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Effective long-term management of invasive non-native species (INNS) in South America is a pressing yet complex task. Critically, the environmental, historical, cultural, and economic idiosyncrasies of the region call for the inclusion of a plurality of views from those sectors of society receiving the negative and positive impacts of INNS. This is a multifaceted, and often daunting, task that can be aided by an early identification of stakeholders – those affected by or with an interest in INNS and their management – accompanied by targeted stakeholder engagement. Here, we report the procedures and results of a stakeholder mapping activity aimed at identifying stakeholders and designing engagement strategies. Using expert knowledge procedures, we compiled comprehensive lists of stakeholders for six case-studies in South America: (i) invasive grasses (Urochloa spp.) in Brazil; (ii) glossy privet (Ligustrum lucidum) in Argentina; (iii) lodgepole and Monterey pines (Pinus contorta and P. radiata) in Argentina; (iv) American mink (Neogale vison) in Argentina and Chile; (v) lodgepole and Monterey pines in Chile; and (vi) German yellow-jacket (Vespula germanica) in Chile. Overall, we identified 250 stakeholders, which, based on their interest and influence, were classified into "context settlers" (2%), "key players" (47%), "crowd" (5%), and "subjects" (49%). We outlined strategies to engage with each of these four groups and for each of our six case-studies. Across case studies, communication with stakeholders was the most common engagement strategy proposed (27%; 19 of 70 strategies), followed by active involvement of stakeholders in INNS research and management (23%). Our results highlight the importance of considering power imbalances, as those stakeholders more likely to benefit from INNS were assessed to have more influence over INNS management relative to local and indigenous communities. Our work illustrates how to identify stakeholders in a rigorous and rapid manner, which should be complemented with the involvement of the stakeholders themselves. [ABSTRACT FROM AUTHOR]

Invasive plant species (IPs) are widespread in forests and cause substantial environmental, economic and social impacts. They occupy native ecological niches, causing local extinctions to the detriment of native biodiversity and disrupting ecosystem services provision. How landscape characteristics may determine the success of IPs remains unclear and, more importantly, how land-use and land-cover changes may result in spatial shifts in the invasion risk. Furthermore, the study of how landscape factors may influence biological invasions has focused on particular species, but not the IPs' community. In this study, we identify and assess landscape variables that influence the presence and distribution of the IPs' community in temperate forests of a global biodiversity hotspot in south-central Chile. We fitted spatially explicit models, combining field-sampling information and landscape variables related to land-use/land-cover, topography, climate, soil characteristics and anthropogenic factors to explain and predict the presence and distribution of the IPs' community. From the whole sampling of plant species, we identified eight plant species classified as IPs: three trees and five shrubs. We used field data from 125 500 × 2 m-transects, in which we registered species richness, abundance and basal area of IPs' community. Distance to forest plantations was the landscape variable with the most substantial influence on IPs' presence and distribution. Richness, abundance and basal area of IPs' trees were higher at shorter distances from forest plantations. The basal area of IPs' trees was the best model explaining the relationship between IPs' community and landscape variables. All descriptors of the IPs' community showed similar spatial patterns: species richness, abundance and tree basal area are higher in more disturbed areas. Our findings contribute to increasing our understanding of the distribution patterns of IPs in forest landscapes. Our models can be suitable tools for designing strategies to prevent, mitigate or make integrated control of the impacts of invasive species in forest landscapes. [ABSTRACT FROM AUTHOR]

Pinus contorta is considered one of the most invasive tree species worldwide, generating significant impacts on biodiversity and ecosystems. In several Patagonian ecosystems in southern Chile, it has escaped from plantations established mainly in the 1970s, and is now invading both forests and treeless environments. In this study, we evaluated the impact of the invasion of P. contorta on microenvironmental conditions in Araucaria araucana forest and Patagonian steppe ecosystems, and assessed how these changes related to the richness and abundance of native and non-native plant species. In each ecosystem, 24 plots of 100 m2 were established along a gradient of P. contorta biomass, where 18 environmental variables and the composition of native and non-native vegetation were measured at a local scale. Our results indicated that increased pine biomass was associated with differences in microclimatic conditions (soil and air temperature, photosynthetically active radiation (PAR), and soil moisture) and soil properties (potassium, nitrate, pH, and litter accumulation). These changes were ecosystem dependent, however, as well as associated with the level of invasion. Finally, the reduction in the richness and abundance of native plants was associated with the changes in soil properties (accumulation of leaf litter, pH, and organic matter) as well as in the microclimate (minimum air temperature, PAR) generated by the invasion of P. contorta. Overall, our results confirm that the invasion of P. contorta impacts microenvironmental conditions (i.e., canopy cover, litter accumulation, minimum air temperature, and maximum soil temperature) and reduces native plant diversity. For future restoration plans, more emphasis should be given to how environmental changes can influence the recovery of invaded ecosystems even after the removal of the living pine biomass (i.e., legacy of the invasion). [ABSTRACT FROM AUTHOR]

Formulating effective management plans for addressing the impacts of invasive non-native species (INNS) requires the definition of clear priorities and tangible targets, and the recognition of the plurality of societal values assigned to these species. These tasks require a multi-disciplinary approach and the involvement of stakeholders. Here, we describe procedures to integrate multiple sources of information to formulate management priorities, targets, and high-level actions for the management of INNS. We follow five good-practice criteria: justified, evidence-informed, actionable, quantifiable, and flexible. We used expert knowledge methods to compile 17 lists of ecological, social, and economic impacts of lodgepole pines (Pinus contorta) and American mink (Neovison vison) in Chile and Argentina, the privet (Ligustrum lucidum) in Argentina, the yellow-jacket wasp (Vespula germanica) in Chile, and grasses (Urochloa brizantha and Urochloa decumbens) in Brazil. INNS plants caused a greater number of impacts than INNS animals, although more socio-economic impacts were listed for INNS animals than for plants. These impacts were ranked according to their magnitude and level of confidence on the information used for the ranking to prioritise impacts and assign them one of four high-level actions—do nothing, monitor, research, and immediate active management. We showed that it is possible to formulate management priorities, targets, and high-level actions for a variety of INNS and with variable levels of available information. This is vital in a world where the problems caused by INNS continue to increase, and there is a parallel growth in the implementation of management plans to deal with them. [ABSTRACT FROM AUTHOR]

Here we present a multi-taxa inventory of naturalized alien species recorded on continental Chile and adjacent marine habitats, including eight taxonomic groups. We identified 1,122 species. These comprise 790 vascular plants (terrestrial and aquatic); 31 nonvascular plants [Bryophyta (mosses), Marchantiophyta (liverworts) and Anthocerotophyta (hornworts)]; 18 marine and freshwater macro and micro algae; 71 fungi; 39 terrestrial vertebrates (amphibians, reptiles, mammals and birds); 108 insects; 37 marine and freshwater invertebrates and vertebrates (6 polychaetes, 3 mollusks and 28 Pisces); and 28 terrestrial gastropods. For all taxonomic groups, naturalized species were found to mainly be distributed in regions with Mediterranean and temperate climates, with few at either extreme of the country. The invasion curves show that naturalized species first underwent a positive increment, followed by an apparent plateau phase, mainly in vascular plants, insects and vertebrates. In fungi, marine and freshwater macro and microalgae, vertebrates and invertebrates, the cumulative number of naturalized species increased sharply starting in the early 20th century; the lack of collections before 1900 is also evident. When considering naturalized species as a whole, this inventory highlights that the rate of new naturalizations consistently increased after 1950, especially for some taxonomic groups such as insects, fungi, and vascular plants. This multi-taxa inventory of naturalized species provides a platform for national reporting on biodiversity indicators and highlights areas where Chile must invest resources to manage biological invasions. [ABSTRACT FROM AUTHOR]

Chile is a global biodiversity hotspot and hosts a large proportion of the southern hemisphere's temperate forests. The Chilean Valdivian temperate forest is a vulnerable ecosystem containing a highly ecologically valuable species assemblage. Productive forest plantations have involved deforestation of, and alien species introduction into, this ecosystem. This process has already severely impacted the western part of Chile (the Coastal Range) and is now occurring in the eastern part (the Andes), with forestry plantations promoted by government subsidies between 1974 and 2012. Archive Landsat satellite imagery classification and Google Earth Engine are used to assess land cover change over a 31-year period with a focus on alien species (Pinaceae and Eucalyptus spp.) spread and native (Nothofagus spp. and Araucaria araucana) deforestation. Results show a clear land cover pattern based on elevation: a higher altitude, relatively undisturbed area dominated by native forest (the Andes), and a lower altitude area where most human activity and related land covers are located (the valley area). The valley is highly dynamic because of constant land cover change due to forestry. Overall, Araucaria araucana cover has decreased over the study period, while Nothofagus spp. has remained relatively stable. Alien Pinaceae has decreased, while Eucalyptus spp. has remained stable. However, the results indicate that change analysis over long periods conceal dynamism. For example, Eucalyptus spp. sharply decreased between the 1980s and the 1990s and surged afterwards. Also, even though Nothofagus spp. cover dominates throughout the study period, change analysis shows a high degree of change in the valley area, indicating newly established Nothofagus spp. patches. Over the study period, long rotation Pinaceae plantations for timber have given way to shorter rotation forestry (alien Eucalyptus spp., native Nothofagus spp.) for pulp and local uses resulting from discontinuation of forest subsidies. In the absence of subsidies, only large-scale plantations can engage in long rotation forestry, as smallholders need the more stable income provided by shorter rotation forestry. Although higher elevations (the Andes) are dominated by native forest, several abandoned alien forest plantations may be the source of biological invasion. In addition, native forest degradation as a result of Araucaria araucana loss and shrub encroachment is occurring. Earth observation methods are key for forest and alien species monitoring and landscape management. They can enhance traditional, ground-based forest surveys and provide continuous and even retrospective monitoring of forest change thanks to the wide availability of current and historical satellite data. • Higher altitude areas are less disturbed and host most of the native forest. • Lower altitude areas have abundant human related land covers and are highly dynamic. • Araucaria araucana cover has decreased between the 1980s and the 2010s. • Araucaria araucana is threatened by alien tree invasion and shrub encroachment. • The end of subsidies could trigger a change from long to short rotation forestry. [ABSTRACT FROM AUTHOR]

Aim: Tree invasions are a threat to biodiversity conservation, and although it is hard to predict the future spread of invasive tree species, there are tools available which could allow some estimations. The magnitude of spatial spread (a proxy of invasiveness) can be predicted from species climatic requirement (climatic niche) and can be represented by species distribution models (SDMs). We aimed to assess whether Acacia dealbata conserves its niche in the new environment of south‐central Chile, and also, to estimate the invasive stage of the species. Location: South‐central area of Chile, between the O'Higgins (34°0″0′S) and Aysen Regions (47°0″0′S). Methods: We used a combination of global, native, and regional data to improve the estimation of the potential distribution of A. dealbata, which has been considered one of the most invasive species of the genus, being registered in at least 34 countries in all the Continents. Results: Our results show that A. dealbata does not conserve its niche in the study area, invading areas with climatic conditions different from those of the native range. It is also not at equilibrium with the environment. According to the global versus regional SDM comparisons, populations present in south‐central Chile present different invasion stages. There are some stable populations, but there are other populations colonizing new areas, occupying unsuitable habitats and some of them are adapting to new climatic conditions. Climatic factors, such as precipitation seasonality, could be acting behind the expansion to new environments, and biotic factors or dispersal limitations could be preventing the species to colonize suitable areas. Main conclusions: The invasion process of A. dealbata is far from stabilizing, and management options should focus on prevention, avoiding, for example, the introduction of the species to Patagonia where the species has not spread yet. More research is needed to complement our results and enhance the development of effective management strategies. [ABSTRACT FROM AUTHOR]

• Forestry tree species are already invading areas where have been planted. • Negative impacts should be address as soon as possible to avoid further damage. • Potential distribution help to allocate efforts and resources efficiently. • New areas are still available for further expansion of existing populations. Non-native tree (NNT) species have been deliberately introduced into new areas. Historically, NNTs were not considered as invasive species, given the benefits they provide, but that has changed during the last decades. The use of non-native trees, whichever purpose they have, should be assessed according to the location and susceptibility of the area to invasion. Forestry trees, such as Pinus and Eucalyptus have been introduced worldwide, being the most planted taxa. Here, we present a complete assessment of the actual invasive stage and potential distribution of most relevant non-native tree species established in the south-Central region of Chile (Pinus radiata, Eucalyptus globulus, Eucalyptus nitens, Pinus contorta and Pseudotsuga menziesii) based on its presences and absences, their global and regional niches, Species Distribution Models (SDM), climatic and anthropogenic variables. None of the studied species maintain the environmental space of their native range. Some of them (P. radiata, E. globulus and E. nitens) have been established under new environmental conditions, thus expanding their niches. Others, such as P. contorta and P. menziesii present a high proportion of stable populations, but there is an even higher proportion of area available for their expansion. Beyond their actual status, all e studied species are capable of expanding their populations into new areas of the studied region. Conservation efforts must be efficiently planned, to reduce these species impacts wherever necessary, but also to avoid their further dispersal into new non-invaded areas. [ABSTRACT FROM AUTHOR]

Abstract: Tree invasions cause important conservation problems, such as changes in plant community composition, reduced regeneration rates of native species, and alteration in landscape structures. One of the most invasive tree genera in the world is Acacia (Fabaceae). In Chile, Acacia dealbata Link is distributed in the mediterranean zone, mostly associated with roadsides and anthropogenic disturbances. In this paper we address the following questions: How does A. dealbata perform across a gradient of native forest and invasive stands? Will it be capable of establishing itself in non-invaded native forests and regenerating under its own canopy in the absence of disturbances? From a contrasting viewpoint, will native species such as Cryptocarya alba (Molina) Looser and Nothofagus obliqua (Mirb.) Oerst be able to survive in an A. dealbata stand, allowing re-colonization of the invaded area? We conducted survival and growth experiments on A. dealbata seedlings in three sites near Concepción, Chile (36°S–72°W) under three conditions: under Acacia stand, in a close native forest and on an intermediate matrix between these two conditions. We compared this to the performance of two native trees (Cryptocarya alba and Nothofagus obliqua). Results showed that A. dealbata and C. alba have high growth and establishment capacities within the native forest, but on the intermediate matrix only A. dealbata can grow and survive. C. alba survives at significantly higher rates than A. dealbata within Acacia stand. Nothofagus obliqua only survive at a very low rate in the native forest. Without disturbances, A. dealbata would be successful on the intermediate matrix and within native forest, while C. alba seems capable of surviving and establishing itself in invaded areas, thereby contributing to recovery and restoration of natural spaces. The interplay between Acacia dealbata and the native vegetation may show us a larger picture of how invasive species are capable of expanding even into forested ecosystems and, furthermore, how we can restore native vegetation and avoid further invasion. [Copyright &y& Elsevier]

Aim Taxonomic comparisons of alien floras across climatically similar regions have been proposed as a powerful approach for increasing our understanding of plant invasions across scales. However, detailed comparisons between the alien biotas of climatically similar regions are scarce. This study aims to compare the taxonomic patterns of alien species richness in mediterranean-type climate areas of central Chile and California, in order to better understand how climatically similar regions converge or diverge in terms of their alien flora. Location Central Chile and California, United States. Methods We compared the alien floras of the state of California in the United States and central Chile, considering within-region variation and taxonomic composition up to the species level. To test for within-region variation, administrative units and counties were grouped within seven latitudinal bands for each region. We tested for differences in the relative contributions of the various origins of the naturalized species to each region. We used a family naturalization index to establish which families had relatively higher numbers of naturalized species in each region. We evaluated the similarity, using cluster analyses with Jaccard’s similarity index, of alien taxa between regions and latitudinal bands using presence–absence matrices at the species, genus and family levels. We used principal components analysis to determine the presence of a compositional gradient including all latitudinal bands. Results We recorded 1212 alien plant species in California and 593 in central Chile, of which 491 are shared between the two regions. These figures include 25 species that are native to California and 37 that are native to Chile. A comparison between the alien floras of central Chile and California reveals three major trends: (1) higher naturalized species diversity for California than for Chile, at all taxonomic levels; (2) differences in the proportion of species according to origin, with America, Africa, Asia and Australia providing a larger number of species in California than in Chile; (3) segregation between regions in terms of taxonomic composition of their alien flora, and a rather weak differentiation within regions; and (4) a trend towards higher similarity between the alien floras of latitudinal bands associated with higher levels of human disturbances. Main conclusions The alien floras of central Chile and California are significantly different, but this difference diminishes in highly disturbed areas. Thus, the current high levels of species movement caused by globalization, together with increasing levels of anthropogenic disturbances, should reduce the differentiation of the alien floras in these regions, increasing overall biotic homogenization. [ABSTRACT FROM AUTHOR]

Natural reserves or protected areas are a keystone of global strategies for biological conservation. With over 18% of its land under protection, Chile faces challenges similar to those in other developing countries. In this paper we describe the history and status of protected areas in Chile, and identify the challenges that the country confronts for improving the conservation and social efficacy of protected areas. Following the modern "pristine" concept of protected areas, Chile created its first protected area in 1907 and the first national park in 1925. Historically, several national and local agencies were in charge of the creation and management of protected areas. In 1984, the national public system of protected areas was created to organize the scattered protected areas around a unified system that seeks to conserve Chilean natural resources. The system has several problems that jeopardize its capability to conserve Chilean biodiversity: insufficient ecosystem representation, inadequate coverage of biodiversity hot-spots, low budgets, and boundary issues. Private protected areas have recently been considered as complementary units to SNASPE. But there are questions about long-term commitment and restriction to development in such areas. The growth of ecotourism may be boosting SNASPE and private reserve initiatives, but it may also threaten the conservation of pristine environments. We propose that protected areas in Chile be part of a comprehensive conservation policy that considers the entire range of natural resources. This policy should also address new ways to conserve biodiversity outside protected area boundaries, bringing both private and public initiatives together. [ABSTRACT FROM AUTHOR]

• Alien pine and eucalyptus plantations cause biological invasions and fragmentation. • Monitoring large areas like the Chilean temperate forests can be challenging. • We used Sentinel-2 to detect alien forest stands within a matrix of native forest. • The relationship between landscape fragmentation and pine cover depends on scale. • Alien pines are a threat for future conservation of the endemic Araucaria araucana. Chile's temperate forest is a global biodiversity hotspot. An upsurge in alien forest plantations has disturbed and fragmented the landscape, promoting biological invasions. The invasion process is not fully understood since monitoring large and inaccessible areas can be prohibitively expensive and logistically challenging using field-based methods alone. Here, a remote sensing approach using Sentinel-2 satellite imagery, fragmentation analysis, and random forest modelling is applied to detect alien tree stands and describe their extent in relation to fragmentation and landscape structure in study areas around Malalcahuello National Reserve and Villarrica National Park. Detailed vegetation maps are produced, with classification accuracies >81% and including four forest classes, two native and two alien. An altitudinal pattern was observed in both sites. At lower altitudes, there was greater total area covered by alien trees and more fragmented native forests than at higher altitudes. However, Villarrica had less alien tree cover than Malalcahuello, but was a more fragmented landscape. Random forest modelling identified that alien pine tree mean patch area was positively correlated with both land cover diversity and Araucaria araucana forest mean patch area in both sites. Given their conservation and cultural relevance, the locations of protected areas need reconsidering to strengthen the protection of A. araucana , which could be outcompeted by alien trees in a context of increasing productive forestry. This is especially urgent in Villarrica, where protected areas already have a substantial presence of alien trees, with most A. araucana found outside protected areas. [ABSTRACT FROM AUTHOR]

Background: Fire has historically shaped the dynamics of Araucaria araucana-Nothofagus pumilio forests in the Andean region of South America. Nevertheless, human disturbances and stronger drought events have resulted in increased occurrence and severity of wildfires. Regardless of their conservation relevance, the initial response to fire of Araucaria-Nothofagus forests has not been well documented. Through this research we tested the hypothesis that plant composition of Araucaria-Nothofagus forest after fire will initially differ in its recovery depending on fire severity, but over time, plant similarity will increase among areas with different levels of fire severity. The study was carried out in old-growth Araucaria-Nothofagus forests in south-central Chile (38°S) that were burned in 2015. We studied vegetation response to three levels of fire severity (low (LS), moderate (MS) and high (HS) severity), also including an adjacent unburned forest (UN), which were assessed one, two and three years after fire. We also evaluated the impact of cattle on plant recovery after fire. We measured species richness, abundance, plant diversity and origin of all vascular plant species. Results: Species richness and plant abundance responded differently to fire severity. Time-since-fire had a significant effect on plant richness and abundance. Plant composition within LS areas was similar to UN areas, but MS and HS areas were significantly less diverse compared to UN and LS. In absence of other major disturbances, similarity in plant composition tended to increase over time across the different levels of fire severity, becoming more similar to UN. We also found a synergetic effect of cattle and fire on species richness. This interaction promotes the establishment of non-native species and limits the recovery of native species following fire. Conclusions: Fire severity had a significant impact on the plant community of old-growth Araucaria-Nothofagus forests, especially in areas of high fire severity, where the native tree N. pumilio was completely absent. Nevertheless, plant communities can recover gradually, depending on fire severity, time-since-fire, and the absence of cattle grazing. Management actions should include the exclusion of cattle within fire-affected areas and afforestation with N. pumilio in areas high fire severity due to its unsuccessful recruitment after fire. [ABSTRACT FROM AUTHOR]

The article focuses on a workshop conducted by the authors in Chile in July 2016 on Scottish-American naturalist John Muir's trip to Chile in 1911. Topics discussed include forest ecology and nature tourism in Chile; protection of important site reserves and increase in conservation value of Araucaria forests in Chile. Other topics include going through sketches of Muir which recorded Araucaria forests and marketing suggestions for conservation of forests.

In recent decades large fires have affected communities throughout central and southern Chile with great social and ecological consequences. Despite this high fire activity, the controls and drivers and the spatiotemporal pattern of fires are not well understood. To identify the large-scale trends and drivers of recent fire activity across six regions in south-central Chile (~32-40° S Latitude) we evaluated MODIS satellite-derived fire detections and compared this data with Chilean Forest Service records for the period 2001-2017. MODIS burned area estimates provide a spatially and temporally comprehensive record of fire activity across an important bioclimatic transition zone between dry Mediterranean shrublands/sclerophyllous forests and wetter deciduous-broadleaf evergreen forests. Results suggest fire activity was highly variable in any given year, with no statistically significant trend in the number of fires or mean annual area burned. Evaluation of the variables associated with spatiotemporal patterns of fire for the 2001-2017 period indicate vegetation type, biophysical conditions (e.g., elevation, slope), mean annual and seasonal climatic conditions (e.g., precipitation) and mean population density have the greatest influence on the probability of fire occurrence and burned area for any given year. Both the number of fires and annual area burned were greatest in warmer, biomass-rich lowland Bío-Bío and Araucanía regions. Resource selection analyses indicate fire 'preferentially' occurs in exotic plantation forests, mixed native-exotic forests, native sclerophyll forests, pasture lands and matorral, vegetation types that all provide abundant, flammable and connected biomass for burning. Structurally and compositionally homogenous exotic plantation forests may promote fire spread greater than native deciduous-Nothofagaceae forests which were once widespread in the southern parts of the study area. In the future, the coincidence of warmer and drier conditions in landscapes dominated by flammable and fuel-rich forest plantations and mixed native-exotic and sclerophyll forests are likely to further promote large fires in south-central Chile., Competing Interests: The authors have declared that no competing interests exist.

Like all obligately ectomycorrhizal plants, pines require ectomycorrhizal fungal symbionts to complete their life cycle. Pines introduced into regions far from their native range are typically incompatible with local ectomycorrhizal fungi, and, when they invade, coinvade with fungi from their native range. While the identities and distributions of coinvasive fungal symbionts of pine invasions are poorly known, communities that have been studied are notably depauperate. However, it is not yet clear whether any number of fungal coinvaders is able to support a Pinaceae invasion, or whether very depauperate communities are unable to invade. Here, we ask whether there is evidence for a minimum species richness of fungal symbionts necessary to support a pine/ectomycorrhizal fungus coinvasion. We sampled a Pinus contorta invasion front near Coyhaique, Chile, using molecular barcoding to identify ectomycorrhizal fungi. We report that the site has a total richness of four species, and that many invasive trees appear to be supported by only a single ectomycorrhizal fungus, Suillus luteus. We conclude that a single ectomycorrhizal (ECM) fungus can suffice to enable a pine invasion.

Nonnative species richness typically declines along environmental gradients such as elevation. It is usually assumed that this is because few invaders possess the necessary adaptations to succeed under extreme environmental conditions. Here, we show that nonnative plants reaching high elevations around the world are not highly specialized stress tolerators but species with broad climatic tolerances capable of growing across a wide elevational range. These results contrast with patterns for native species, and they can be explained by the unidirectional expansion of nonnative species from anthropogenic sources at low elevations and the progressive dropping out of species with narrow elevational amplitudes--a process that we call directional ecological filtering. Independent data confirm that climatic generalists have succeeded in colonizing the more extreme environments at higher elevations. These results suggest that invasion resistance is not conferred by extreme conditions at a particular site but determined by pathways of introduction of nonnative species. In the future, increased direct introduction of nonnative species with specialized ecophysiological adaptations to mountain environments could increase the risk of invasion. As well as providing a general explanation for gradients of nonnative species richness and the importance of traits such as phenotypic plasticity for many invasive species, the concept of directional ecological filtering is useful for understanding the initial assembly of some native floras at high elevations and latitudes.