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The globalization of trade and increased human mobility have facilitated the introduction and spread of nonnative species, posing significant threats to biodiversity and human well-being. As centers of global trade and human populations, cities are foci for the introduction, establishment, and spread of nonnative species. We present a global synthesis of urban characteristics that drive biological invasions within and across cities, focusing on four axes: (a) connectivity, (b) physical properties, (c) culture and socioeconomics, and (d) biogeography and climate. Urban characteristics such as increased connectivity within and among cities, city size and age, and wealth emerged as important drivers of nonnative species diversity and spread, while the relative importance of biogeographic and climate drivers varied considerably. Elaborating how these characteristics shape biological invasions in cities is crucial for designing and implementing strategies to mitigate the impacts of invasions on ecological systems and human well-being. [ABSTRACT FROM AUTHOR]

Understanding the relationship between biodiversity and ecosystem functioning (BEF) is crucial to predicting the consequences of ongoing global biodiversity loss. However, what drives BEF relationships in natural ecosystems under globally changing conditions remains poorly understood.To address this knowledge gap, we applied a trait‐based approach to data from coastal dune plant communities distributed along a natural environmental stress gradient. Specifically, we compared the relative importance of below‐ground and above‐ground traits in predicting productivity, decomposition, water regulation, carbon stock and nutrient pools, and tested how these BEF relationships were modulated by environmental stress and the presence of rare species that are typically excluded from experimental systems.Below‐ground traits were just as important as above‐ground traits in driving ecosystem functioning. Moreover, despite having low abundances, rare species positively influenced ecosystem multifunctionality (EMF). However, most biodiversity effects became weaker as environmental stress increased.Our study shows that to understand variation in ecosystem functioning we must consider below‐ground traits as much as above‐ground ones. Moreover, it highlights the importance of conserving rare species for maintaining EMF. However, our findings also suggest that rapid global change could dampen the positive effects of diversity on ecosystem functioning. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Question: Specialization refers to the degree of niche breadth of a species. Generalist species are able to persist in a broad range of habitats, whereas specialist species are adapted to a restricted range of environmental conditions. Cities host a great heterogeneity of habitats with variable degrees of human impact. This is generally reflected in the functional composition of the urban floras. The aim of our study is thus to explore whether the degree of functional specialization of urban plant assemblages varies among habitats subject to different degrees of human impact and disturbance regimes. Location: Thirty‐two cities in Central Europe with more than 100,000 inhabitants. Methods: We used a data set containing plots of urban floras sampled in seven habitat types within each city: historical city square, boulevard, residential area with a compact building pattern, residential area with an open building pattern, city park, early successional site, and mid‐successional site. These habitats differ in the level of human impact, ranging from moderately urbanized suburban habitats to the most urbanized habitats in the city center. For each plot, we calculated a recently introduced specialization index, which is based on the application of concentration measures to Grime's community‐level mean CSR strategies. Results: Along the urbanization gradient, from peripheral to central habitats, we observed a marked intensification in the degree of functional specialization of urban habitats, which is primarily attributable to an increase in the selection of ruderal species. Conclusions: Urban ecosystems are characterized by a wide variety of human impacts that affect the functioning of the resident species. Considering cities as heterogeneous systems is thus of paramount importance for understanding the mechanisms that drive the assembly of urban floras. [ABSTRACT FROM AUTHOR]

Trait‐based ecology has already revealed main independent axes of trait variation defining trait spaces that summarize plant adaptive strategies, but often ignoring intraspecific trait variability (ITV). By using empirical ITV‐level data for two independent dimensions of leaf form and function and 167 species across five habitat types (coastal dunes, forests, grasslands, heathlands, wetlands) in the Italian peninsula, we found that ITV: (i) rotated the axes of trait variation that define the trait space; (ii) increased the variance explained by these axes and (iii) affected the functional structure of the target trait space. However, the magnitude of these effects was rather small and depended on the trait and habitat type. Our results reinforce the idea that ITV is context‐dependent, calling for careful extrapolations of ITV patterns across traits and spatial scales. Importantly, our study provides a framework that can be used to start integrating ITV into trait space analyses. [ABSTRACT FROM AUTHOR]