AimWe hypothesized that mechanisms underlying beta diversity in rivers would differ between gradients where (1) natural stressors result in progressive species turnover with high specificity and (2) anthropogenic stressors result in the loss of specialist taxa thus giving rise to nestedness.Location Great Britain, the Iberian Peninsula and the Himalayan Mountains. MethodsWe analysed five datasets describing benthic macroinvertebrates sampled along natural (elevation, salinity) and anthropogenic (acidity, metals, land use) stress gradients. Predictions were tested by fitting models relating species richness and beta-diversity components (total, turnover and nestedness dissimilarities) to putative stress intensity (i.e. the degree to which a particular environmental constraint filters species occurrence). ResultsStress intensity accounted for most of the variability in species richness (r 2 = 0.64-0.93), which declined with increasing stress. Dissimilarity in community composition between locations increased with the difference in stress intensity for all datasets. For natural stressors, beta-diversity patterns mainly reflected species turnover, whilst for anthropogenic stressors beta diversity mainly reflected nesting of subsets of species as stress intensity increased. Main conclusionsOur results support the hypothesis that natural and anthropogenic stressors generate contrasting patterns in beta diversity that arise through different mechanisms.

Aim  In aquatic ecosystems, standing (lentic) and running (lotic) waters differ fundamentally in their stability and persistence, shaping the comparative population genetic structure, geographical range size and speciation rates of lentic versus lotic lineages. While the drivers of this pattern remain incompletely understood, the suite of traits making up the ability of a species to establish new populations is instrumental in determining such differences. Here we explore the degree to which the association between habitat type and geographical range size results from differences in dispersal ability or fundamental niche breadth in the members of the Enochrus bicolor complex, an aquatic beetle clade with species across the lentic–lotic divide.Location  Western Mediterranean, with a special focus on North Africa, the Iberian Peninsula and Sicily.Methods  DNA sequences for four loci were obtained from species of the E. bicolor complex and analysed using phylogenetic inference. Dispersal and establishment abilities were assessed in lentic–lotic species pairs of the complex, using flight wing morphometrics and thermal tolerance ranges as surrogates, respectively.Results  There were clear differences in range size between the lotic and lentic taxa of the complex, which appears to have had a lotic origin with two transitions to standing waters. Only small differences were observed in temperature tolerance and acclimation ability between the two lotic–lentic sister species studied. By contrast, wing morphometrics revealed clear, consistent differences between lotic and lentic Enochrus species pairs, the latter having a higher dispersal capacity.Main conclusions  We hypothesize that there have been two habitat shifts from lotic to lentic waters, which have allowed marked expansions in geographical range size in western Mediterranean species of the E. bicolor complex. Differences in dispersal rather than in establishment ability appear to underlie differences in geographical range extent, as transitions to lentic waters were associated with changes in wing morphology, but not in thermal tolerance range. In this lineage of water beetles, selection for dispersal in geologically short‐lived lentic systems has driven the evolution of larger range sizes in lentic taxa compared with those of their lotic relatives.

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Under global change, the ion concentration of aquatic ecosystems is changing worldwide. Many freshwater ecosystems are being salinized by anthropogenic salt inputs, whereas many naturally saline ones are being diluted by agricultural drainages. This occurs concomitantly with changes in other stressors, which can result in additive, antagonistic or synergistic effects on organisms. We reviewed experimental studies that manipulated salinity and other abiotic stressors, on inland and transitional aquatic habitats, to (i) synthesize their main effects on organisms' performance, (ii) quantify the frequency of joint effect types across studies and (iii) determine the overall individual and joint effects and their variation among salinity–stressor pairs and organism groups using meta-analyses. Additive effects were slightly more frequent (54%) than non-additive ones (46%) across all the studies ( n = 105 responses). However, antagonistic effects were dominant for the stressor pair salinity and toxicants (44%, n = 43), transitional habitats (48%, n = 31) and vertebrates (71%, n = 21). Meta-analyses showed detrimental additive joint effects of salinity and other stressors on organism performance and a greater individual impact of salinity than the other stressors. These results were consistent across stressor pairs and organism types. These findings suggest that strategies to mitigate multiple stressor impacts on aquatic ecosystems should prioritize restoring natural salinity concentrations. This article is part of the theme issue ‘Salt in freshwaters: causes, ecological consequences and future prospects’.

Salinity is one of the most important drivers of the distribution, abundance and diversity of organisms. Previous studies on the evolution of saline tolerance have been mainly centred on marine and terrestrial organisms, while lineages inhabiting inland waters remain largely unexplored. This is despite the fact that these systems include a much broader range of salinities, going from freshwater to more than six times the salinity of the sea (i.e. >200 g/L). Here, we study the pattern and timing of the evolution of the tolerance to salinity in an inland aquatic lineage of water beetles (Enochrus species of the subgenus Lumetus, family Hydrophilidae), with the general aim of understanding the mechanisms by which it was achieved. Using a time-calibrated phylogeny built from five mitochondrial and two nuclear genes and information about the salinity tolerance and geographical distribution of the species, we found that salinity tolerance appeared multiple times associated with periods of global aridification. We found evidence of some accelerated transitions from freshwater directly to high salinities, as reconstructed with extant lineages. This, together with the strong positive correlation found between salinity tolerance and aridity of the habitats in which species are found, suggests that tolerance to salinity may be based on a co-opted mechanism developed originally for drought resistance.

1The Segura River Basin is one of the most arid and regulated zones in the 2 Mediterranean as well as Europe that includes four hydrologic river types, according to 3 their natural flow regime: main stem rivers, stable streams, seasonal streams and 4 temporary streams. The relationships between flow regime and fluvial and riparian 5 habitats were studied at reference and hydrologically-altered sites for each of the four 6 types. Flow regime alteration was assessed using two procedures: 1) an indirect index, 7 derived from variables associated with the main hydrologic pressures in the basin, and 82) reference and altered flow series analyses using the Indicators of Hydrologic 9 Alteration (IHA) and the Indicators of Hydrologic Alteration in Rivers (IAHRIS). 10Habitats were characterized using the River Habitat Survey (RHS) and its derived 11Habitat Quality Assessment (HQA) score, whereas riparian condition was assessed 12 using the Riparian Quality Index (RQI) and an inventory of riparian native/exotic 13 species. Flow stability and magnitude were identified as the main hydrologic drivers of 14 the stream habitats in the Segura Basin. Hydrologic alterations were similar to those 15 described in other Mediterranean arid and semiarid areas where dams have reduced flow 16 magnitude and variability and produced the inversion of seasonal patterns. Additionally, 17 the Segura Basin presented two general trends: an increase in flow torrentiality in main 18 stems and an increase in temporality in seasonal and temporary streams. With the 19 indirect alteration index, main stems presented the highest degree of hydrologic 20 alteration, which resulted in larger channel dimensions and less macrophytes and 21 mesohabitats. However, according to the hydrologic analyses, the seasonal streams 22 presented the greatest alteration, which was supported by the numerous changes in 23 habitat features. These changes were associated with a larger proportion of uniform 24 banktop vegetation as well as reduced riparian native plant richness and mesohabitat 25 density. Both stream types presented consequent reductions in habitat and riparian 26 quality as the degree of alteration increased. However, stable streams, those least 27 impacted in the basin, and temporary streams, which are subject to great hydrologic 28 stress in reference conditions, showed fewer changes in physical habitat due to 29 hydrologic alteration. This study clarifies the relationships between hydrologic regime 30 and physical habitat in Mediterranean basins. The hydrologic and habitat indicators that 31 respond to human pressures and the thresholds that imply relevant changes in habitat 32 and riparian quality presented here will play a fundamental role in the use of holistic 33 frameworks when developing environmental flows on a regional scale. 34 2