Large decline of birds in Sahelian rangelands due to loss of woody cover and soil seed bank

摘要

The large and widespread decline of European migratory birds spending the northern winter in the Sahel suggests - on top of adverse changes in the breeding quarters - pivotal changes in African overwintering areas. This paper attempts to answer three questions related to the sub-Saharan region: (1) can a change in the woody vegetation explain the decline of migratory birds feeding in trees, (2) ditto for the ground vegetation and bird species searching for food on the ground, and (3) are African bird species also in decline? The analysis is confined to the western Sahel (annual rainfall 100-400 mm, 15.5-18 degrees N and 14.7-16 degrees W), a region intensively used as rangeland but too dry for agriculture. The woody cover, largely stable before 1970, declined in the driest zone by 90% between the 1960s and 2000s, and by 40% in the more humid zone where the woody cover was already low in the 1960s. The woody vegetation changed in many places from an Acacia savanna into an open savanna with scattered bushes and few trees, concomitant with a shift in species composition. These changes took place during a prolonged drought (1969-1992), and were aggravated by increased grazing pressure after the construction of boreholes as evident from the loss of woody cover close to boreholes. A comparison of bird composition and densities in grazed and ungrazed areas and in three study sites intermittently surveyed between 1960 and 1994, with our surveys in the same sites in 2014-2015, revealed about 80% losses for birds feeding on the ground. The increased grazing pressure of livestock must have caused a large reduction of the soil seed bank and most likely also of insects. Between 15 and 16 degrees W an estimated 8 million arboreal birds, mainly migrants, and 30 million ground-feeding birds, mainly granivorous residents, lost their habitat. Assuming that this zone is representative for the Sahelian rangelands as a whole, 1.5-2.0 billion birds have lost their habitat in half a century.