Community convergence across biogeographically distinct regions suggests the existence of key, repeated evolutionary mechanisms relating community characteristics to the environment. However, convergence studies at the community level often involve only qualitative comparisons of the environment and may fail to identify which environmental variables drive community structure. We tested the hypothesis that the biological traits of fish communities on two continents (Europe and North America) are similarly related to environmental conditions. Specifically, from observations of individual fish made at the local scale (few m2) within French streams, we generated habitat preference models linking traits of fish species to local scale hydraulic conditions (Froude number). Using this information, we then predicted how hydraulics and geomorphology at the larger stream reaches scale (length of streams including several pool-riffle sequences) should quantitatively influence the trait composition of entire communities. Trait composition for fish in stream reaches with low Froude number at low flow or high proportion of pools was predicted as non-benthic, large, fecund, long-lived, non-streamlined, and weak swimmers. We tested our predictions in contrasting stream reaches in France (n=11) and Virginia, USA (n=76), using analyses of covariance to quantify the relative influence of continent versus physical variables on fish traits. Results indicated that trait proportions in the communities differed across continents (up to 55 % of the variance in each trait explained), but, within continents, they were also related to the hydraulic and geomorphic variables (up to 54 % of the variance within continents explained). In particular, a synthetic description of fish traits was well predicted from the Froude number independently of the continent. In addition, the effect of physical variables was comparable across continents for most traits, confirming our predictions qualitatively and quantitatively. Therefore, despite phylogenetic and historical differences between continents, fish communities of France and Virginia exhibit convergence in biological traits related to hydraulics and geomorphology. This convergence reflects morphological and behavioral adaptations to physical stress in streams. This study supports the existence of a habitat template for ecological strategies. Some key quantitative variables that define this habitat template can be identified by characterizing how individual organisms use their physical environment, and by using dimensionless physical variables which are suited to reveal common energetic properties in different systems. Overall, quantitative tests of community convergence are efficient tools to demonstrate that some community traits are predictable from environmental features.

Les traits des communautés de poissons (morphologie, comportement, reproduction) répondent de façon comparables aux conditions hydrauliques dans des tronçons de cours d'eau français et nord-americains. Ces observations sont quantitativement prévisibles par des observations faites en France à une autre échelle : celle des microhabitats au sein des tronçons de cours d'eau.