Changes in Sahelian bird biodiversity and tree density over a five-year period in northern Nigeria

摘要

Capsule Two widespread censuses of already degraded woodland in the Sahel in Northern Nigeria separated by five years (2002 and 2007) showed relatively small further changes in tree and bird density.Aims To determine how change in tree density affects African and Palearctic migrant bird density in already degraded Sahelian woodland.Methods We compared differences in the number of bird species and their abundance with differences in the size and number of woody plants between two seasons separated by five years at 16 degraded woodland sites across 250 km east-west in northern Nigeria.Results Densities of large trees (>5 m) decreased, but densities of smaller trees and shrubs increased, partly as a consequence of growth and because larger trees have been preferentially removed or partly cut down; tree densities were, however, initially low. Of 37 common bird species, 16% had higher densities and 22% had lower densities, in the second study period. The density of Common Whitethroats, Northern Wheatears and Yellow Wagtails was correlated with the number and size of trees, but not that of Lesser Whitethroat and Subalpine Warbler. Overall there was relatively little turnover in species between the two surveys and if (the initially low) tree density was halved, only approximately 2.1% of the average total for number of bird species per 0.2 ha point would be lost.Conclusion Few large changes in bird species' density were found, possibly because: (1) all habitats sampled were already at relatively low tree density-initial reductions from high tree density or near removal of all trees may cause larger changes in local bird densities; (2) reduction in the number of large trees was partially compensated by a more lush development of the vegetation as a whole in the second study period; and (3) the bird species remaining in degraded Sahelian woodland in northern Nigeria are already well adapted to habitat change and low tree density.