Understanding how species respond to climate change is critical for forecasting the future dynamics and distribution of pests, diseases and biological diversity. Although ecologists have long acknowledged species' direct physiological and demographic responses to climate, more recent work suggests that these direct responses can be overwhelmed by indirect effects mediated via other interacting community members. Theory suggests that some of the most dramatic impacts of community change will probably arise through the assembly of novel species combinations after asynchronous migrations with climate. Empirical tests of this prediction are rare, as existing work focuses on the effects of changing interactions between competitors that co-occur today. To explore how species' responses to climate warming depend on how their competitors migrate to track climate, we transplanted alpine plant species and intact plant communities along a climate gradient in the Swiss Alps. Here we show that when alpine plants were transplanted to warmer climates to simulate a migration failure, their performance was strongly reduced by novel competitors that could migrate upwards from lower elevation; these effects generally exceeded the impact of warming on competition with current competitors. In contrast, when we grew the focal plants under their current climate to simulate climate tracking, a shift in the competitive environment to novel high-elevation competitors had little to no effect. This asymmetry in the importance of changing competitor identity at the leading versus trailing range edges is best explained by the degree of functional similarity between current and novel competitors. We conclude that accounting for novel competitive interactions may be essential to predict species' responses to climate change accurately.

Extreme climatic events (ECEs) – such as unusual heat waves, hurricanes, floods, and droughts – can dramatically affect ecological and evolutionary processes, and these events are projected to become more frequent and more intense with ongoing climate change. However, the implications of ECEs for biological invasions remain poorly understood. Using concepts and empirical evidence from invasion ecology, we identify mechanisms by which ECEs may influence the invasion process, from initial introduction through establishment and spread. We summarize how ECEs can enhance invasions by promoting the transport of propagules into new regions, by decreasing the resistance of native communities to establishment, and also sometimes by putting existing non‐native species at a competitive disadvantage. Finally, we outline priority research areas and management approaches for anticipating future risks of unwanted invasions following ECEs. Given predicted increases in both ECE occurrence and rates of species introductions around the globe during the coming decades, there is an urgent need to understand how these two processes interact to affect ecosystem composition and functioning.

Climate change and biological invasions are primary threats to global biodiversity that may interact in the future. To date, the hypothesis that climate change will favour non-native species has been examined exclusively through local comparisons of single or few species. Here, we take a meta-analytical approach to broadly evaluate whether non-native species are poised to respond more positively than native species to future climatic conditions. We compiled a database of studies in aquatic and terrestrial ecosystems that reported performance measures of non-native (157 species) and co-occurring native species (204 species) under different temperature, CO 2 and precipitation conditions. Our analyses revealed that in terrestrial (primarily plant) systems, native and non-native species responded similarly to environmental changes. By contrast, in aquatic (primarily animal) systems, increases in temperature and CO 2 largely inhibited native species. There was a general trend towards stronger responses among non-native species, including enhanced positive responses to more favourable conditions and stronger negative responses to less favourable conditions. As climate change proceeds, aquatic systems may be particularly vulnerable to invasion. Across systems, there could be a higher risk of invasion at sites becoming more climatically hospitable, whereas sites shifting towards harsher conditions may become more resistant to invasions.

The enemy release hypothesis is a common explanation for species invasions, suggesting that introduced species benefit from leaving behind natural enemies in the native range. However, any such advantage may attenuate over time. In this study, we test a prediction of this more dynamic enemy release hypothesis: that non-native plant species that became established longer ago exhibit stronger negative feedbacks with the soil. Consistent with declining enemy release over time, we found increasingly negative soil feedbacks for species established longer ago in New Zealand. Negative soil feedbacks were also stronger for more widespread species, but weaker for more locally abundant species, suggesting that species accumulate negative interactions as they spread and can be locally regulated by these interactions. We also present data to support the common assumption that relatives have similar impacts on and responses to soil communities. Together, these data highlight the dynamic nature of novel interactions arising from species introductions.

Shifts in species' phenology in response to climate change have wide-ranging consequences for ecological systems. However, significant variability in species' responses, together with limited data, frustrates efforts to forecast the consequences of ongoing phenological changes. Herein, we use a case study of three North American plant communities to explore the implications of variability across levels of organisation (within and among species, and among communities) for forecasting responses to climate change. We show how despite significant variation among species in sensitivities to climate, comparable patterns emerge at the community level once regional climate drivers are accounted for. However, communities differ with respect to projected patterns of divergence and overlap among their species' phenological distributions in response to climate change. These analyses and a review of hypotheses suggest how explicit consideration of spatial scale and levels of biological organisation may help to understand and forecast phenological responses to climate change.

Darwin acknowledged contrasting, plausible arguments for how species invasions are influenced by phylogenetic relatedness to the native community. These contrasting arguments persist today without clear resolution. Using data on the naturalization and abundance of exotic plants in the Auckland region, we show how different expectations can be accommodated through attention to scale, assumptions about niche overlap, and stage of invasion. Probability of naturalization was positively related to the number of native species in a genus but negatively related to native congener abundance, suggesting the importance of both niche availability and biotic resistance. Once naturalized, however, exotic abundance was not related to the number of native congeners, but positively related to native congener abundance. Changing the scale of analysis altered this outcome: within habitats exotic abundance was negatively related to native congener abundance, implying that native and exotic species respond similarly to broad scale environmental variation across habitats, with biotic resistance occurring within habitats.

To predict the threat of biological invasions to native species, it is critical that we understand how increasing abundance of invasive alien species (IAS) affects native populations and communities. The form of this relationship across taxa and ecosystems is unknown, but is expected to depend strongly on the trophic position of the IAS relative to the native species. Using a global metaanalysis based on 1,258 empirical studies presented in 201 scientific publications, we assessed the shape, direction, and strength of native responses to increasing invader abundance. We also tested how native responses varied with relative trophic position and for responses at the population vs. community levels. As IAS abundance increased, native populations declined nonlinearly by 20%, on average, and community metrics declined linearly by 25%. When at higher trophic levels, invaders tended to cause a strong, nonlinear decline in native populations and communities, with the greatest impacts occurring at low invader abundance. In contrast, invaders at the same trophic level tended to cause a linear decline in native populations and communities, while invaders at lower trophic levels had no consistent impacts. At the community level, increasing invader abundance had significantly larger effects on species evenness and diversity than on species richness. Our results show that native responses to invasion depend critically on invasive species’ abundance and trophic position. Further, these general abundance–impact relationships reveal how IAS impacts are likely to develop during the invasion process and when to best manage them.

Many non‐native plants in the US have become problematic invaders of native and managed ecosystems, but a new generation of invasive species may be at our doorstep. Here, we review trends in the horticultural trade and invasion patterns of previously introduced species and show that novel species introductions from emerging horticultural trade partners are likely to rapidly increase invasion risk. At the same time, climate change and water restrictions are increasing demand for new types of species adapted to warm and dry environments. This confluence of forces could expose the US to a range of new invasive species, including many from tropical and semiarid Africa as well as the Middle East. Risk assessment strategies have proven successful elsewhere at identifying and preventing invasions, although some modifications are needed to address emerging threats. Now is the time to implement horticulture import screening measures to prevent this new wave of plant invasions.