Aim In response to climate change, species distribution shifts resulting from local extinctions, colonizations and variations in population abundances potentially lead to community‐level reorganizations. Here, we assess changes over time in stream fish communities, quantify the extent to which these changes are attributable to population declines or increases and identify their main drivers. Location France. Time period 1980–2012. Major taxa studied Stream fish species. Methods We used abundance‐monitoring data to quantify changes in composition and uniqueness for 332 stream fish communities between a cold historical period (1980–1993) and a warm contemporary period (2004–2012). Then, we used a model‐averaging procedure to test the impacts of factors related to climate, land use and non‐native species density and their interacting effects in shaping community reorganization. Results We observed biotic homogenization over time in stream fish communities, although some communities experienced differentiation. Changes in composition mainly resulted from population declines and were favoured by an increase in temperature seasonality and in non‐native species density. Population declines decreased with fragmentation and changes in non‐native species density, whereas population increases were negatively driven by changes in precipitation and positively by fragmentation. Our results provide evidence that environmental changes can interact with other factors (e.g., upstream–downstream, fragmentation intensity) to determine community reorganization. Main conclusions In the context of global change, fish assemblage reorganizations mainly result from population declines of species. These reorganizations are spatially structured and driven by both climatic and human‐related stressors. Here, we emphasize the need to take into account several components of global change, because the interplay between stressors might play a key role in the ongoing biodiversity changes.

Ecological theory suggests that communities are not random combinations of species but rather the results of community assembly processes filtering and sorting species that are able to coexist together. To date, such processes (i.e., assembly rules) have been inferred from observed spatial patterns of biodiversity combined with null model approaches, but relatively few attempts have been made to assess how these processes may be changing through time. Specifically, in the context of the ongoing biodiversity crisis and global change, understanding how processes shaping communities may be changing and identifying the potential drivers underlying these changes become increasingly critical. Here, we used time series of 460 French freshwater fish communities and assessed both functional and phylogenetic diversity patterns to determine the relative importance of two key assembly rules (i.e., habitat filtering and limiting similarity) in shaping these communities over the last two decades. We aimed to (a) describe the temporal changes in both functional and phylogenetic diversity patterns, (b) determine to what extent temporal changes in processes inferred through the use of standardized diversity indices were congruent, and (c) test the relationships between the dynamics of assembly rules and both climatic and biotic drivers. Our results revealed that habitat filtering, although already largely predominant over limiting similarity, became more widespread over time. We also highlighted that phylogenetic and trait-based approaches offered complementary information about temporal changes in assembly rules. Finally, we found that increased environmental harshness over the study period (especially higher seasonality of temperature) led to an increase in habitat filtering and that biological invasions increased functional redundancy within communities. Overall, these findings underlie the need to develop temporal perspectives in community assembly studies, as understanding ongoing temporal changes could provide a better vision about the way communities could respond to future global changes.

Aim: Multiple hypotheses predict how gradients of nutrient availability, plant biomass, and temperature shape trophic pyramids. We aim to disentangle the simultaneous influence of those factors and their indirect effects on trophic structure and individual trophic levels. Location: United States. Time period: 2017. Major taxa studied: Invertebrates. Methods: To examine differences in trophic pyramid shape and abundance within trophic levels and across ecological gradients, we conducted 54 standardized surveys of invertebrate communities in North American grasslands. We tested for the direct and indirect effects of plant biomass, temperature, sodium (Na), other essential elements (e.g. N, P, and K), and toxic heavy metals, (e.g. Ar and Pb) in plant tissue on both individual trophic levels, and trophic pyramid shape, estimated as the community trophic mean (CTM). Results: Plant sodium increased CTM, indicating that high plant sodium concentrations are associated with top-heavy invertebrate trophic pyramids. Sites with higher plant biomass had higher proportions of herbivores compared to higher trophic levels. Finally, increasing temperature resulted in more top-heavy trophic pyramids. Overall, plant biomass, temperature, and plant chemistry directly and indirectly affected the abundances within different trophic levels, highlighting the complexity of factors regulating trophic structure. Main conclusions: Trophic structure of grassland invertebrate communities is strongly influenced by plant sodium, plant biomass, and to a lesser extent, temperature. Grasslands occupy 30% of the Earth's terrestrial surface and are an imperiled ecosystem due to conversion to row crop agriculture. As biogeochemistry and temperature in the Anthropocene are increasingly modified, our results have considerable implications for the trophic structure of future grassland communities.

In response to climate change, earlier phenological events have been reported for a large range of taxa such that phenological shifts are considered as one of the fingerprints of the effect of climate change on organisms. Evidence further suggests that changes in the timing of phenological events might decouple biotic interactions due to differential phenological adjustment among interacting species, ultimately leading to population declines. Nonetheless, few studies have investigated how climate-driven changes in the timing of phenological events influence population abundances. In this study, we investigated how two environmental variables known to influence the migration timing of freshwater fish (i.e. water discharge and temperature) directly or indirectly influenced abundances of 21 fish species using daily time series gathered at four sites located in France over a period spanning from 9 to 21 years. We found no evidence for long-term trends in migration timing or fish abundances over time. Using piecewise structural equation models, we demonstrate that inter-annual variations in abundances were driven by inter-annual variations in temperature through variations in migration timing. Overall, our results suggest that climate change may concomitantly influence different biological aspects (e.g. phenology, abundance) of fish species. We argue that considering different responses to climate change is paramount if we are to improve our understanding of how organisms and populations are influenced by climate change in order to set-up efficient conservation strategies.

Biodiversity encompasses multiple facets, among which taxonomic, functional and phylogenetic aspects are the most often considered. Understanding how those diversity facets are distributed and what are their determinants has become a central concern in the current context of biodiversity crisis, but such multi‐faceted measures over large geographical areas are still pending. Here, we measured the congruence between the biogeographical patterns of freshwater fish morphological, ecological and phylogenetic diversity across Europe and identified the natural and anthropogenic drivers shaping those patterns. Based on freshwater fish occurrence records in 290 European river catchments, we computed richness and evenness for morphological, ecological and phylogenetic diversity using standardized effect sizes for each diversity index. We then used linear models including climatic, geo‐morphological, biotic and human‐related factors to determine the key drivers shaping freshwater fish biodiversity patterns across Europe. We found a weak spatial congruence between facets of diversity. Patterns of diversity were mainly driven by elevation range, climatic seasonality and species richness while other factors played a minor role. Finally, we found that non‐native species introductions significantly affected diversity patterns and influenced the effects of some environmental drivers. Morphological, ecological and phylogenetic diversity constitute complementary facets of fish diversity rather than surrogates, testifying that they deserve to be considered altogether to properly assess biodiversity. Although the same environmental and anthropogenic factors overall explained those diversity facets, their relative influence varied. In the current context of global change, non‐native species introductions may also lead to important reshuffling of assemblages resulting in profound changes of diversity patterns.

Aim Understanding how species assemble into assemblages and identifying the determinants of assembly processes remains a key challenge in ecology. Within assemblages, functional trait dispersion can be used to infer assembly processes, but this inference could depend on the trait considered. Here, using both single and multiple trait‐based approaches, we analysed dispersion patterns for alpha (i.e., related to niche partitioning) and beta (i.e., related to environmental tolerance) traits in freshwater fish assemblages and characterized how trait dispersion patterns vary along environmental gradients at large spatial scale. Location Western Palaearctic, 290 river catchments. Time period Contemporary. Major taxa studied Freshwater fish. Methods Based on freshwater fish occurrence records in 290 European river catchments, we computed dispersion indices (mean pairwise distance) using standardized effect sizes for each single trait and multiple traits. We then used linear models including climatic, geo‐morphological, biotic and human‐related factors to determine the key drivers shaping freshwater fish dispersion patterns across Europe. Results We highlighted spatial variation in trait dispersion, with both underdispersion and overdispersion simultaneously observed for a given trait, but also distinct patterns of trait dispersion, even within beta and alpha traits. We provided evidence that elevation range and current and past climatic conditions mainly structured trait dispersion patterns. Finally, our results revealed that spatial patterns in trait dispersion based on multiple traits were less pronounced than those based on individual traits. Main conclusions Our results highlighted that traits showed different spatial and environmental patterns, reflecting different ecological patterns. This could lead to potential problems when using functional indices computed on multiple traits and challenges their relevance to describe diversity patterns and to infer the assembly processes shaping community structure.

of multiple functions in different meta-ecosystems, 2) functional trait analogues across taxa, 3) dimension responses and effects on gradients of meta-ecosystems and 4) matching of functionally analogous traits between trophic levels.

Thresholds and tipping points are frequently used concepts to address the risks of global change pressures and their mitigation. It is tempting to also consider them to understand biodiversity change and design measures to ensure biotic integrity. Here, we argue that thresholds and tipping points do not work well in the context of biodiversity change for conceptual, ethical, and empirical reasons. Defining a threshold for biodiversity change (a maximum tolerable degree of turnover or loss) neglects that ecosystem multifunctionality often relies on the complete entangled web of species interactions and invokes the ethical issue of declaring some biodiversity dispensable. Alternatively defining a threshold for pressures on biodiversity might seem more straightforward as it addresses the causes of biodiversity change. However, most biodiversity change appears to be gradual and accumulating over time rather than reflecting a disproportionate change when transgressing a pressure threshold. Moreover, biodiversity change is not in synchrony with environmental change, but massively delayed through inertia inflicted by population dynamics and demography. In consequence, formulating environmental management targets as preventing the transgression of thresholds is less useful in the context of biodiversity change, as such thresholds neither capture how biodiversity responds to anthropogenic pressures nor how it links to ecosystem functioning. Instead, addressing biodiversity change requires reflecting the spatiotemporal complexity of altered local community dynamics and temporal turnover in composition leading to shifts in distributional ranges and species interactions.