Dispersal, defined as the movement of individuals among local communities in a landscape, is a central regional determinant of metacommunity dynamics in ecosystems. Whereas both natural and anthropogenic ecosystem fragmentations can limit dispersal, previous attempts to measure such limitations have faced considerable context dependency, due to a combination of spatial extent and associated environmental variability, the wide range of dispersal modes, and abilities of organisms or variation in network topologies. Therefore, the role dispersal plays compared to local environmental filtering in explaining metacommunity dynamics remains unclear in fragmented dendritic ecosystems. We quantified α‐ and β‐diversity components of invertebrate metacommunities across 10 fragmented headwater stream networks and tested the hypothesis that dispersal is the primary determinant of biodiversity organisation in these dynamic and spatially constrained ecosystems. Alpha‐diversity was much lower in intermittent than perennial reaches, even long after rewetting, indicating an overwhelming effect of drying including a legacy effect on local communities. Beta‐diversity was never correlated with environmental distances but predominantly explained by spatial distances accounting for river network fragmentation. The nestedness proportion of β‐diversity was considerable and reflected compositional differences where communities from intermittent reaches were subsets of perennial reaches. Altogether, these results indicate dispersal as the primary process shaping metacommunity dynamics in these 10 headwater stream networks, where local communities recurrently undergo extinction and recolonisation events. This challenges previous conceptual views that local environment filtering is the main driver of headwater stream metacommunities. As river networks become increasingly fragmented due to global change, our results suggest that some freshwater ecosystems currently driven by local environment filtering could gradually become dispersal‐limited. In this perspective, shifts from perennial to intermittent flow regimes represent ecological thresholds that should not be crossed to avoid jeopardising river biodiversity, functional integrity, and the ecosystem services they provide to society.

In the Anthropocene, river networks are globally threatened by human activities, creating dramatic consequences for aquatic biodiversity. Permanent and temporary fragmentation by manmade structures and drying events, respectively, are both increasing in rivers worldwide. Although both of these fragmentation types can limit species dispersal, their individual and combined effects in shaping metacommunity dynamics at both local (site-level) and regional (network-level) scales have not been explored. Here, we examined whether processes structuring aquatic invertebrate metacommunities vary through time in response to flow variability in a river network affected by drying and severe fragmentation by manmade structures. We also compared the relative influences of permanent and temporary fragmentation on metacommunity dynamics and hypothesized that permanent fragmentation would be the primary determinant of metacommunity dynamics. We conducted an intensive sampling effort (30 sampling sites  6 dates) and measured 12 local environmental variables and 4 spatial distances to assess the relative importance of niche-and dispersal-based processes on benthic invertebrate metacommunities across hydrological phases. Spatial distances considered here were overland, network, and 2 fragmentation distances integrating the permanent fragmentation by manmade structures and temporary fragmentation by drying. We used Mantel tests to identify relationships between community dissimilarities and environmental and spatial distance matrices. We identified temporal variability in metacommunity processes with a predominant role of dispersal and no effect of niche-based processes. Metacommunities were shaped primarily by permanent fragmentation, whereas we detected no effect of fragmentation by network-scale drying. Dispersal-based metacommunity processes varied over time, following the wet-dry cycles that characterize dynamic river networks. Our results suggest the importance of key hydrological phases that should be incorporated into conservation perspectives. In addition, we highlight the need to quantify context dependency in metacommunity studies to optimize biodiversity conservation strategies in river-network ecosystems.

In group living, species spatial distribution results from responses to environmental heterogeneity and/or mutual interactions between individuals. These mutual interactions can be regulated by genetic and/or epigenetic factors. In this study, we focus on genetic factors and investigate how the spatial distribution of some individuals colonizing a new environment is influenced by inbreeding.Our biological model is Tetranychus urticae, a phytophagous mite considered as a major pest of many cultivated plants. Groups of T. urticae were composed of individuals from successive inbreeding (sister-brother sib-mating). Our results show that the interindividual distances increase with inbreeding. Indeed, inbreeding level seems to be an important factor affecting the intra-plant spatial distribution of mites. These results confirm that mites have the capability to discriminate their kin and, moreover, that they are able to accurately perceive differences between close relatives from sib-mating lines.

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Tetranychus urticae (Acari: Tetranychidae) is a phytophagous mite that forms colonies of several thousand individuals. Like spiders, every individual produces abundant silk strands and is able to construct a common web for the entire colony. Despite the importance of this silk for the biology of this worldwide species, only one previous study suggested how to visualize it. To analyze the web structuration, we developed a simple technique to dye T. urticae'silk on both inert and living substrates. Fluorescent brightener 28 (FB) (Sigma F3543) diluted in different solvents at different concentrations regarding the substrate was used to observe single strands of silk. On glass lenses, a 0.5% dimethyl sulfoxide solution was used and on bean leaves, a 0.1% aqueous solution. A difference of silk deposit was observed depending the substrate: rectilinear threads on glass lenses and more sinuous ones on bean leaves. This visualizing technique will help to carry out future studies about the web architecture and silk used by T. urticae. It might also be useful for the study of other silk-spinning arthropods.