Most studies of invasive species have been in highly modified, lowland environments, with comparatively little attention directed to less disturbed, high‐elevation environments. However, increasing evidence indicates that plant invasions do occur in these environments, which often have high conservation value and provide important ecosystem services. Over a thousand non‐native species have become established in natural areas at high elevations worldwide, and although many of these are not invasive, some may pose a considerable threat to native mountain ecosystems. Here, we discuss four main drivers that shape plant invasions into high‐elevation habitats: (1) the (pre‐)adaptation of non‐native species to abiotic conditions, (2) natural and anthropogenic disturbances, (3) biotic resistance of the established communities, and (4) propagule pressure. We propose a comprehensive research agenda for tackling the problem of plant invasions into mountain ecosystems, including documentation of mountain invasion patterns at multiple scales, experimental studies, and an assessment of the impacts of non‐native species in these systems. The threat posed to high‐elevation biodiversity by invasive plant species is likely to increase because of globalization and climate change. However, the higher mountains harbor ecosystems where invasion by non‐native species has scarcely begun, and where science and management have the opportunity to respond in time.

Summary 1.In many European agricultural landscapes, species richness is declining considerably. Studies performed at a very large spatial scale are helpful in understanding the reasons for this decline and as a basis for guiding policy. In a unique, large-scale study of 25 agricultural landscapes in seven European countries, we investigated relationships between species richness in several taxa, and the links between biodiversity and landscape structure and management. 2. We estimated the total species richness of vascular plants, birds and five arthropod groups in each 16-km 2 landscape, and recorded various measures of both landscape structure and intensity of agricultural land use. We studied correlations between taxonomic groups and the effects of landscape and land-use parameters on the number of species in different taxonomic groups. Our statistical approach also accounted for regional variation in species richness unrelated to landscape or land-use factors. 3. The results reveal strong geographical trends in species richness in all taxonomic groups. No single species group emerged as a good predictor of all other species groups. Species richness of all groups increased with the area of semi-natural habitats in the landscape. Species richness of birds and vascular plants was negatively associated with fertilizer use. 4. Synthesis and applications. We conclude that indicator taxa are unlikely to provide an effective means of predicting biodiversity at a large spatial scale, especially where there is large biogeographical variation in species richness. However, a small list of landscape and land-use parameters can be used in agricultural landscapes to infer large-scale patterns of species richness. Our results suggest that to halt the loss of biodiversity in these landscapes, it is important to preserve and, if possible, increase the area of semi-natural habitat.

Despite intensive research, we still have no general understanding of why plant invasions occur. Many different mechanisms of plant invasions have been proposed, but studies designed to investigate them often produce inconsistent results. It remains unclear whether this unsatisfying state of affairs reflects the complexity of the real world (in which every invasion is unique) or the failure to identify the key processes driving most plant invasions. Here we argue that greater generalization is possible, but only if we recognize that the ecological and evolutionary processes enabling a species to advance into a new area change during the course of an invasion. In our view, an invasion can often usefully be subdivided into a primary phase, in which the abundance of an often preadapted species increases rapidly (typically in resource-rich, disturbed habitats), and a secondary phase, in which further spread is contingent upon plastic responses or genetic adaptation to new ecological circumstances. We present various examples to show how this partitioning of the invasion phase sensu stricto produces new hypotheses about the processes underlying plant invasions. Some of these hypotheses can be conveniently tested by investigating plant invasions along strong environmental gradients such as those that occur in mountainous regions.

Aim To investigate how species richness and similarity of non-native plants varies along gradients of elevation and human disturbance.Location Eight mountain regions on four continents and two oceanic islands. MethodsWe compared the distribution of non-native plant species along roads in eight mountainous regions. Within each region, abundance of plant species was recorded at 41-84 sites along elevational gradients using 100-m 2 plots located 0, 25 and 75 m from roadsides. We used mixed-effects models to examine how local variation in species richness and similarity were affected by processes at three scales: among regions (global), along elevational gradients (regional) and with distance from the road (local). We used model selection and information criteria to choose best-fit models of species richness along elevational gradients. We performed a hierarchical clustering of similarity to investigate human-related factors and environmental filtering as potential drivers at the global scale. ResultsSpecies richness and similarity of non-native plant species along elevational gradients were strongly influenced by factors operating at scales ranging from 100 m to 1000s of km. Non-native species richness was highest in the New World regions, reflecting the effects of colonization from Europe. Similarity among regions was low and due mainly to certain Eurasian species, mostly native to temperate Europe, occurring in all New World regions. Elevation and distance from the road explained little of the variation in similarity. The elevational distribution of non-native species richness varied, but was always greatest in the lower third of the range. In all regions, non-native species richness declined away from roadsides. In three regions, this decline was steeper at higher elevations, and there was an interaction between distance and elevation. Main conclusionsBecause non-native plant species are affected by processes operating at global, regional and local scales, a multi-scale perspective is needed to understand their patterns of distribution. The processes involved include global dispersal, filtering along elevational gradients and differential establishment with distance from roadsides.

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Due to altered ecological and evolutionary contexts, we might expect the responses of alien plants to environmental gradients, as revealed through patterns of trait variation, to differ from those of the same species in their native range. In particular, the spread of alien plant species along such gradients might be limited by their ability to establish clinal patterns of trait variation. We investigated trends in growth and reproductive traits in natural populations of eight invasive Asteraceae forbs along altitudinal gradients in their native and introduced ranges (Valais, Switzerland, and Wallowa Mountains, Oregon, USA). Plants showed similar responses to altitude in both ranges, being generally smaller and having fewer inflorescences but larger seeds at higher altitudes. However, these trends were modified by region-specific effects that were independent of species status (native or introduced), suggesting that any differential performance of alien species in the introduced range cannot be interpreted without a fully reciprocal approach to test the basis of these differences. Furthermore, we found differences in patterns of resource allocation to capitula among species in the native and the introduced areas. These suggest that the mechanisms underlying trait variation, for example, increasing seed size with altitude, might differ between ranges. The rapid establishment of clinal patterns of trait variation in the new range indicates that the need to respond to altitudinal gradients, possibly by local adaptation, has not limited the ability of these species to invade mountain regions. Studies are now needed to test the underlying mechanisms of altitudinal clines in traits of alien species.

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