Testing large-scale hypotheses using surveys: the effects of land use on the habitats, invertebrates and birds of Himalayan rivers.

Abstract

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. In this paper, 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 are illustrated. Surveys were done of 180 streams and their catchments in NW India and Nepal in 1994-96. Land use in the region was 33% terraced agriculture, with associated vegetation of scrub or rough pasture, broadleaved or mixed forests, coniferous forest, sal (Shorea robusta) forest, and alpine vegetation. Statistical analyses of the data collected were carried out 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. 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. No clear effects were detected of land use on stream chemistry, but terracing was accompanied by significantly increased abundances of benthic dipterans, ephemeropterans and total aquatic invertebrates. 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 (1 species) and negatively (4 species), but only in the case of the one species affected positively did the presence of terracing per se affect occurrence unequivocally; effects on other species were either small or confounded by altitude. The data do not refute the hypothesis that catchment land use affects Himalayan river ecology, but the data on the regional consequences for river birds were equivocal. It is suggested 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.

Key words

• agriculture
• alpine vegetation
• altitude
• aquatic animals
• aquatic invertebrates
• aquatic organisms
• biodiversity
• catchment hydrology
• coniferous forests
• deciduous forests
• drainage
• forests
• habitats
• human activity
• impact
• land use
• methodology
• mixed forests
• mountain areas
• mountain forests
• pastures
• rivers
• scrub
• streams
• surveys
• terracing
• vegetation
• water quality
• watersheds
• wild animals
• wild birds
• wildlife
• land use change