Predictive modeling is used increasingly often in freshwater bioassessment. These models are routinely applied across broad geographical areas, yet the influence of spatial extent on model performance has not been assessed directly. We examined the influence of changing geographical extent on the performance of River InVertebrate Prediction and Classification System (RIVPACS)–type predictive models of benthic macroinvertebrates in boreal headwater streams representative of reference conditions. Separate models were constructed for 2 hierarchical extents: within ecoregions (middle boreal and north boreal) and across ecoregions. We assessed the accuracy and precision of the models using root mean squared error (RMSE) of the observed/expected (O/E) ratio of the number of predicted taxa. We compared the RMSE of the within-ecoregion models to that of the across-ecoregions models. We tested the models using 2 thresholds for probability of species occurrence (Pt): Pt = 0 (all predicted taxa) and Pt = 0.5 (only common taxa). We also assessed the importance of catchment vs local habitat variables as predictors of macroinvertebrate fauna. Within the 2 ecoregions, region-specific models performed better than the across-ecoregions model with Pt = 0 , but the difference in performance of models with different regional extents decreased when Pt = 0.5. Catchment area and stream slope were the only consistently significant predictor variables, but overall, only minor differences were found in the relative importance of catchment vs local attributes. Models with Pt = 0.5 performed better than those with Pt = 0 ; this result suggests that models based only on common taxa might perform more consistently than models based on all taxa in the assessment of headwater streams. However, the expected number of taxa would then become very low (mean: 8.0–12.6 with Pt = 0.5), which might impede detection of human impact. Alternative approaches that do not rely solely on taxon occurrences but also incorporate aspects of assemblage composition might be needed for assessment of headwater streams.
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