The concept of metacommunity (i.e., a set of local communities linked by dispersal) has gained great popularity among community ecologists. However, metacommunity research mostly addresses questions on spatial patterns of biodiversity at the regional scale, whereas conservation planning requires quantifying temporal variation in those metacommunities and the contributions that individual (local) sites make to regional dynamics. We propose that recent advances in diversity-partitioning methods may allow for a better understanding of metacommunity dynamics and the identification of keystone sites. We used time series of the 2 components of beta diversity (richness and replacement) and the contributions of local sites to these components to examine which sites controlled source-sink dynamics in a highly dynamic model system (an intermittent river). The relative importance of the richness and replacement components of beta diversity fluctuated over time, and sample aggregation led to underestimation of beta diversity by up to 35%. Our literature review revealed that research on intermittent rivers would benefit greatly from examination of beta-diversity components over time. Adequately appraising spatiotemporal variability in community composition and identifying sites that are pivotal for maintaining biodiversity at the landscape scale are key needs for conservation prioritization and planning. Thus, our framework may be used to guide conservation actions in highly dynamic ecosystems when time-series data describing biodiversity across sites connected by dispersal are available.

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Water scarcity is a global threat to freshwater biodiversity, but connecting variation in streamflow to viability of imperiled faunas remains a challenge. Here we combined time‐series modeling techniques on long‐term ecohydrological data to quantify flow–ecology relationships on native and non‐native riverine fish in the American Southwest, and simulate likely fish trajectories and “quasi‐extinction” risks in the near future. Streamflow has been declining conspicuously over the past 30 years in the Colorado and Rio Grande river basins, and year‐to‐year variation in streamflow influences the covariation between native and non‐native fish abundance. Risks of decline are high (>50%) for nearly three‐quarters of the modeled native species, and current trends in streamflow increase quasi‐extinction risk for natives (+8.5%) but reduce this risk for non‐natives (–5.9%). Hydrological changes need to be mitigated if we are to slow down the rapid replacement of native biodiversity with non‐native species in American Southwest rivers.

1. Due to their ubiquity and highly efficient passive dispersal, benthic diatoms are an ideal group in which to explore the drivers of community spatial structure and the ultimate links between such patterns and species autoecology. However, studies addressing these issues have largely overlooked unicellular taxa to date. 2. We examined the spatial order (i.e. nestedness) of a diatom meta-community of 122 lotic sites in the north-east Iberian Peninsula and the relationship between the contributions of individual species to nestedness and their niche breadths [measured using outlying mean index (OMI) analyses]. 3. We observed a significantly nested pattern in the meta-community (T = 14.05, NODF = 22.57), although idiosyncratic species accounted for a substantial fraction (34%) of the global species pool. Hydrological stability was identified as the main driver of nestedness. Highly stable, moderately stable and intermittent sites differed in nestedness and in alpha diversity, with intermittent sites being the most species poor and the least nested. 4. We also observed a significant association between nestedness and niche attributes, with idiosyncratic species exhibiting wider distributions and larger niche breadths compared with nested species. Overall, our results indicate that diatom communities inhabiting hydrologically stable rivers present a higher level of order in community spatial pattern and a higher proportion of specialist taxa than communities in intermittent streams. 5. Because recent hydroclimatic models predict an increase in stream intermittency in the Mediterranean region, our findings may help anticipate future diatom biodiversity patterns resulting from global change. We suggest that the current need to protect a diverse network of sites (as opposed to a few species-rich sites) will be even greater in the near future because local communities are not only probably to become taxonomically and functionally poorer but also less predictable, making it more difficult to implement efficient programmes for biodiversity conservation at the regional scale.

Summary Dams fragment river systems worldwide, and Mediterranean‐climate rivers, characterised by highly seasonal hydrographs and adapted biotas, are particularly impacted by flow regulation. Whereas the effects of flow regulation on hydrology, sediment transport and biodiversity have long been examined, responses at the food‐web level remain understudied. Environmental variation is a key control of food‐web structure. Thus, we predicted that flow regulation would impact food‐chain length (FCL) via changes in the flow variation regime, and we tested this prediction in a set of flow unregulated to completely regulated reaches in a Mediterranean river basin. In each reach, we characterised flow variation, together with two other putative controls of FCL (productivity and habitat size). We combined community data with carbon and nitrogen stable isotopes to estimate food‐chain length, and Bayesian mixing models allowed estimates of dietary proportions of consumers. Flow variation was paramount in controlling FCL in the studied river network, and this same control largely explained the degree of omnivory among top predators. Thus, omnivory mechanisms were the main proximate structural mechanism allowing shifts in food‐web structure and linking disturbance regimes to FCL. Our results suggest that flow regulation in Mediterranean rivers may impact food‐web structure even when no significant changes in community composition are observed. If highly variable Mediterranean streams become increasingly affected by flow regulation, the resulting more stable conditions could enhance intraguild predation and thus lengthen riverine food chains.

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Summary We analysed taxa lists from 447 individual wetlands from several ecoregions across the world using nestedness and similarity‐based multivariate analyses. We examined how similar wetland assemblages are across regions, whether variation in assemblages is ordered (nested) or unpredictable (idiosyncratic), whether individual taxa occur predictably or unpredictably across wetland habitats, and if any of these patterns differed between temporary‐ and permanent water habitats. We found that macroinvertebrate assemblages were highly nested (N = 0.947), but unexpectedly 37 of the 40 most widespread taxa (>10% occurrence) were idiosyncratic. Of the 447 wetlands, we identified 277 that shared more than 40% similarities in assemblages, were mostly nested, and clustered together in ordination space, and thus could be considered a core set of wetlands in terms of assemblage structure. Assemblages in the 170 wetlands outside this core (mostly idiosyncratic) tended to be depauperate sites in arid or high elevation areas, or alternatively taxonomically rich sites supporting numerous lotic or lacustrine organisms. The ‘Core’ itself split into two main parts, one comprised of wetlands from semi‐arid or mild climate areas dominated by strong flying insects, and the second comprised of wetlands from wetter, more northerly areas where non‐insects with passive dispersal were very prevalent. Climate and geology appear to be major controls on macroinvertebrate distributions across the set of 447 wetlands. Hydrology (temporary versus permanent) of wetlands was a lesser control on assemblage structure over the set of 447 wetlands. That wetlands are dominated by about 40 widely‐occurring macroinvertebrate taxa, and those taxa tend to occur idiosyncratically, suggests that overall assemblages across wetlands may share many similarities, but some of widespread taxa may still be missing from many individual wetlands. Why these otherwise fairly ubiquitous taxa do not occur in specific wetlands may shed important light on how those wetlands are controlled ecologically; in other words, do sites lack specific factors required by these taxa?