Summary Relating species traits to habitat characteristics can provide important insights into the structure and functioning of stream communities. However, trade‐offs among species traits make it difficult to predict accurately the functional diversity of freshwater communities. Many authors have pointed to the value of working with groups of organisms as similar as possible in terms of relationships among traits and have called for definition of groups of organisms with similar suites of attributes. We used multivariate analyses to examine separately the relationships among 11 biological traits and among 11 ecological traits of 472 benthic macroinvertebrate taxa (mainly genera). The main objective was to demonstrate (1) potential trade‐offs among traits; (2) the importance of the different traits to separate systematic units or functional groupings; and (3) uniform functional groups of taxa that should allow a more effective use of macroinvertebrate biological and ecological traits. We defined eight groups and 15 subgroups according to a biological trait ordination which highlighted size (large to small), reproductive traits (K to r strategists), food (animal to plant material) and feeding habits (predator to scraper and/or deposit feeder) as ‘significant’ factors determining the ordination of taxa. This ordination partly preserved phylogenetic relationships among groups. Seven ecological groups and 13 ecological subgroups included organisms with combinations of traits which should be successively more adequate in habitats from the main channel to temporary waters, and from the crenon to the potamic sections of rivers, and to systems situated outside the river floodplain. These gradients corresponded to a gradual shift from (1) rheophilic organisms that lived in the main channel of cold oligotrophic mountain streams to (2) animals that preferred eutrophic habitats of still or temporary waters in lowlands. The groups with similar ecological traits had a more diverse systematic structure than those with similar biological traits. Monitoring and assessment tools for the management of water resources are generally more effective if they are based on a clear understanding of the mechanisms that lead to the presence or absence of species groups in the environment. We believe that groups with similar relationships among their species traits may be useful in developing tools that measure the functional diversity of communities.

The historical trend of aggradation in stream beds of Europe';s mountain and piedmont areas has been reversed since the beginning of the twentieth century to a general tendency towards narrowing and incision. After an overview of some causes and geomorphological effects of river bed incision, and a description of some case studies in alpine rivers, this paper aims to assess how the phenomenon affects the ecosystems of alluvial plains. The questions addressed are, in particular, the effect of river bed incision on sediment redistribution during floods, habitat diversity and the water table; and the consequences of these changes for the distribution and diversity of biological communities. The effect of incision on floodplain vegetation, instream invertebrate communities, groundwater and former channel ecosystems and fish communities are considered. It has been found that incision rarely affects the biocoenoses directly, but indirect changes in habitat conditions are significant. At the scale of the whole hydrosystem, the predicted impact of incision is a decrease in habitat heterogeneity, and, thereby, in biodiversity. At the ecosystem scale, the negative effect on biodiversity is higher in habitats that are less connected to the running water of the river. ©1997 by John Wiley & Sons, Ltd.

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