Community Invasibility Research Articles

Predicting Microstegium vimineum invasion in natural plant communities of the southern Blue Ridge Mountains, USA

Shade-tolerant non-native invasive plant species may make deep incursions into natural plant communities, but detecting such species is challenging because occurrences are often sparse. We developed Bayesian models of the distribution of Microstegium vimineum in natural plant communities of the southern Blue Ridge Mountains, USA to address three objectives: (1) to assess local and landscape factors that influence the probability of presence of M. vimineum; (2) to quantify the spatial covariance error structure in occurrence that was not accounted for by the environmental variables; and (3) to synthesize our results with previous findings to make inference on the spatial attributes of the invasion process. Natural plant communities surrounded by areas with high human activity and low forest cover were at highest risk of M. vimineum invasion. The probability of M. vimineum presence also increased with increasing native species richness and soil pH, and decreasing basal area of ericaceous shrubs. After accounting for environmental covariates, evaluation of the spatial covariance error structure revealed that M. vimineum is invading the landscape by a hierarchical process. Infrequent long-distance dispersal events result in new nascent sub-populations that then spread via intermediate- and short-distance dispersal, resulting in 3-km spatial aggregation pattern of sub-populations. Containment or minimisation of its impact on native plant communities will be contingent on understanding how M. vimineum can be prevented from colonizing new suitable habitats. The hierarchical invasion process proposed here provides a framework to organise and focus research and management efforts.

Invasive, non-indigenous corals in a tropical rocky shore environment: No evidence for generalist predation

The biological invasion of rocky shore communities modifies species composition and the interaction between community elements. Two species, Tubastraea coccinea Lesson, 1829 and Tubastraea tagusensis Wells, 1982 have been identified as non-indigenous and invasive to the tropical rocky shores of southeast Brazil. The aim of the current study was to investigate predator–prey interactions between a guild of native potential generalist predators and the non-indigenous corals using total exclusion and partial cages versus controls on a tropical rocky shore in the southwest Atlantic. Despite generalist predators being present in natural densities on the experimental system and substantial settlement occurring during the 2year study (average density of Tubastraea spp. was 11.29 ind. 240cm−2) no significant differences were found in density or size of corals between treatments. Dominant groups in the accompanying biological community (ABC) were encrusting calcareous and turf algae, the sponge Iotrochota sp. and barnacles but ABC height and biomass and sediment mass did not differ between treatments. Due to transport and invasion of new communities these corals are thought to have escaped from predation despite a diverse suite of native potential generalist predators. Both corals have been shown to possess bioactive chemicals which predatory fish avoid and this would seem to be the mechanism of predator avoidance. The successful invasion by Tubastraea spp. in the southwest Atlantic is at least in part due to highly reduced (or nil) predation compared to the native range.

Effects of human mediated disturbances on exotic forest insect diversity in a Chilean mediterranean ecosystem

At the current rate of exchange of goods and people among geographic areas, the introduction of insect species into new habitats represents an increasing threat to insect diversity. The situation is especially acute in Mediterranean ecosystems where the high human population density incurs multiple sources of disturbance and high propagule pressure. In this study, we characterize the relationship between native and exotic forest insect richness and evaluate how human-mediated disturbances can influence this relationship in the Mediterranean central Chile. Exotic and native species richness were positively correlated across the study area, suggesting similar effect of environmental variables on both assemblages over large scales. When the effect of human-mediated disturbances was evaluated using generalized linear and additive models, we found that native richness, human population density and habitat diversity were the most important variables affecting exotic richness. Moreover, we detected strong nonlinearities in the effect of some variables. For instance, the influence of human population density on the exotic richness followed a threshold function, where below 1,000 hab/km2, the proportion of exotics in the community grew rapidly with increasing human density, but above this threshold density, human population did not produce further increases in exotic richness. Two important conclusions arise from these results: first, there is a positive effect of human-mediated disturbances on the exotic richness in central Chile, and second, the key role that human population density has on the invasibility of insect communities in rural and semi-rural Mediterranean areas.

Niche dimensionality links biodiversity and invasibility of microbial communities

SummaryBiodiversity is a central factor driving community invasibility. Diverse communities exploit resources more efficiently, leaving less free niche space available to invaders. Niche partitioning, however, is only possible in complex resource environments, and we hypothesized that resource richness drives the biodiversity–invasibility relationship.We tested the effect of two biodiversity indices, taxonomic richness and functional dissimilarity, on the invasibility ofPseudomonas fluorescenscommunities in microcosms of varying resource richness, herein used as a proxy for niche dimensionality, because differentP. fluorescensgenotypes differed in their ability to use those resources.Invader success was negatively correlated with the diversity of the resident community, with functional dissimilarity being of greater significance than taxonomic richness. Varied niche dimensionality revealed different mechanisms determining community invasibility: at low niche dimensionality, invasibility was driven by the presence of particular genotypes (identity effect) rather than by the biodiversity of the resident community. At high niche dimensionality, functional dissimilarity increased community productivity and reduced invasion, most likely through complementarity effects.The results show that functionally dissimilar bacterial strains efficiently exploit their environment, reducing the resources available for invasive species. These findings call for the preservation of functionally dissimilar taxa to warrant resistance of communities against invasive species, in particular, in environments of high niche dimensionality.

Eco-Evolutionary Community Dynamics: Covariation between Diversity and Invasibility across Temperature Gradients

Understanding biodiversity gradients is a long-standing challenge, and progress requires theory unifying ecology and evolution. Here, we unify concepts related to the speed of evolution, the influence of species richness on diversification, and niche-based coexistence. We focus on the dynamics, through evolutionary time, of community invasibility and species richness across a broad thermal gradient. In our framework, the evolution of body size influences the ecological structure and dynamics of a trophic network, and organismal metabolism ties temperature to eco-evolutionary processes. The framework distinguishes ecological invasibility (governed by ecological interactions) from evolutionary invasibility (governed by local ecology and constraints imposed by small phenotypic effects of mutation). The model yields four primary predictions: (1) ecological invasibility declines through time and with increasing temperature; (2) average evolutionary invasibility across communities increases and then decreases through time as the richness-temperature gradient flattens; (3) in the early stages of diversification, richness and evolutionary invasibility both increase with increasing temperature; and (4) at equilibrium, richness does not vary with temperature, yet evolutionary invasibility decreases with increasing temperature. These predictions emerge from the "evolutionary-speed" hypothesis, which attempts to account for latitudinal species richness gradients by invoking faster biological rates in warmer, tropical regions. The model contrasts with predictions from other richness-gradient hypotheses, such as "niche conservatism" and "species energy." Empirically testing our model's predictions should help distinguish among these hypotheses.

Invasibility of a nutrient-poor pasture through resident and non-resident herbs is controlled by litter, gap size and propagule pressure.

Since inference concerning the relative effects of propagule pressure, biotic interactions, site conditions and species traits on the invasibility of plant communities is limited, we carried out a field experiment to study the role of these factors for absolute and relative seedling emergence in three resident and three non-resident confamilial herb species on a nutrient-poor temperate pasture. We set up a factorial field experiment with two levels each of the factors litter cover (0 and 400 g m−2), gap size (0.01 and 0.1 m2) and propagule pressure (5 and 50 seeds) and documented soil temperature, soil water content and relative light availability. Recruitment was recorded in spring and autumn 2010 and in spring 2011 to cover initial seedling emergence, establishment after summer drought and final establishment after the first winter. Litter alleviated temperature and moisture conditions and had positive effects on proportional and absolute seedling emergence during all phases of recruitment. Large gaps presented competition-free space with high light availability but showed higher temperature amplitudes and lower soil moisture. Proportional and absolute seedling recruitment was significantly higher in large than in small gaps. In contrast, propagule pressure facilitated absolute seedling emergence but had no effects on proportional emergence or the chance for successful colonisation. Despite significantly higher initial seedling emergence of resident than non-resident species, seed mass and other species-specific traits may be better predictors for idiosyncratic variation in seedling establishment than status. Our data support the fluctuating resource hypothesis and demonstrate that the reserve effect of seeds may facilitate seedling emergence. The direct comparison of propagule pressure with other environmental factors showed that propagule pressure affects absolute seedling abundance, which may be crucial for species that depend on other individuals for sexual reproduction. However, propagule batch size did not significantly affect the chance for successful colonisation of disturbed plots.

Complex interactions between spatial pattern of resident species and invasiveness of newly arriving species affect invasibility

Understanding the factors that affect establishment success of new species in established communities requires the study of both the ability of new species to establish and community resistance. Spatial pattern of species within a community can affect plant performance by changing the outcome of inter-specific competition, and consequently community invasibility. We studied the effects of spatial pattern of resident plant communities on fitness of genotypes from the native and introduced ranges of two worldwide invasive species, Centaurea stoebe and Senecio inaequidens, during their establishment stage. We experimentally established artificial plant mixtures with 4 or 8 resident species in intra-specifically aggregated or random spatial patterns, and added seedlings of genotypes from the native and introduced ranges of the two target species. Early growth of both S. inaequidens and C. stoebe was higher in aggregated than randomly assembled mixtures. However, a species-specific interaction between invasiveness and invasibility highlighted more complex patterns. Genotypes from native and introduced ranges of S. inaequidens showed the same responses to spatial pattern. By contrast, genotypes from the introduced range of C. stoebe did not respond to spatial pattern whereas native ones did. Based on phenotypic plasticity, we argue that the two target species adopted different strategies to deal with the spatial pattern of the resident plant community. We show that effects of spatial pattern of the resident community on the fitness of establishing species may depend on the diversity of the recipient community. Our results highlight the need to consider the interaction between invasiveness and invasibility in order to increase our understanding of invasion success.

The invasion of plant communities following extreme weather events under ambient and elevated temperature

Although the problem of plant invasions is expected to increase with climate change, there is as yet little experimental evidence, in particular, for the effects of extreme weather events. We established communities of European meadow species, which were subjected to warming and extreme event (drought and deluge) treatments in a factorial design at an experimental garden in Zurich, Switzerland. Phylogenetically matched pairs of native and alien species (Bromus erectus, B. inermis, Trifolium pratense, T. hybridum, Lactuca serriola, and Conyza canadensis) were introduced into the communities to test if invader performance is favored by warming and extreme events, and if alien invaders perform better than native colonizers. With a warming of on average 0.3 °C, a higher cover of native plant communities was observed, while drought decreased cover in the short-term and lowered biomass. Germination, survival, and growth of the introduced species were lower under elevated temperature. Survival of all pairs and growth of Trifolium was greater in drought pots, while deluge had no effect. While the alien species showed a faster rate of increase in the number of leaves, mortality of alien species was greater than of native species. Overall, the performance of the focal species varied much more among taxonomic groups than native/alien provenances. The results suggest that with climate change, different types of extreme events will differ in the severity of their effects on native plant communities. Meanwhile, the effects of climate change on plant invasions are more likely to operate indirectly through the impacts on native vegetation.

Biotic and abiotic constraints to a plant invasion in vegetation communities of Tierra del Fuego

AbstractThe biotic resistance theory relates invader success to species richness, and predicts that, as species richness increases, invasibility decreases. The relationship between invader success and richness, however, seems to be positive at large scales of analysis, determined by abiotic constraints, and it is to be expected that it is negative at small scales, because of biotic interactions. Moreover, the negative relationship at small scales would be stronger within species of the same functional group, because of having similar resource exploitation mechanisms. We studied the relationship between the cover of a worldwide invader of grasslands, Hieracium pilosella L., and species richness, species diversity and the cover of different growth forms at two different levels of analysis in 128 sites during the initial invasion process in the Fuegian steppe, Southern Patagonia, Argentina. At regional level, the invader was positively correlated to total (r = 0.28, P = 0.003), exotic (r = 0.273, P = 0.004), and native species richness (r = 0.210, P = 0.026), and to species diversity (r = 0.193, P = 0.041). At community level, we found only a weak negative correlation between H. pilosella and total richness (r = −0.426, P = 0.079) and diversity (r = −0.658, P = 0.063). The relationship between the invader and other species of the same growth form was positive both at regional (r = 0.484, P < 0.001) and community (r = 0.593, P = 0.012) levels. Consequently, in the period of establishment and initial expansion of this exotic species, our results support the idea that invader success is related to abiotic factors at large scales of analysis. Also, we observed a possible sign of biotic constraint at community level, although this was not related to the abundance of species of the same growth form.

How invader traits interact with resident communities and resource availability to determine invasion success

Competition for limited resources is considered a key factor controlling invasion success. Resource availability can be viewed in either the long or short‐term. Long‐term availability depends on the baseline nutrient availability in the ecosystem and how those conditions shape the ecological community. Short‐term resource availability fluctuates with disturbances that alter nutrient availability and/or the density and composition of the ecological community.We investigated how species’ traits interact with short and long‐term resource availability to determine the outcome of invasions. We manipulated long‐term baseline resource availability, disturbance intensity, disturbance frequency, and propagule pressure in a fully factorial design using protist microcosms. Our results show that short and long‐term resource availability and the direct mortality from disturbance interact with the traits of resident community members and traits of invaders to determine community invasibility. While competitively dominant invaders with slow growth rates may suffer rather than benefit from short‐term resource fluctuations, quickly growing but competitively inferior invaders can benefit from both the resource fluctuations and the heterogeneity in community composition created by disturbance. Our findings empirically synthesize two explanations for invasion success, namely short‐term resource fluctuations and long‐term resource availability, and highlight the importance of considering traits of invaders and residents, such as growth rate and competitive ability, in the context of productivity and disturbance gradients. This species’ traits approach could resolve idiosyncratic results from natural systems undergoing disturbance and invasion that do not follow patterns predicted by traditional invasion frameworks.

Congress summary: EMAPI 10: Bridging approaches to plant invasions

news and update ISSN 1948-6596 congress summary EMAPI 10: Bridging approaches to plant invasions 10th International Conference on the Ecology and Management of Alien Plant Invasions (EMAPi 10) – Stellenbosch, South Africa, 23-27th August 2009 “Invasion biology” has developed considerably in the 17 years since the first meeting on the Ecology and Management of Alien Plant Invasions (EMAPI) in 1992. Biological invasions provide a broad um- brella under which an increasingly wide variety of interdisciplinary studies are conducted. Nowhere was this more clearly illustrated than at this year’s EMAPI 10 conference hosted by the Centre of Ex- cellence for Invasion Biology in Stellenbosch, South Africa. The 31 sessions covered a broad spectrum of topics from understanding ecological and evolutionary mechanisms driving invasions, predicting future invasion patterns and dynamics, to designing and implementing effective manage- ment strategies. More than 230 presenters and attendants came from all continents to discuss the state of the art of invasion ecology and manage- ment. Until now invasion biology has to a large extent been concerned with identifying general patterns and processes, for example particular plant traits or natural enemy release, to explain why, and ultimately to aid in predicting which, taxa become invasive (Richardson and Pysek 2006). Some novel approaches to these problems were discussed during the conference such as the use of altitudinal gradients to tease apart the rela- tive roles of climate, habitat and anthropogenic influences as drivers of invasion using multi-scale, multi-regional comparisons (Pauchard et al. 2009). The conference also reflected the growing impor- tance of an evolutionary perspective. Indeed, a large proportion of the more theoretical presenta- tions addressed evolutionary questions using mo- lecular approaches, for example to reconstruct invasion pathways and assess levels of genetic variation in introduced populations. There was also recognition that climate change will impact patterns of invasion and community invasibility. Several studies used niche modelling approaches to predict some of these changes, but apart from a presentation by Bruce Osborne few attempts to experimentally assess synergistic impacts of cli- mate change and invasion were presented. De- spite the advances, there was generally a lack of presentations which tested or introduced novel theory and mechanisms of invasion. To an extent this might reflect the difficulty of making broad generalisations across invasions. This might also represent a shift towards the use of plant inva- sions as tools to address basic ecological and evo- lutionary questions (e.g. Sax et al. 2007), for ex- ample the evolution of species range margins or breeding systems, rather than as a research topic in its own right. Although theory and management of inva- sions are usually treated separately in the litera- ture, EMAPI meetings have always provided a platform for discussion of the interaction between science and application. One of the main points discussed in the meeting was to what extent we should worry about controlling invasive species in countries with lower resources and where other conservation issues seem to be more urgent (Nunez and Pauchard 2009). However, plenary addresses by Arne Witt and Sue Milton empha- sised the negative impacts of invasive species on human livelihood in sub-Saharan Africa, for exam- ple as a drain on scarce water resources. This is also the message of the South African “Working for Water” programme (http://www.dwaf.gov.za/ wfw/) which employs annually approximately 20,000 people from underprivileged sectors of society in the control of invasive species. As noted by David Richardson, the chief-organizer, this meeting included a large number of managers coming from many developing countries of Africa. This unique opportunity to share experiences be- tween scientists and practitioners should be en- couraged in all scientific meetings, especially when dealing directly with conservation themes. Another important message we can extract from the meeting is the need to clarify the goals and implications of our management actions. © 2009 the authors; journal compilation © 2009 The International Biogeography Society — frontiers of biogeography 1.2, 2009

Book review: Fifty years of Elton

news and update ISSN 1948‐6596 book review Fifty years of Elton Fifty Years of Invasion Ecology: The Legacy of Charles Elton, by David M. Richardson (ed.) 2011, Wiley‐Blackwell, 432 pp. ISBN: 978‐1‐4443‐3586‐6 Price: £95.00 (Hardback) / £45.00 (Paperback); http://www.wiley.com/ The year 1958 marked the publishing of The Ecol‐ ogy of Invasions by Animals and Plants (henceforth EIAP) by Charles Elton. In this book, Elton examined the introduction and subsequent success of numerous non‐native species and put forth a set of hypotheses about why certain spe‐ cies become invasive (invasiveness) and why cer‐ tain communities may be more susceptible to in‐ vasion (invasibility). Fifty years after EIAP was pub‐ lished, David Richardson organized a symposium in Stellenbosch, South Africa, that brought to‐ gether 137 people from over 14 different coun‐ tries to discuss how the study of biological inva‐ sions has progressed since the publication of EIAP. Fifty Years of Invasion Ecology (henceforth Fifty Years), edited by Richardson, grew out of that symposium and provides a comprehensive exami‐ nation of what knowledge has been gleaned in the past 50 years by studying the subset of organisms that have become prolific when moved, by acci‐ dent or purpose, out of their native range. Some might find it strange for an entire book to be devoted to updates on the ideas pro‐ posed in one monograph, particularly when that monograph was written for a lay audience. Yet anyone who has read EIAP knows how captivating the book is, how many ideas about community invasibility were set out in it, and how the book set the groundwork for a field of biological inquiry that now generates thousands of publications per year. Whether or not Elton founded the field of invasion biology (and this is covered by a chapter by Daniel Simberloff), it is clear that EIAP was an “important milestone in the history of invasion ecology” (Richardson page xiii), and following up on the ideas Elton set forth and species he case‐ studied was a worthwhile endeavour. The book is split into seven parts, some of which are stronger than others. The first part is one of the strongest – it provides a historical per‐ spective on Elton, the field of invasion biology, and the idea of “nativeness.” The chapters in the next two parts, “Evolution and Current Dimen‐ sions of Invasion Ecology” and “New Takes on In‐ vasion Patterns” don’t hang together as well, but I’m not sure where I would have put these chap‐ ters either. The meat of the book is called “The Nuts and Bolts of Invasion Biology,” and it is these nine chapters that people interested in determin‐ ing the state of knowledge on ecological and evo‐ lutionary explanations of invasion success will be most keen to read. Almost all of the chapters cite very recent literature, including papers and books published in 2010. The fifth part contains two chapters on “poster‐child invaders”, while the penultimate part marks new directions and technologies being used to detect and evaluate the spread and im‐ pact of invasive species. This part has one of the more polemical chapters in the book, by Mark Davis. He critiques invasion ecology for relying on niche‐based paradigms of community assembly and for overstating conclusions. Although this chapter is a rearrangement of one in his 2009 book Biological Invasions, I liked the inclusion of a chapter in Fifty Years that cautions against falling into the same “mess” that permeates community ecology. The link to Elton and EIAP are tenuous for some chapters, but that doesn’t matter because almost every chapter provides a comprehensive, synthetic review of a topic of interest to ecolo‐ gists. Interested in the state of the diversity‐ invasibility debate? Read chapter 10 by Jason Fridley. Interested in new techniques to detect invasive species at the point of entry? Read chap‐ ter 22 on the possibility of hand‐held devices to perform DNA barcoding on the spot. In addition, several chapters discuss whether propagule pres‐ sure, the number of times and number of indi‐ viduals of a species that are introduced, can ex‐ © 2011 the authors; journal compilation © 2011 The International Biogeography Society — frontiers of biogeography 3.2, 2011

Commentary: Effects of community invasion across multiple spatial scales

commentary: Effects of community invasion across multiple spatial scales

Microbial-mediated feedbacks of leaf litter on invasive plant growth and interspecific competition

Feedbacks between plants and soil microbes can play an important role in competition between potential invaders and the resident community. However, the role of saprophytic soil microbes is poorly understood because research largely focuses on the role of specific soil-borne pathogens. Our aim was to examine the role of plant-saprophyte feedbacks in soil processes (decomposition and enzyme activities) and plant competition. We preconditioned a common soil in microcosms by decomposing litter of four species; two exotic invasive species (Microstegium vimineum and Berberis thunbergii) and two native species (Viburnum acerifolium and Vaccinium corymbosum). We then replaced the litter with either the same species’ litter or a different species’ litter on the preconditioned soil. We measured the effect of preconditioning on subsequent litter decomposition, microbial community structure (phospholipid fatty acids) and function (soil enzyme activities and decomposition). We then grew Berberis and Viburnum seedlings in preconditioned soils under intraspecific and interspecific competition to determine whether litter preconditioning had a feedback effect on competition. Changes in microbial community structure during preconditioning persisted through time and altered subsequent soil enzyme activities and litter decomposition. These changes also affected the growth rate of two shrub species, but because both shrubs grew best in soil that previously contained Berberis litter, competition between these species was not directly affected. Plant litter creates a legacy that influences the future structure of the microbial community even after that litter is gone. This legacy effect has functional consequences on decomposition and plant growth, and could be an important but under-appreciated factor in soil and plant community ecology. Further study is needed to determine how these consequences affect plant community composition and invasibility.

Effects of tree sapling diversity and nutrient addition on herb-layer invasibility in communities of subtropical species

Exotic species are assumed to alter ecosystem functioning. However, little is known of the relationships within vertically structured plant communities such as forests, where tree saplings interact with herbaceous species, especially in the early phases of succession. This relationship was tested in a common garden experiment which assessed the impacts on tree saplings and herbaceous species following nutrient addition and the introduction of exotic herb species. The experiment was established in South- East China using four broad-leaved tree species (Elaeocarpus decipiens, Schima superba, Castanea henryi and Quercus serrata) to study the relationships between tree sapling diversity, herb-layer productivity and invasibility. Tree saplings were planted in monoculture and in mixtures of two and four species. A full factorial design was applied, within which species composition was crossed with nutrient and exotic seed-addition treatments. The seed-addition treatment included mixtures of seeds from eight exotic herb species, and herb community attributes were assessed after a four month growing season. Results indicate that certain tree species negatively affect native as well as exotic herbs; however, the high productivity of native herbs had a stronger negative impact on exotic species than tree saplings. Nutrient addition increased the productivity of exotic herbs but had no effect on native herbs. Remarkably, exotic species introduction had a negative feedback effect on the growth of tree saplings, which highlights the potential of exotic herbs to diminish tree recruitment. Although tree saplings reduced invasive effects on the herb-layer during the earliest phase of forest succession, nutrient addition had a more profound and opposite effect on these invaders.

Invasion by the leguminous tree Acacia dealbata (Mimosaceae) reduces the native understorey plant species in different communities

Acacia dealbata Link (Mimosaceae) is a woody leguminous tree introduced to Europe from Australia that has become a serious environmental problem in north-western Spain, where it forms dense monospecific patches threatening native-plant biodiversity. We describe the effects of invasion by A. dealbata on understorey plant composition in shrubland, oak- and pine-forest communities in north-western Spain. Plant species richness and composition, plant density and total plant cover were measured and compared among patches with different invasion status (invaded, transition and non-invaded patches) in each studied community. A clear effect of the community type and invasion on the understorey vegetation was observed. In general, composition of shrubland was different from that of oak and pine forests. We found significant effects of invasion status on species richness, plant density and total plant cover; values were significantly lower in invaded than non-invaded patches. Invasion by A. dealbata also was associated with changes in species composition. In total, eight native species, including the endemic Daboecia cantabrica (thuds) K. Koch, were confined to non-invaded patches and were replaced by other natives in invaded and transition patches. Our results suggest that although A. dealbata represented a serious threat to all of the study communities, the severity of the impact depended on the community type.

A meta-analysis of the ecological significance of density in tree invasions

Species richness, resource availability, and disturbance are the primary factors considered in assessing the invasibility of plant communities. Nonetheless, the density of individuals in a community is a common and easy trait to measure. The ecological significance of the density of both native and invasive tree species was assessed using a systematic review and formal meta-analysis. The densities of recipient communities and invasive exotic tree species in novel ranges were identified in the published literature. In addition, we compared by means of a meta-analysis: (i) densities of invasive versus native species in invaded communities; (ii) densities of native species in invaded versus uninvaded communities; and (iii) densities of invasive species along distance gradients from initial locus of invasion. Invasive trees were found at higher densities than native species in recipient communities. Invasions by woody species were also recorded in communities with relatively low densities of natives suggesting that (i) low density forests may be more susceptible to invasion and/or (ii) density of the recipient community may be reduced during the invasion process. In addition, comparison of native species densities between invaded and uninvaded stands from the same community suggests that invasive trees negatively affect density of native trees once established. Therefore, the widely reported low density and often richness of native plants in invaded communities cannot be directly linked to ecosystem susceptibility to invasion without considering concomitant impacts. These findings suggest that density is a key preliminary determinant or factor which should be considered when assessing tree invasion dynamics.

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

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

Extreme climatic events change the dynamics and invasibility of semi-arid annual plant communities

Extreme climatic events represent disturbances that change the availability of resources. We studied their effects on annual plant assemblages in a semi-arid ecosystem in north-central Chile. We analysed 130 years of precipitation data using generalised extreme-value distribution to determine extreme events, and multivariate techniques to analyse 20 years of plant cover data of 34 native and 11 exotic species. Extreme drought resets the dynamics of the system and renders it susceptible to invasion. On the other hand, by favouring native annuals, moderately wet events change species composition and allow the community to be resilient to extreme drought. The probability of extreme drought has doubled over the last 50 years. Therefore, investigations on the interaction of climate change and biological invasions are relevant to determine the potential for future effects on the dynamics of semi-arid annual plant communities.

Testing the mutualism disruption hypothesis: physiological mechanisms for invasion of intact perennial plant communities

Soil resources derived from mutualistic arbuscular mycorrhizal fungi (AMF) play a critical role in the physiological function of many native plant species. Allelopathic plant invasion studies have revealed declines in AMF inoculation potential of invaded soils, and lost opportunities for plants to form new AMF associations. Yet, if allelochemicals also kill AMF external hyphae already associated with plant roots, this mutualism disruption should result in physiological stress for native plants. We previously demonstrated that forest soils infested with garlic mustard (Alliaria petiolata), an allelopathic invader, exhibit reduced fungal hyphal abundance. Here, we demonstrate for the first time that treatment with garlic mustard tissue reduces soil respiration rates and diminishes physiological function of false Solomon's seal (Maianthemum racemosum), an AMF-dependant forest understory native. Treated plants exhibited reduced stomatal conductance and photosynthesis relative to controls, consistent with the proposed loss of AMF function. Such physiological declines, if sustained over several growing seasons, could decrease native understory perennials' growth rates and increase their susceptibility to environmental stresses. These data provide an explicit mechanism that can help explain the loss of established native perennials from invaded mature forests. We propose that the physiological costs of mutualism disruption may be a widespread but previously untested mechanism enhancing the invasion of undisturbed ecosystems by allelopathic species.