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Coinvasive ectomycorrhizal (ECM) fungi allow Pinaceae species to invade regions otherwise lacking compatible symbionts, but ECM fungal communities permitting Pinaceae invasions are poorly understood. In the context of Pinaceae invasions on Isla Victoria, Nahuel Huapi National Park, Argentina, we asked: what ECM fungi are coinvading with Pinaceae hosts on Isla Victoria; are some ECM fungal species or genera more prone to invade than others; and are all ECM fungal species that associate with Northern Hemisphere hosts also nonnative, or are some native fungi compatible with nonnative plants? We sampled ECMs from 226 Pinaceae host plant individuals, both planted individuals and recruits, growing inside and invading from plantations. We used molecular techniques to examine ECM fungal communities associating with these trees. A distinctive subset of the ECM fungal community predominated far from plantations, indicating differences between highly invasive and less invasive ECM fungi. Some fungal invaders reported here have been detected in other locations around the world, suggesting strong invasion potential. Fungi that were frequently detected far from plantations are often found in early-successional sites in the native range, while fungi identified as late-successional species in the native range are rarely found far from plantations, suggesting a means for predicting potential fungal coinvaders.

Roads are known to act as corridors for dispersal of plant species. With their variable microclimate, role as corridors for species movement and reoccurring disturbance events, they show several characteristics that might influence range dynamics of both native and non-native species. Previous research on plant species ranges in mountains however seldom included the effects of roads. To study how ranges of native and non-native species differ between roads and adjacent vegetation, we used a global dataset of plant species composition along mountain roads. We compared average elevation and range width of species, and used generalized linear mixed models (GLMMs) to compile their range optimum and amplitude. We then explored differences between roadside and adjacent plots based on a species’ origin (native vs non-native) and nitrogen and temperature affinity.Most non-native species had on average higher elevational ranges and broader amplitudes in roadsides. Higher optima for non-native species were associated with high nitrogen and temperature affinity. While lowland native species showed patterns comparable to those in non-native species, highland native species had significantly lower elevational ranges in roadsides compared to the adjacent vegetation.We conclude that roadsides indeed change the elevational ranges of a variety of species. These changes are not limited to the expansion of non-native species along mountain roads, but also include both upward and downward changes in ranges of native species. Roadsides may thus facilitate upward range shifts, for instance related to climate change, and they could serve as corridors to facilitate migration of alpine species between adjacent high-elevation areas. We recommend including the effects of mountain roads in species distribution models to fine-tune the predictions of range changes in a warming climate.

Tree invasions have escalated in importance in the last few decades (more species, greater area invaded, more types of impacts, increasing complexity of management challenges), and are increasingly studied from many perspectives. This research spans many disciplines, including ecology, population biology, genetics, remote sensing, ecological modelling, risk analysis, resource economics and, increasingly, the humanities. There has been substantial progress in understanding patterns and processes, but many unanswered questions remain. Only a few invasive trees have been well studied, many of them in only a small part of their invasive range. Invasive trees often have substantial impacts, especially when they invade formerly treeless vegetation. Trees have several features that make them useful for understanding key aspects of biological invasions (the determinants of invasiveness and invasibility), but also the full spectrum of human perceptions and values that frames biological invasions as an environmental problem. This editorial provides background and summarizes the main outputs from a workshop held in Argentina in September 2012 that set out to summarize current knowledge on key topics and to determine the most important challenges facing researchers and managers. The sixteen papers in the special issue of Biological Invasions span disciplines, geographic regions and taxa and provide novel insights on pathways and historical perspectives, detection and monitoring, determinants of invasiveness, function and impact, and the many challenges that face managers.

There is a strong bias concerning the regions of the globe where research on biological invasions is conducted, with notably lower representation of developing countries. However, in developing countries, effective management strategies to control invasions could be more beneficial in conserving global biodiversity since these countries tend to have larger, highly diverse natural habitats. Lower levels of development are seen as an obstacle to tackling biological invasions, but little thought is given to the advantages of developing countries in dealing with invasive species. We analyzed differences between developed and developing countries regarding the problem of invasive species and their historical and current patterns of international trade, disturbance levels and land use, research and monitoring, control and mitigation, and social awareness. Developed nations have some advantages, especially in levels of social awareness and means for controlling and studying exotics, but developing nations also enjoy important advantages given their lower levels of international trade and the availability of low-cost labor. Also, there is evidence that the process of economic development, which results in more efficient ways to transform landscapes and increases international trade, is strongly associated with increasing rates of biological invasion. Differences in data quality and availability between developed and developing countries make comparative analyses of biological invasions a difficult task. Thus, these differences creates a challenge in forming global strategies to deal with invasions. There have been calls for creating international plans to deal with invasive species, but we believe that it is important first to acknowledge the challenges and understand both the advantages and disadvantages of developing countries.

Human activity is creating a global footprint by changing the climate, altering habitats and reshuffling the distribution of species. The movement of species around the globe has led to the naturalization and accumulation of multiple non-native species within ecosystems, which is frequently associated with habitat disturbance and changing environmental conditions. However, interactions among species will also influence community composition, but little is known about the full range of direct and indirect interactions among native and non-native species. Here, we show through a meta-analysis of 1,215 pairwise plant interactions between 274 vascular plant species in 21 major habitat types that interactions between non-native plants are asymmetrical with interactions between non-native and native plants. Non-native plants were always bad neighbours, but the negative effect of non-natives on natives was around two times greater than the effect of non-natives on other non-natives. In contrast, the performance of non-native plants was five times higher in the presence of a neighbouring native plant species than in the presence of a neighbouring non-native plant species. Together, these results demonstrate that invaded plant communities may accumulate additional non-native species even if direct interactions between non-natives species are negative. Put another way, invasions may be more likely to lead to more invasions, requiring more active management of ecosystems by promoting native species restoration to undermine invasive positive feedback and to assist native species recovery in invaded ecosystems. Invading plant species have direct and indirect effects on both native and other non-native species. This meta-analysis finds that while non-native plants negatively affect all their neighbours, they affect natives around twice as harshly as other non-natives.

Why some species become invasive while others do not remains an elusive question. It has been proposed that invasive species should depend less on mutualisms, because their spread would then be less constrained by the availability of mutualistic partners. We tested this idea with the genus Pinus, whose degree of invasiveness is known at the species level (being highly and negatively correlated with seed size), and which forms obligate mutualistic associations with ectomycorrhizal fungi (EMF). Mycorrhizal dependence is defined as the degree to which a plant needs the mycorrhizal fungi to show the maximum growth. In this regard, we use plant growth response to mycorrhizal fungi as a proxy for mycorrhizal dependence. We assessed the responsiveness of Pinus species to EMF using 1,206 contrasts published on 34 species, and matched these data with data on Pinus species invasiveness. Surprisingly, we found that species that are more invasive depend more on mutualisms (EMF). Seedling growth of species with smaller seeds benefited more from mutualisms, indicating a higher dependence. A higher reliance on EMF could be part of a strategy in which small-seeded species produce more seeds that can disperse further, and these species are likely to establish only if facilitated by mycorrhizal fungi. On the contrary, big-seeded species showed a lower dependence on EMF, which may be explained by their tolerance to stressful conditions during establishment. However, the limited dispersal of larger seeds may limit the spread of these species. We present strong evidence against a venerable belief in ecology that species that rely more on mutualisms are less prone to invade, and suggest that in certain circumstances greater reliance on mutualists can increase spread capacity.

PurposePinaceae (pine family) trees are native to the Northern Hemisphere and their invasion into the Southern Hemisphere is a growing problem threatening biological diversity. Pinaceae are ectomycorrhizal (ECM) and their invasions are facilitated by non-native and co-invasive ECM fungi. Nothofagaceae species (southern beeches) are dominant overstory trees across large swaths of the Southern Hemisphere and are the only widespread ECM trees native to southern South America (SSA). This observational study investigates the in situ impact of Pinaceae invasions upon native soil fungi associated with Nothofagaceae hosts in SSA.MethodsWe performed soil nutrient testing and metabarcode sequencing of fungi in the rhizosphere of Nothofagus antarctica and Nothofagus dombeyi invaded by Pinaceae trees to determine whether co-invasive fungi might impact native soil fungi. Sampling transects extended from invasions into adjacent Nothofagus stands without invasive Pinaceae.ResultsThe fungal community composition of the Nothofagaceae rhizosphere was dominated by plant-associated Mortierellaceae OTUs in metabarcode data. Mortierellaceae OTU relative abundance was significantly reduced near invasions of Pinus contorta (Pinaceae). Invasions of Pseudotsuga menziesii (Pinaceae) and Pinus contorta were associated with reduced relative abundance of Nothofagus-associated ECM OTUS in the Nothofagus rhizosphere. Pinus contorta invasions were also associated with reduced soil organic matter, total carbon, total phosphorus, and total nitrogen.ConclusionFurther empirical study is warranted to investigate the hypothesis that Mortierellaceae and Pinaceae-specific /suillus-rhizopogon ECM fungi compete for nutrients bound in soil organic matter. Such competition may have potential long-term legacy effects upon post-invasion restoration efforts and implications for Pinaceae invasions globally.

Invasive woody species produce significant ecological and economic threats globally, disrupting native biodiversity and ecosystem functions. Detecting factors that favour nonnative species recruitment could help design management strategies to reduce their negative impacts. In the Patagonian steppe, nonnative pines affect larger areas, which is a growing problem. The native community is distributed in patches dominated by cushion shrubs, with vertical and deep roots, and tussock grasses, with shallow roots. Cushion shrubs may facilitate pine recruitment by generating a nurse effect. Meanwhile, shallow roots of grasses could function as a recruitment filter as soil growing space is already occupied. We monitored Pinus contorta saplings in 4m2 plots. We determined distance to nearest seed source, quantified shrub and grass cover and neighbouring root biomass under each sapling. 82% of saplings recruited next to shrubs (shrub cover = 37%), up to 200 m from seed source. These saplings were 20% taller and had 80% less neighbouring root biomass than next to grasses. Only 10% recruited next to grasses (grass cover = 19%), only up to 50 m from the seed source. The remaining 8% recruited in bare ground (bare ground cover = 44%). Shrubs facilitate pine recruitment by promoting nonnative conifer growth and reducing belowground competition, while grasses and bare ground inhibit it. Our results can be used to help design invasive pine management strategies, such as promoting high-density sheep grazing (holistic ranching), which could reduce the abundance of both pines and shrub species around plantations.

The ideal-weed hypothesis predicts that invasive plants should be less dependent on mutualisms. However, evidence in favor of or against this hypothesis comes mainly from observational studies. Here, we experimentally tested this hypothesis using a two-factor greenhouse experiment, comparing the seedling growth response of different Pinus species (varying in invasiveness) to ectomycorrhizal fungal inoculation. Most species showed no response until they were 6 mo old, at which point inoculation increased growth between 10 and 260% among the different species. This growth response was higher for species with lower seed mass, higher dispersal ability, higher Z score (a proxy for invasiveness) and higher number of naturalized regions, all of which correspond to higher invasiveness. Our results show that timing is a crucial factor when comparing mycorrhizal dependency of different species. Dependence on mutualistic microorganisms could be part of a strategy in which invasive species produce smaller seeds, in greater number, which can disperse further, but where seedlings are more reliant on mycorrhizas to improve access to water, nutrients, and protection from pathogens. Our results suggest that reliance on mutualisms may enhance, rather than limit, nonnative species in their ability to spread, establish, and colonize.

Knowing which species will become invasive has been the holy grail of invasion biology. A survey of woody plants was conducted in 2001 on an island (Isla Victoria) where 135 non-native woody plant species had been introduced 80 years previously. The survey showed that nearly 10% of introduced species had spread further than 100 m from the original sites. For some introduced species on this island longer time lags between introduction and invasion (or further spread) could be at play. To test if new invasions have begun since the original survey or if ongoing invasions have spread further, we repeated the survey a decade later. The proportion of introduced species that became invasive did not increase between surveys, suggesting that longer time lags may not reveal major invasions from new species on this island. However, we found that the relative frequency of taller individuals (above 2 m) has increased, suggesting a change in age structure of these invasive populations. Further, we found that woody invader densities and maximum heights have increased since the original survey, suggesting that woody plant invasion is progressing, and for some species even accelerating. These results highlight the importance of research on how long invasions of long-lived woody plants can take, providing key data to guide long-term monitoring of sites with multiple non-native plant introductions.