▪ Abstract  In the search to identify factors that make some plant species troublesome invaders, many studies have compared various measures of native and alien invasive plant performance. These comparative studies provide insights into the more general question “Do alien invasive plants usually outperform co-occurring native species, and to what degree does the answer depend on growing conditions?” Based on 79 independent native-invasive plant comparisons, the alien invaders were not statistically more likely to have higher growth rates, competitive ability, or fecundity. Rather, the relative performance of invaders and co-occurring natives often depended on growing conditions. In 94% of 55 comparisons involving more than one growing condition, the native's performance was equal or superior to that of the invader, at least for some key performance measures in some growing conditions. Most commonly, these conditions involved reduced resources (nutrients, light, water) and/or specific disturbance regimes. Independently of growing conditions, invaders were more likely to have higher leaf area and lower tissue construction costs (advantageous under high light and nutrient conditions) and greater phenotypic plasticity (particularly advantageous in disturbed environments where conditions are in frequent flux). There appear to be few “super invaders” that have universal performance advantages over co-occurring natives; rather, increased resource availability and altered disturbance regimes associated with human activities often differentially increase the performance of invaders over that of natives.

The assumption that climatic niche requirements of invasive species are conserved between their native and invaded ranges is key to predicting the risk of invasion. However, this assumption has been challenged recently by evidence of niche shifts in some species. Here, we report the first large-scale test of niche conservatism for 50 terrestrial plant invaders between Eurasia, North America, and Australia. We show that when analog climates are compared between regions, fewer than 15% of species have more than 10% of their invaded distribution outside their native climatic niche. These findings reveal that substantial niche shifts are rare in terrestrial plant invaders, providing support for an appropriate use of ecological niche models for the prediction of both biological invasions and responses to climate change.

Assessing whether the climatic niche of a species may change between different geographic areas or time periods has become increasingly important in the context of ongoing global change. However, approaches and findings have remained largely controversial so far, calling for a unification of methods. Here, we build on a review of empirical studies of invasion to formalize a unifying framework that decomposes niche change into unfilling, stability, and expansion situations, taking both a pooled range and range-specific perspective on the niche, while accounting for climatic availability and climatic analogy. This framework provides new insights into the nature of climate niche shifts and our ability to anticipate invasions, and may help in guiding the design of experiments for assessing causes of niche changes.

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.

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.altitudinal gradient | dispersal | invasibility | nestedness | Rapoport effect S everal factors are known to shape species richness patterns along elevational gradients, notably energetic constraints on primary productivity and species-area relationships (1, 2). However, these factors are often highly correlated, making it difficult to assign causality, especially because species richness patterns are the result of both contemporary and historical ecological and evolutionary forces. High-elevation floras are typically composed of species with narrow climatic ranges and specialized ecophysiological adaptations to low temperatures, such as low stature, slow growth rates, and freezing resistance (3). Because richness gradients emerge from the overlap of individual species ranges, some authors have generated null models for richness patterns by assuming that species ranges are placed at random within a bounded elevational domain (4, 5). This usually produces a mid-domain effect, with richness peaking at mid-elevations where the overlap of species ranges is greatest. Indeed, such mid-elevation peaks do occur, and at least some of them can be explained by the overlap at ecotones of species adapted to different parts of the gradient (6).Although there is a long tradition of studies on elevational richness patterns of native species, little is known about similar phenomena in nonnative species. Nearly 1,000 nonnative plant species have been recorded from mountains throughout t...