Aim Metacommunity assembly mechanisms have been traditionally considered stable through time. However, in highly dynamic systems with varying local environmental conditions and patch connectivity, communities are likely to experience temporal shifts in their assembly mechanisms. Here, we used a set of perennial (PR) and intermittent (IR) rivers to assess if assembly mechanisms vary seasonally in response to flow intermittence. Location Mediterranean climate region (100,000 km2), Spain. Methods We used a modelling approach to assess the relative effect of environmental sorting and dispersal‐based processes on aquatic invertebrate metacommunities within and across river types at four distinct hydrological periods. We used local environmental variables to assess environmental sorting, and considered geographical, network and topographical distances as different dispersal surrogates. Linear mixed effect models accounting for the non‐independence of pairwise distances were used to assess the relationships between community dissimilarity and distance matrices. Results Assembly mechanisms were more temporally stable in PR than in IR. In PR, community dissimilarities were equally related to environmental and geographical distances suggesting codominance of species sorting and dispersal‐based assembly mechanisms. In IR, environmental distance best explained community dissimilarities during the dry period when flow cessation imposes strong environmental sorting, whereas metacommunity organization was much more stochastic during the rewetting period when high flows may randomly reorganize communities. Dispersal processes dominated assembly mechanisms between PR and IR during the rewetting period suggesting an increase in recolonization processes linking both river types following the dry period. Geographical and topographical distances best explained community variability, suggesting that overland dispersal dominates in river networks fragmented by drying events. Main conclusions Aquatic invertebrate metacommunity assembly mechanisms vary seasonally in response to changes in hydrological conditions. The temporal dimension should be better incorporated into metacommunity studies in highly dynamic systems such as intermittent rivers.

Metacommunity studies commonly use spatial distances or, more recently, landscape resistance measures between study sites as a surrogate for connectivity. However, local communities are connected to many other sources of colonisation than the sites included in a study, and the availability and distance to potential colonisation sources may be better surrogates for dispersal than distances to other sampling sites. Here we test the effect of habitat connectivity on the assembly of stream‐riffle macroinvertebrates with different dispersal abilities, after controlling for habitat heterogeneity and among‐site distances (62 km on average). We used a null model approach to compare observed community dissimilarity to random expectation. Significant deviations from randomness were expected due to the hierarchical structure of river networks with their increasing flux of organisms from headwaters to mainstem reaches. We found a gradual shift in dispersal‐based processes driving assembly mechanisms, from dispersal limitation in the isolated headwater streams to randomness in connected headwater and isolated mid‐order streams, and to mass effects in the most connected mid‐order streams. Weak flyers were constrained by dispersal limitation in the most isolated sites, whereas strong flyers were not restricted by the river network structure and were mainly assembled through mass effects. The approach taken was sufficient to unravel the importance of dispersal and habitat connectivity on community assembly and may therefore be particularly well suited to other large data sets with isolated sites (i.e. low geographical density of sites).

Rapid shifts in biotic communities due to environmental variability challenge the detection of anthropogenic impacts by current biomonitoring programs. Metacommunity ecology has the potential to inform such programs, because it combines dispersal processes with niche-based approaches and recognizes variability in community composition. Using intermittent rivers—prevalent and highly dynamic ecosystems that sometimes dry—we develop a conceptual model to illustrate how dispersal limitation and flow intermittence influence the performance of biological indices. We produce a methodological framework integrating physical- and organismal-based dispersal measurements into predictive modeling, to inform development of dynamic ecological quality assessments. Such metacommunity-based approaches could be extended to other ecosystems and are required to underpin our capacity to monitor and protect ecosystems threatened under future environmental changes.

Dispersal is an essential process in population and community dynamics, but is difficult to measure in the field. In freshwater ecosystems, information on biological traits related to organisms’ morphology, life history and behaviour provides useful dispersal proxies, but information remains scattered or unpublished for many taxa. We compiled information on multiple dispersal-related biological traits of European aquatic macroinvertebrates in a unique resource, the DISPERSE database. DISPERSE includes nine dispersal-related traits subdivided into 39 trait categories for 480 taxa, including Annelida, Mollusca, Platyhelminthes, and Arthropoda such as Crustacea and Insecta, generally at the genus level. Information within DISPERSE can be used to address fundamental research questions in metapopulation ecology, metacommunity ecology, macroecology and evolutionary ecology. Information on dispersal proxies can be applied to improve predictions of ecological responses to global change, and to inform improvements to biomonitoring, conservation and management strategies. The diverse sources used in DISPERSE complement existing trait databases by providing new information on dispersal traits, most of which would not otherwise be accessible to the scientific community.

Aim To analyse temporal metacommunity dynamics in river networks in relation to hydrological conditions and dispersal. Location Fifteen river reaches from the Llobregat, Besòs and Foix catchments in the North‐Eastern Iberian Peninsula. Taxon Aquatic macroinvertebrates belonging to 99 different families. Methods We sampled aquatic macroinvertebrate communities during spring in 20 consecutive years. We built two environmental distances (one related with water chemistry and another one with river flow regime) and two spatial distances (network distance and topographic distance). Then we used Mantel tests (accounting for spatial autocorrelation) to relate macroinvertebrate dissimilarity with environmental and spatial distances. Additionally, we determined the dry and wet years using the Standardized Precipitation Index (SPI) and we classified macroinvertebrate families based on their ability to fly and to drift. Finally, we ran a linear regression model including the correlation value (r) of each Mantel test as response variable and distance type (environmental or spatial), SPI, dispersal mode, their pairwise interactions and a three‐way interaction as predictor variables. Results Metacommunity organization varied over time and it was significantly affected by precipitation, which can be related to river network connectivity. The environmental filters, mainly the flow regime, were generally more important than the spatial filters in explaining community dissimilarity over the study period. However, this depended on the dispersal abilities of the organisms. Network fragmentation due to flow intermittence during the dry years significantly reduced the dispersal capacity of strong aerial dispersers, leading to spatially structured metacommunities. For strong drift dispersers, community dissimilarity patterns were generally best explained by environmental filters regardless of SPI. Main conclusions A significant temporal variation in metacommunity organization can be expected in highly dynamic systems (e.g. Mediterranean rivers) and it might depend on the dispersal modes and abilities of the organisms, since they determine the response to changes in environmental and landscape filters.

Disturbance events govern how the biodiversity of ecological communities varies in both space and time. In freshwater ecosystems, there is evidence that local and regional‐scale drivers interact to influence ecological responses to drying disturbances. However, most research provides temporal snapshots at the local scale, whereas few studies encompass a gradient of drying severity spanning multiple years. Using a dataset of rare spatiotemporal extent and detail, we demonstrate how independent and interacting local and regional‐scale factors drive shifts in the α and β diversities of communities in dynamic river ecosystems. We examined aquatic invertebrate assemblage responses to hydrological variability (as characterized by monthly observations of instream conditions) at 30 sites over a 12‐year period encompassing typical years and two severe drought disturbances. Sites varied in their disturbance regimes and hydrological connectivity at both local (i.e. site‐specific) and regional (i.e. river catchment) scales. Whereas α diversity was mainly influenced by local factors including flow permanence and the temporal extent of ponded and dry conditions, both temporal and spatial β diversities also responded to regional‐scale metrics such as the spatial extent of flow and hydrological connectivity. We observed stronger local negative responses for taxa with lower capacities to tolerate drying (i.e. resistance) and/or to recover after flow resumes (i.e. resilience), whereas taxa with functional traits promoting resilience made an increasing contribution to spatial β diversity as hydrological connectivity declined. As droughts increase in extent and severity across global regions, our findings highlight the functional basis of taxonomic responses to disturbance and connectivity, and thus advance understanding of how drying disturbances shape biodiversity in river networks. Our identification of the role of regional hydrological factors could inform catchment‐scale management strategies that support ecosystem resilience in a context of global change.

Ecological communities can remain stable in the face of disturbance if their constituent species have different resistance and resilience strategies. In turn, local stability scales up regionally if heterogeneous landscapes maintain spatial asynchrony across discrete populations—but not if large‐scale stressors synchronize environmental conditions and biological responses. Here, we hypothesized that droughts could drastically decrease the stability of invertebrate metapopulations both by filtering out poorly adapted species locally, and by synchronizing their dynamics across a river network. We tested this hypothesis via multivariate autoregressive state‐space (MARSS) models on spatially replicated, long‐term data describing aquatic invertebrate communities and hydrological conditions in a set of temperate, lowland streams subject to seasonal and supraseasonal drying events. This quantitative approach allowed us to assess the influence of local (flow magnitude) and network‐scale (hydrological connectivity) drivers on invertebrate long‐term trajectories, and to simulate near‐future responses to a range of drought scenarios. We found that fluctuations in species abundances were heterogeneous across communities and driven by a combination of hydrological and stochastic drivers. Among metapopulations, increasing extent of dry reaches reduced the abundance of functional groups with low resistance or resilience capacities (i.e. low ability to persist in situ or recolonize from elsewhere, respectively). Our simulations revealed that metapopulation quasi‐extinction risk for taxa vulnerable to drought increased exponentially as flowing habitats contracted within the river network, whereas the risk for taxa with resistance and resilience traits remained stable. Our results suggest that drought can be a synchronizing agent in riverscapes, potentially leading to regional quasi‐extinction of species with lower resistance and resilience abilities. Better recognition of drought‐driven synchronization may increase realism in species extinction forecasts as hydroclimatic extremes continue to intensify worldwide.

Air temperature at the northernmost latitudes is predicted to increase steeply and precipitation to become more variable by the end of the 21st century, resulting in altered thermal and hydrological regimes. We applied five climate scenarios to predict the future (2070-2100) benthic macroinvertebrate assemblages at 239 near-pristine sites across Finland (ca. 1200 km latitudinal span). We used a multitaxon distribution model with air temperature and modeled daily flow as predictors. As expected, projected air temperature increased the most in northernmost Finland. Predicted taxonomic richness also increased the most in northern Finland, congruent with the predicted northwards shift of many species' distributions. Compositional changes were predicted to be high even without changes in richness, suggesting that species replacement may be the main mechanism causing climate-induced changes in macroinvertebrate assemblages. Northern streams were predicted to lose much of the seasonality of their flow regimes, causing potentially marked changes in stream benthic assemblages. Sites with the highest loss of seasonality were predicted to support future assemblages that deviate most in compositional similarity from the present-day assemblages. Macroinvertebrate assemblages were also predicted to change more in headwaters than in larger streams, as headwaters were particularly sensitive to changes in flow patterns. Our results emphasize the importance of focusing protection and mitigation on headwater streams with high-flow seasonality because of their vulnerability to climate change.