Land use and the conservation status of raptors in Botswana.

摘要

Records of 25,298 diurnal raptors observed in 1762 spot counts and 55,577 km of roadside counts in Botswana during 1991-1995 were analysed as a function of distance from the boundary of conservation areas (national parks or game reserves in the subtropical woodland zone). Spot counts were more sensitive and efficient than roadside counts. Overall there was a dramatic decline in raptor abundance once >15 km outside conservation areas, more so in the dry than in the wet season. Different groups had different patterns: numbers of large resident eagles dropped on land >50 km from protected areas to <10% of numbers in the core of protected areas. Numbers in the peripheral 30 km inside protected areas were already >40% lower than in the core, indicatinga massive edge effect on populations inside reserves. Migrants were less sensitive; the Wahlberg's (Aquila wahlbergi) and steppe eagle (A. nipalensis), which one would expect to belong to a sensitive group because of their large size, were indifferent to land use factors. The latter species had a pattern strikingly different from the resident tawny eagle (A. rapax). Numbers of scavenging vultures peaked near the interface between conservation areas and unprotected land, but the white-headed vulture (Trigonoceps occipitalis) followed the pattern of the large eagles. This appears to be the single most sensitive species, occurring anywhere >15 km outside protected areas at <10% of its capacity. Species with a preference for human-modified habitat were more abundant outside conservation areas, as expected. The widespread depletion of biomass and biodiversity (including potential prey) following from structural changes to vegetation caused by overgrazing by livestock seems to be the main force behind thepoor status of raptors on unprotected land. Large buffer zones with special management are needed outside parks to ensure the integrity of conservation areas.