Abstract
Habitat suitability and the distinct mobility of species depict fundamental keys for explaining and understanding the distribution of river fishes. In recent years, comprehensive data on river hydromorphology has been mapped at spatial scales down to 100 m, potentially serving high resolution species-habitat models, e.g., for fish. However, the relative importance of specific hydromorphological and in-stream habitat variables and their spatial scales of influence is poorly understood. Applying boosted regression trees, we developed species-habitat models for 13 fish species in a sand-bed lowland river based on river morphological and in-stream habitat data. First, we calculated mean values for the predictor variables in five distance classes (from the sampling site up to 4000 m up- and downstream) to identify the spatial scale that best predicts the presence of fish species. Second, we compared the suitability of measured variables and assessment scores related to natural reference conditions. Third, we identified variables which best explained the presence of fish species. The mean model quality (AUC = 0.78, area under the receiver operating characteristic curve) significantly increased when information on the habitat conditions up- and downstream of a sampling site (maximum AUC at 2500 m distance class, +0.049) and topological variables (e.g., stream order) were included (AUC = +0.014). Both measured and assessed variables were similarly well suited to predict species’ presence. Stream order variables and measured cross section features (e.g., width, depth, velocity) were best-suited predictors. In addition, measured channel-bed characteristics (e.g., substrate types) and assessed longitudinal channel features (e.g., naturalness of river planform) were also good predictors. These findings demonstrate (i) the applicability of high resolution river morphological and instream-habitat data (measured and assessed variables) to predict fish presence, (ii) the importance of considering habitat at spatial scales larger than the sampling site, and (iii) that the importance of (river morphological) habitat characteristics differs depending on the spatial scale.
Highlights
Species distribution or habitat models are widely used in conservation planning and the management of natural systems to: (i) statistically analyse species’ ecological needs based on empirical data from a number of sampling sites, and (ii) predict species’ presence for large areas based on species-habitat relationships
Most of the upstream river network are small graveldominated lowland streams (LAWA river type 16, [11]) belonging to the hyporhithral region according to Illies [12] with Leuciscus leuciscus, Phoxinus phoxinus and Salmo trutta as key fish species
The modelling algorithm did not converge and failed to compute two models (T. tinca–measured variables, distance 2500 m; G. aculeatus–assessed variables, distance 0 m), that were excluded from further analysis
Summary
Species distribution or habitat models are widely used in conservation planning and the management of natural systems to: (i) statistically analyse species’ ecological needs based on empirical data from a number of sampling sites, and (ii) predict species’ presence for large areas based on species-habitat relationships. Hydromorphological assessments of the physical habitats (e.g. the German method of the Länderarbeitsgemeinschaft Wasser (LAWA) [3]) typically map channel dimensions, indicators for morphodynamics, channel-bed and bank features, substrates and the structure of the riparian zone These data can be used to investigate and model the distribution of stream biota such as fish in relation to the hydromorphological habitat conditions. Besides increasing the predictive power of the models, such species-habitat relationships at larger spatial scales might reflect or approximate the minimum spatial extent of a specific habitat required that contains suitable spawning substrates, littoral nurseries for larvae and juveniles, as well as feeding grounds and overwintering habitats for all age groups of a species This is of special importance in river rehabilitation, e.g. for planning and dimensioning stepping stone habitats or the necessary spatial extent of successful restoration measures
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