While biodiversity loss is undeniably a global phenomenon, an increase in taxonomic richness has recently been reported from some ecosystems and spatial scales. A striking increase in abundance and/or species richness has been documented from temperate rivers over the last 25 years, with many of the expanding species (i.e. winners) being native species. However, the lack of repeatedly collected local environmental data prevents the exploration of their niche dynamics and also makes it difficult to distinguish between possible causes.

We fill this gap by using species occurrence data from 65 pristine Czech rivers sampled in 1997-2000 and 2015. The same methods were used for sampling macroinvertebrates and measuring environmental parameters in both periods. We selected 43 winners, defined as taxonomically validated and originally non-rare native macroinvertebrate species whose occupancy increased by at least six sites between the time periods. We searched for consistent patterns of niche dynamics (i.e. (stability, expansion, and restriction) among species that might contribute most to the overall increase in species richness. Using several biological traits, we also compared the winners with the other 253 taxa collected to look for differences.

Analysis of the occurrence data showed that niche stability was by far the predominant pattern of the niche dynamics. This clearly indicates that the winners fill their original niche, with a limited contribution of niche shift or expansion, depending on the species. As no significant differences in either temperature preferences or the other biological traits were found between the winners and the other taxa, there is no unique set of functional traits that explain the success of the winners.

The observed mechanism of filling the original niche space by the spreading native species not only explains the increase in local species richness, but also contributes to support the hypothesis of a climate-driven increase in ecosystem energy flow from a new perspective. The increased metabolism of the system may relax interspecific competition allowing to carry more individuals and species, even without the need for an increase in nutrients and ecosystem recovery.

Data on aquatic macroinvertebrates and local environmental variables were collected from 65 streams of 2–7th stream Strahler order and altitudes of 150–800 m a.s.l. across all main river basins of the Czech Republic (map available in Zhai et al. 2023). The sites represented the least modified sites in terms of physical conditions, water quality and riparian zone and floodplain characteristics met the requirements for type-specific reference conditions according to the Water Framework Directive, 2000/60/EC30 (Nijboer et al. 2004). The initial selection of these sites also considered their long-term water quality development prior to the first sampling campaign (Kokeš et al. 2006), which resulted in the exclusion also of streams affected by acidification in the 1980s. This selection is the reason why both high-elevation streams and large lowland rivers are poorly represented in the dataset as a fraction of them fulfil the requirements to be considered pristine or at least near natural in case of low-altitude rivers.

The sampling of macroinvertebrates was done during spring (April–May) and autumn (September–October) in two periods: 1997–2000 (initial period of the first sampling campaign) and 2015 (final period, the last sampling), using the same standardized methodology of a multi-habitat sampling method. For details on data collection and sample processing see Zhai et al. (2023). Data from spring and autumn samples of the same period were pooled for each site. The identification was done to species whenever possible, except for Diptera, some Oligochaeta and Coleoptera larvae that were identified to genera. Due to uncertainties of identifications in the initial period, all Chironomidae taxa had to be excluded from the dataset and several species of Ephemeroptera, Plecoptera, Trichoptera, and Oligochaeta had to be pooled into operational taxonomic units. This study did not require ethical approval, nor were any licenses or permits needed to carry out the work.

Three criteria were applied to define the winners suitable for temporal niche change exploration: (a) unequivocal and reliable identification to species level, (b) occurrence frequency in the initial period of ≥6 (i.e., 10% frequency), (c) increase in the occurrence frequency in the terminal period at least by 6 sites. From the total of 296 taxa, these three criteria were met by 43 of them (Table S1, Fig. S1b of this study). On the contrary, there were only five species that could be considered “losers” based on the same numerical criteria. Because such rarity of losers in our study system, we did not analyse their niche dynamic.

To define the environmental niches of winners we included three groups of environmental variables: (i) local environmental parameters, as biological oxygen demand (BOD5), concentrations of total phosphorus (TP), nitrate-nitrogen (NO₃-N), and ammonium-nitrogen (NH₄-N), roughness of bed substrate (Phi), and share of riffles (Riff), all counted as means of the values measured six times during the year of macroinvertebrate sampling; (ii) proxies of land use (from 250 m wide river side strip across 15 km of stream network above the site), i.e. areas of unfavourable surfaces (Un_surf) and forested area (Fore), computed based on the information from the CORINE Land Cover system (see Zhai et al. 2023 for details on how the parameters were measured); and (iii) climatic variables, such as mean air temperatures in January and July (T_Jan, T_Jul), and annual precipitation (Prec), which were calculated based on values of daily averages in the year of sampling and two consecutive years prior to sampling, using gridded data provided by the Czech Hydrometeorological Institute (Polášek et al. 2017).

Each of 296 taxa included in the analysis was characterised by seven biological traits describing body size, feeding behaviour, locomotion, respiration, dispersal strategy and potential, and life cycle (Table S1 of this study). Trait data were taken from published sources (Tachet et al. 2000; Sarremejane et al. 2020) and online databases (Schmidt-Kloiber and Hering 2015; Polášek et al. 2017). Traits with 29 categories were coded using the fuzzy approach to assign the affinity of species to a particular trait category (Chenevet et al. 1994). We also tested whether winners were more warm-adapted than the other species using the approach of Haase et al. (2019), to associate expansion of winners with warming. This approach defines the temperature niche of species based on their affinities to river zones (Illies and Botosaneanu, 1963). For more details and literature, see Haase et al. (2019). The data were available for all winners and for 232 of 253 remaining taxa.

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