Predicting community characteristics from habitat conditions: fluvial fishand hydraulics

1. One current approach to the prediction of community characteristics is t o use models of key local-scale processes (e.g, niche dimensions) affecting individuals and to estimate the effects of these attributes over larger sc ales. We tested this approach, focusing on how the hydraulic habitat struct ures fluvial fish communities. 2. We used a recent statistical habitat model to predict fish community cha racteristics in eleven reaches in the Rhone river basin in France. Predicti ons were made 'blindly' since most reaches were not used to calibrate the m odel. The model reflects species preferences for local hydraulics. We made predictions of the fish community from the local hydraulic conditions found in the reaches under low flow conditions. The overall abundance and the re lative abundance (both as indices) of fish species, specific size classes a nd species traits (i.e, reproductive, trophic, morphological and others) we re predicted. We summarized our predictions of the relative abundance of sp ecies as two 'community structure indices' using Principal Component Analys is. 3. Our predictions from low-flow hydraulics were compared with long-term ob servations of fish communities. The relative abundance of species actually observed depended largely on zoogeographic factors within the Rhone basin w hich could not be predicted by the model. The model predicted 13% of the va riance in the indices of relative abundance at the species level and 23% of this variance at the trait level for all zoogeographic regions combined. H owever, when focused on reaches within a geographic region, the model expla ined up to 47% of the same variance. Therefore, geographic regions act as ' filters' on the relative abundance of species, but hydraulics do affect fis h communities within a given geographical context. 4. For the synthetic 'community structure indices', we obtained good predic tions from hydraulics independently of the geographical context (variance e xplained up to 95%). These indices were linked to simple key hydraulic char acteristics of river reaches (Froude and/or Reynolds number). The indices e nabled interpretations of the links between hydraulics, geomorphology, disc harge and community patterns. These links were consistent with existing kno wledge of species and their traits. 5. In addition to the above validations, the habitat model partly explained the observed effects of impoundment on fish communities. 6. The present results show that stream hydraulics strongly impact fish com munity structure. Consequently, our findings confirm that community charact eristics can be predicted using models of the local-scale habitat requireme nts of the species forming the community.