooting and no‐rooting treatments. By contrast, we found that the proportion of non‐native seedlings in the soil samples was double that in the droppings, and over 80% of the germinated seeds were native species in both samples. Lastly, an effect size test showed that soil disturbance by wild boar rather than endozoochorous dispersal facilitates plant invasions. These results have implications for both the native and introduced ranges of wild boar, where rooting disturbance may facilitate community composition shifts.     

Comparison of invertebrate herbivores on native and non‐native Senecio species: Implications for the enemy release hypothesis

The enemy release hypothesis posits that non‐native plant species may gain a competitive advantage over their native counterparts because they are liberated from co‐evolved natural enemies from their native area. The phylogenetic relationship between a non‐native plant and the native community may be important for understanding the success of some non‐native plants, because host switching by insect herbivores is more likely to occur between closely related species. We tested the enemy release hypothesis by comparing leaf damage and herbivorous insect assemblages on the invasive species Senecio madagascariensis Poir. to that on nine congeneric species, of which five are native to the study area, and four are non‐native but considered non‐invasive. Non‐native species had less leaf damage than natives overall, but we found no significant differences in the abundance, richness and Shannon diversity of herbivores between native and non‐native Senecio L. species. The herbivore assemblage and percentage abundance of herbivore guilds differed among all Senecio species, but patterns were not related to whether the species was native or not. Species‐level differences indicate that S. madagascariensis may have a greater proportion of generalist insect damage (represented by phytophagous leaf chewers) than the other Senecio species. Within a plant genus, escape from natural enemies may not be a sufficient explanation for why some non‐native species become more invasive than others.     

Testing multiple pathways for impacts of the non‐native Black‐headed Weaver Ploceus melanocephalus on native birds in Iberia in the early phase of invasion

Not all non‐native species have strong negative impacts on native species. It is desirable to assess whether a non‐native species will have a negative impact at an early stage in the invasion process, when management options such as eradication are still available. Although it may be difficult to detect early impacts of non‐native species, it is necessary to ensure that management decisions can be based on case‐specific scientific evidence. We assess the impacts of a non‐native bird, the Black‐headed Weaver Ploceus melanocephalus, at an early stage in its invasion of the Iberian Peninsula. To do this we identify potential pathways by which competition for shared resources by Black‐headed Weavers could lead to population declines in two ecologically similar native species, and generate hypotheses to test for evidence of competition along these pathways. Black‐headed Weavers could potentially impact native species by displacing them from nesting habitat, or by reducing habitat quality. We found no evidence for either potential competition pathway, suggesting that Black‐headed Weavers do not currently compete with the two native species. However, it is possible that mechanisms that currently allow coexistence may not operate once Black‐headed Weavers reach higher population densities or different habitats.     

Pine invasions: climate predicts invasion success; something else predicts failure

Aim Explaining why some invasions fail while others succeed is a prevailing question in invasion biology. Different factors have been proposed to explain the success or failure of exotics. Evidence suggests that climate similarities may be crucial. We tested this using 12 species of the genus Pinus that have been widely planted and shown to be highly invasive. Pinus is among the best‐studied group of exotic species and one that has been widely introduced world‐wide, so we were able to obtain data on invasive and non‐invasive introductions (i.e. unsuccessful invasions; areas where after many decades of self‐sowing seeds there is no invasion). Location World‐wide. Methods We developed species distribution models for native ranges using a maximum entropy algorithm and projected them across the globe. We tested whether climate‐based models were able to predict both invasive and non‐invasive introductions. Results Appropriate climatic conditions seem to be required for these long‐lived species to invade because climates accurately predicted invasions. However, climate matching is necessary, but not sufficient to predict the fate of an introduction because most non‐invasive introductions were predicted to have triggered an invasion. Main conclusions Other factors, possibly including biotic components, may be the key to explaining why some introductions do not become invasions, because many areas where Pinus is not invading were predicted to be suitable for invasion based solely on climate.     

Niche shifts during the global invasion of the Asian tiger mosquito, Aedes albopictus Skuse (Culicidae), revealed by reciprocal distribution models

Aim Niche‐based distribution models are often used to predict the spread of invasive species. These models assume niche conservation during invasion, but invasive species can have different requirements from populations in their native range for many reasons, including niche evolution. I used distribution modelling to investigate niche conservatism for the Asian tiger mosquito (Aedes albopictus Skuse) during its invasion of three continents. I also used this approach to predict areas at risk of invasion from propagules originating from invasive populations. Location Models were created for Southeast Asia, North and South America, and Europe. Methods I used maximum entropy (Maxent ) to create distribution models using occurrence data and 18 environmental datasets. One native model was created for Southeast Asia; this model was projected onto North America, South America and Europe. Three models were created independently for the non‐native ranges and projected onto the native range. Niche overlap between native and non‐native predictions was evaluated by comparing probability surfaces between models using real data and random models generated using a permutation approach. Results The native model failed to predict an entire region of occurrences in South America, approximately 20% of occurrences in North America and nearly all Italian occurrences of A. albopictus. Non‐native models poorly predict the native range, but predict additional areas at risk for invasion globally. Niche overlap metrics indicate that non‐native distributions are more similar to the native niche than a random prediction, but they are not equivalent. Multivariate analyses support modelled differences in niche characteristics among continents, and reveal important variables explaining these differences. Main conclusions The niche of A. albopictus has shifted on invaded continents relative to its native range (Southeast Asia). Statistical comparisons reveal that the niche for introduced distributions is not equivalent to the native niche. Furthermore, reciprocal models highlight the importance of controlling bi‐directional dispersal between native and non‐native distributions.     

Local and landscape correlates of non‐native species invasion in restored wetlands

Vulnerability of natural communities to invasion by non‐native plants has been linked to factors such as recent disturbance and high resource availability, suggesting that recently restored habitats may be especially invasible. Because non‐native plants can interfere with restoration goals, monitoring programs should anticipate which sites are most susceptible to invasion and which species are likely to become problematic at a site. Restored sites of larger area and those with high rates of propagule input should have higher species richness of both natives and non‐natives, leading to a positive correlation between the two. However, in restored wetlands, urbanization, riparian landscape settings, and nitrogen enrichment likely favor non‐native relative to native species. We sampled 28 restored wetlands in Illinois, USA, modeled the responses of native richness, non‐native richness and non‐native cover to local and landscape predictors with linear regression, and modeled the presence/absence of 21 non‐native species with logistic regressions. Unexpectedly, native and non‐native richness were uncorrelated, suggesting different responses to environmental factors. Native richness declined with increasing available soil nitrogen and urbanization in the surrounding landscape. Non‐native richness, the richness of non‐natives relative to natives, and the likelihood of invasion by several individual invasive species decreased with increasing distance from the city of Chicago, likely in response to decreasing non‐native propagule pressure. Total cover of non‐natives, however, as well as cover by non‐native Phalaris arundinacea, increased with nitrogen availability. Our results indicate that although non‐native richness was better predicted by factors related to propagule pressure, non‐native species dominance was more closely related to local abiotic factors. Non‐native richness in restoration sites may be beyond the control of restoration practitioners, and furthermore, may be of limited relevance for conservation goals. In contrast, limiting the relative dominance of non‐natives should be a restoration priority and may be achievable through management of nutrient availability.     

Subtle differences in two non‐native congeneric beach grasses significantly affect their colonization,spread, and impact

Comparisons of congeneric species have provided unique insights into invasion ecology. Most often, non‐native species are compared to native ones to look for traits predicting invasion success. In this study, we examine a different facet of congeneric comparisons in which both species are non‐native. Ecological variability among non‐native congeners might 1) lead to the inhibition or facilitation of either species’ ability to colonize and spread, 2) result in larger cumulative impacts due to synergies between species, and 3) depend on the physical context of the invaded habitat. To explore these possibilities, we studied the distribution and abundance of two non‐native beach grasses: European beach grass Ammophila arenaria and American beach grass Ammophila breviligulata, their interaction with one another, and their biotic and physical impacts on dune ecosystems of the Pacific coast of North America. We found that over a two‐decade period, A. breviligulata has increased its dominance over A. arenaria on dunes where it was originally planted in 1935 and has actively spread to new sites formerly dominated by A. arenaria. Our results also show that dune plant species richness was lower at A. breviligulata sites, although there was an increase in the native beach grass Elymus mollis. More significantly, we found that the two grass species are associated with significantly different foredune shapes that are likely controlled by a combination of variability in sand supply along the coast and subtle differences in the congeners’ morphology and growth form. These differences have significant implications for the coastal protection services of dunes to humans and the conservation of native species. They provide a cautionary tale on the impacts of introducing novel species based purely on analogy with closely related species.     

Linking disturbance and resistance to invasion via changes in biodiversity: a conceptual model and an experimental test on rocky reefs

Biological invasions threaten biodiversity worldwide. Nonetheless, a unified theory linking disturbance and resistance to invasion through a mechanistic understanding of the changes caused to biodiversity is elusive. Building on different forms of the disturbance‐biodiversity relationship and on the Biotic Resistance Hypothesis (BRH), we constructed conceptual models showing that, according to the main biodiversity mechanism generating invasion resistance (complementary vs. identity effects), disturbance can either promote or hinder invasion. Following the Intermediate Disturbance Hypothesis (IDH), moderate levels of disturbance (either frequency or intensity) are expected to enhance species richness. This will promote invasion resistance when complementarity is more important than species identity. Negative effects of severe disturbance on invasion resistance, due to reductions in species richness, can be either overcompensated or exacerbated by species identity effects, depending on the life‐traits becoming dominant within the native species pool. Different invasion resistance scenarios are generated when the diversity‐disturbance relationship is negative or positive monotonic. Predictions from these models were experimentally tested on rocky reefs. Macroalgal canopies differing in species richness (1 vs. 2 vs. 3) and identity, were exposed to either a moderate or a severe pulse disturbance. The effects of different canopy‐forming species on the seaweed, Caulerpa cylindracea, varied from positive (Cystoseira crinita) to neutral (Cystoseira barbata) to negative (Cystoseira compressa). After 2 years, severely disturbed plots were monopolized by C. compressa and supported less C. cylindracea. Our study shows that the effects of disturbance on invasion depend upon its intensity, the main mechanism through which biodiversity generates invasion resistance and the life‐traits selected within the native species pool. Disturbance can sustain invasion resistance when promoting the dominance of competitively subordinate species possessing traits that allow outperforming invaders.     

Evaluating dominance as a component of non-native species invasions

Many studies have quantified plant invasions by determining patterns of non‐native species establishment (i.e. richness and absolute cover). Until recently, dominance has been largely overlooked as a significant component of invasion. Therefore, we re‐examined a 6‐year data set of 323 0.1 ha plots within 18 vegetation types collected in the Grand Staircase‐Escalante National Monument from 1998 to 2003, including dominance (i.e. relative cover) in our analyses. We specifically focused on the non‐native species Bromus tectorum, a notable dominant annual grass in this system. We found that non‐native species establishment and dominance are both occurring in species‐rich, mesic vegetation types. Therefore, non‐native species dominance may result despite many equally abundant native species rather than a dominant few, and competitive exclusion does not seem to be a primary control on either non‐native species establishment or dominance in this study. Unlike patterns observed for non‐native species establishment, relative non‐native species cover could not be predicted by native species richness across vegetation types (R² < 0.001; P = 0.45). However, non‐native species richness was found to be positively correlated with relative non‐native species cover and relative B. tectorum cover (R² = 0.46, P < 0.01; R² = 0.17, P < 0.01). Analyses within vegetation types revealed predominantly positive relationships among these variables for the correlations that were significant. Regression tree analyses across vegetation types that included additional biotic and abiotic variables were a little better at predicting non‐native species dominance (PRE = 0.49) and B. tectorum dominance (PRE = 0.39) than at predicting establishment. Land managers will need to set priorities for control efforts on the more productive, species‐rich vegetation types that appear to be susceptible to both components of invasion.     

Movement Behavior of Native and Invasive Small Mammals Shows Logging May Facilitate Invasion in a Tropical Rain Forest

Invasive species pose one of the greatest threats to biodiversity. This study investigates the extent to which human disturbance to natural ecosystems facilitates the spread of non‐native species, focusing on a small mammal community in selectively logged rain forest, Sabah, Borneo. The microhabitat preferences of the invasive Rattus rattus and three native species of small mammal were examined in three‐dimensional space by combining the spool‐and‐line technique with a novel method for quantifying fine‐scale habitat selection. These methods allowed the detection of significant differences for each species between the microhabitats used compared with alternative, available microhabitats that were avoided. Rattus rattus showed the greatest preference for heavily disturbed habitats, and in contrast to two native small mammals of the genus Maxomys, R. rattus showed high levels of arboreal behavior, frequently leaving the forest floor and traveling through the understory and midstory forest strata. This behavior may enable R. rattus to effectively utilize the complex three‐dimensional space of the lower strata in degraded forests, which is characterized by dense vegetation. The behavioral flexibility of R. rattus to operate in both terrestrial and arboreal space may facilitate its invasion into degraded forests. Human activities that generate heavily disturbed habitats preferred by R. rattus may promote the establishment of this invasive species in tropical forests in Borneo, and possibly elsewhere. We present this as an example of a synergistic effect, whereby forest disturbance directly threatens biodiversity and indirectly increases the threat posed by invasive species, creating habitat conditions that facilitate the establishment of non‐native fauna.     

Soil characteristics of Rocky Mountain National Park grasslands invaded by Melilotus officinalis and M. alba

Aim Invasion of nitrogen‐fixing non‐native plant species may alter soil resources and impact native plant communities. Altered soils may be the driving mechanism that provides a suitable environment to facilitate future invasions and decrease native biodiversity. We hypothesized that Melilotus invasion would increase nitrogen availability and produce soil microclimate and biochemical changes, which could in turn alter plant species composition in a montane grassland community. Location Our research addressed the effects of white and yellow sweet clover (Melilotus officinalis and M. alba) invasion on soil characteristics and nitrogen processes in the montane grasslands in Rocky Mountain National Park. Methods We sampled soil in replicate sites of Melilotus‐invaded and control (non‐invaded) patches within disturbed areas in montane grassland habitats. Soil composites were analysed for available nitrogen, net nitrogen mineralization, moisture, carbon/nitrogen (C : N ratio), texture, organic matter and pH. Data were recorded at three sample dates during the growing seasons of 1998 and 1999. Results Contrary to our expectations, we observed lower nitrogen availability and mineralization in invaded patches, and differences in soil moisture content and soil C : N. Soil C : N ratios were higher in invaded plots, in spite of the fact that Melilotus had the lowest C : N ratios of other plant tissue analysed in this study. Main conclusions These findings provide land managers of natural areas with a better perspective on the possibilities of nitrogen‐fixing species impact on soil nutrient levels.     

Biogeographical contrasts to assess local and regional patterns of invasion: a case study with two reciprocally introduced exotic maple trees

Quantitative comparisons of distribution and abundance of exotic species in their native and non‐native ranges represent a first step when studying invaders. However, this approach is rarely applied 2 particularly to tree species. Using biogeographical contrasts coupled with regional dispersal surveys, we assessed whether two exotic maple tree species, Acer negundo and Acer platanoides, can be classified as invasive in the non‐native regions surveyed. We also examined the importance of biogeography in determining the degree of invasion by exotic species using this reciprocal approach. Local‐scale surveys were conducted in a total of 34 forests to compare density, relative abundance, age structure of native and introduced populations, and whether the two introduced maple species negatively affected native tree species density. Regional‐scale surveys of a total of 136 forests were then conducted to assess distribution in the introduced regions. Introduced populations of A. negundo were denser than populations measured in their native range and negatively related to native tree species density. Age structure did not differ between regions for this species. At the regional scale, this species has invaded most of the riparian corridors sampled in France. Conversely, the density of A. platanoides introduced populations was similar to that of native populations and was not related to native tree species density. Although seedling recruitment was higher away than at home, this species has invaded only 9% of the forests sampled in southern Ontario, Canada. Although reported invasive, these two exotic maple species differed in their relative demographic parameters and regional spread. Acer negundo is currently invasive in southern France while A. platanoides is not aggressively invasive in southern Ontario. Importantly, this study effectively demonstrates that biogeography through structured contrasts provide a direct means to infer invasion of exotic species.     

Anomalous droughts,not invasion,decrease persistence of native fishes in a desert river

Changing climate extremes and invasion by non‐native species are two of the most prominent threats to native faunas. Predicting the relationships between global change and native faunas requires a quantitative toolkit that effectively links the timing and magnitude of extreme events to variation in species abundances. Here, we examine how discharge anomalies – unexpected floods and droughts – determine covariation in abundance of native and non‐native fish species in a highly variable desert river in Arizona. We quantified stochastic variation in discharge using Fourier analyses on >15 000 daily observations. We subsequently coupled maximum annual spectral anomalies with a 15‐year time series of fish abundances (1994–2008), using Multivariate Autoregressive State‐Space (MARSS) models. Abiotic drivers (discharge anomalies) were paramount in determining long‐term fish abundances, whereas biotic drivers (species interactions) played only a secondary role. As predicted, anomalous droughts reduced the abundances of native species, while floods increased them. However, in contrast to previous studies, we observed that the non‐native assemblage was surprisingly unresponsive to extreme events. Biological trait analyses showed that functional uniqueness was higher in native than in non‐native fishes. We also found that discharge anomalies influenced diversity patterns at the meta‐community level, with nestedness increasing after anomalous droughts due to the differential impairment of native species. Overall, our results advance the notion that discharge variation is key in determining community trajectories in the long term, predicting the persistence of native fauna even in the face of invasion. We suggest this variation, rather than biotic interactions, may commonly underlie covariation between native and non‐native faunas, especially in highly variable environments. If droughts become increasingly severe due to climate change, and floods increasingly muted due to regulation, fish assemblages in desert rivers may become taxonomically and functionally impoverished and dominated by non‐native taxa.