Evidence is accumulating of major vegetation changes such as shrub encroachment in many savanna regions of the world, including in Africa.  Its causes are still understood only poorly, as the determinants of tree-grass coexistence are complex, and little is known about the consequences of this fundamental ecosystem shift on biodiversity.  The objective of this work was to assess the impact of changes in woody cover on bird communities in Southern African savannas at various scales.  I combined information on temporal changes in bird occurrence, remote sensing and data on bird species distribution along gradients of increasing woody cover in several savannas in southern Africa.

Results/Conclusions

At the local scale, bird species richness is maximal at relatively low tree densities (20% tree cover in mesic savanna), which tends to confirm the role of savanna trees as keystone structures. An increase in shrub cover results in an increase in bird species richness up to intermediate shrub cover (40% shrub cover in arid savanna).  However, shrub encroachment results in a dramatic shift of bird community composition, and a decrease of species richness at the landscape scale.  Temporal changes in vegetation cover and bird occurrence during the last decade suggest that shrub encroachment could be one of the main drivers of bird population dynamics in southern African savannas, although it is likely to interact with other global drivers such as temperature increase.  If current trends carry on, the persistence of bird species associated with open savanna might be jeopardized at the scale of southern Africa.  In arid savanna, bird species respond to changes in the vegetation structure rather than vegetation composition.  In mesic savanna, bird species seem to respond to different scales and different features depending on their life-history traits.  Management policies should prioritize the maintenance of large patches of grassland and landscape connectivity between patches of open savanna.