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Niche and neutral processes drive community assembly and metacommunity dynamics, but their relative importance might vary with the spatial scale. The contribution of niche processes is generally expected to increase with increasing spatial extent at a higher rate than that of neutral processes. However, the extent to what community composition is limited by dispersal (usually considered a neutral process) over increasing spatial scales might depend on the dispersal capacity of composing species. To investigate the mechanisms underlying the distribution and diversity of species known to have great powers of dispersal (hundreds of kilometres), we analysed the relative importance of niche processes and dispersal limitation in determining beta-diversity patterns of aquatic plants and cladocerans over regional (up to 300 km) and continental (up to 3300 km) scales. Both taxonomic groups were surveyed in five different European regions and presented extremely high levels of beta-diversity, both within and among regions. High beta-diversity was primarily explained by species replacement (turnover) rather than differences in species richness (i.e. nestedness). Abiotic and biotic variables were the main drivers of community composition. Within some regions, small-scale connectivity and the spatial configuration of sampled communities explained a significant, though smaller, fraction of compositional variation, particularly for aquatic plants. At continental scale (among regions), a significant fraction of compositional variation was explained by a combination of spatial effects (exclusive contribution of regions) and regionally-structured environmental variables. Our results suggest that, although dispersal limitation might affect species composition in some regions, aquatic plant and cladoceran communities are not generally limited by dispersal at the regional scale (up to 300 km). Species sorting mediated by environmental variation might explain the high species turnover of

1. The relative role of environmental and regional processes in determining patterns of biodiversity may vary across spatial scales. In inland aquatic communities, macrophytes and zooplankton have the potential to disperse over large spatial scales (hundreds of kilometres), but the role of environmental and regional processes in determining species richness (SR) at different spatial scales is poorly understood. 2. To assess the relative importance of environmental and regional processes acting on both regional (within-region) and continental (among-region) scales, we surveyed 139 waterbodies in five geographic regions (Spain, Italy, the Netherlands, Scotland and Norway) and in each region measured environmental variables, descriptors of connectivity and SR of two aquatic taxa, namely aquatic plants and cladocerans. 3. Analyses using boosted regression trees and generalised linear models, integrated with multiple imputation of missing data, indicated that both environmental and regional processes were related to local SR. 4. Total phosphorus was the main environmental correlate of SR, showing a unimodal relationship with both taxonomic groups. Conductivity and lake depth were also related to cladoceran SR, and Secchi depth was related to aquatic plant richness. 5. The SR of aquatic plants and cladocerans were significantly correlated, but this relationship was mainly indirect (i.e. determined by the effect of common environmental variables). 6. The limited role of connectivity and space (lack of spatial autocorrelation in model residuals) relative to environment suggests that dispersal limitation is not as important as the environment in explaining richness patterns for both groups within regions. However, region identity strongly influenced SR, which suggests an important effect of biogeographic factors (e.g. the level of endemicity; the number of unique species in each region) and/or dispersal limitation at the continental scale. 7. We conclude that environmental f

We studied genetic differentiation of two subarctic Daphnia species (subgenus Hyalodaphnia; Cladocera: Anomopoda) in relation to ecological and morphological diversification. Daphnia longispina and the recently discovered species Daphnia umbra are genetically differentiated based on mitochondrial 12S rDNA and internal transcribed spacer (ITS) regions. Genetic differentiation of 12S rDNA among the two sister taxa is in the range of differentiation among other Hyalodaphnia species (uncorrected genetic distance = 0.11). Despite frequent interspecific hybridization among Daphnia (Hyalodaphnia) species, we found no interspecific hybrids of D. umbra and D. longispina. D. umbra is for the first time recorded to occur in Northern Finland and Russia (Pechora Delta), and to cooccur in neighboring sympatry with Daphnia longispina across a subarctic region in northern Finland. Species are ecologically differentiated: D. umbra occurred at higher elevations, in larger and deeper water bodies than D. longispina. Species did not differ significantly in levels of ultraviolet-protective melanin pigmentation but varied with regard to environmental preferences, such as fish predation and levels of total dissolved organic carbon (DOC). These findings argue that ecological differentiation and divergent selection might have caused speciation or at least are responsible for the maintenance of reproductive isolation among subarctic Daphnia (Hyalodaphnia) species.