Résumé
1. Piecemeal changes in land use might have cumulative effects on regional biodiversity. However, this hypothesis is difficult to test experimentally at the scales involved, so alternative approaches are required. Here, we illustrate some of the strengths and weaknesses of surveys for evaluating the effects of land use on rivers and river birds over a large area of the Himalayan mountains. 2. We surveyed 180 streams and their catchments in north-west India and Nepal in 1994-96. We then used analysis of covariance (ancova), multiple linear regression and multiple logistic regression to assess how stream habitat structure, stream chemistry, aquatic invertebrate abundance and the occurrence of river birds were affected by land use after accounting for altitudinal pattern. 3. Streams draining terraced catchments differed significantly in habitat structure from other streams. They had more physical modifications, wider channels, fewer cascades, finer substrata and simpler riparian vegetation with fewer trees. We detected no clear effects of land use on stream chemistry, but terracing was accompanied by significantly increased abundances of benthic dipterans, ephemeropterans and total aquatic invertebrates. 4. River bird occurrence was best explained by altitude, and secondarily by habitat structure. Some of the habitat features influenced by terracing significantly affected birds both positively (grey wagtail Motacilla cinerea) and negatively (little forktail Enicurus scouleri, river chat Chaimarrornis leucocephalus, brown dipper Cinclus pallasii, plumbeous redstart Rhyacornis fuliginosus). However, only in the grey wagtail did the presence of terracing per se affect occurrence unequivocally; effects on other species were either small or confounded by altitude. 5. We cannot refute the hypothesis that catchment land use affects Himalayan river ecology, but our data on the regional consequences for river birds were equivocal. We suggest that large-scale surveys, although providing one of the few pragmatic methods of assessing large anthropogenic effects on ecosystems, will need careful design to factor out potential confounds if they are to be used to test hypotheses robustly. They should also be supported where possible with process studies, intervention studies and model applications to independent data.