Abundance and diversity of insectivorous birds along a riparian altitudinal gradient in the arid southern Karoo, South Africa

Riparian corridors are key resources in arid areas, supporting greater productivity and bird diversity than surrounding landscapes. Rivers in arid areas generally rise in better-watered highlands so that river corridors link highland and lowland, mesic and arid habitats. Geology and land use generally also change with altitude. We conducted a pilot study along an altitudinal gradient in a riparian corridor to test the hypothesis that invertebrates and insectivorous bird abundance increased with decreasing altitude and with increasing nutrient loading in the river. Aquatic, aerial and terrestrial invertebrates did not increase uniformly along this gradient, but insectivorous bird abundance was strongly correlated with invertebrate abundance.