Changes in bird communities in Swaziland savannas between 1998 and 2008 owing to shrub encroachment

Abstract

Aim: This study investigates changes in bird communities between 1998 and 2008 in four savanna sites in Swaziland and the extent to which shrub encroachment is responsible for these changes. Location: Swaziland, southern Africa. Methods: Generalized estimated equations were used to estimate changes in bird species occurrence between 1998 and 2008. Remote sensing of aerial photographs/satellite images was used to assess vegetation changes during the same period. We assessed the role of shrub encroachment for bird communities by testing the relationship between change in species occurrence and species habitat using a general linear model. We also estimated species richness, colonization and extinction and used general linear models to test the effects of vegetation changes on these parameters. Results: More than half of the bird species showed a significant change in occurrence between 1998 and 2008: 32 species increased and 29 decreased. Change in species occurrence was significantly explained by species habitat. Species significantly increasing were mainly associated with wooded savanna, whereas species significantly decreasing were mainly associated with open savanna. Species richness decreased significantly, and this decrease was significantly explained by shrub cover increase at the plot scale (from 24% to 44% on average). Extinction at the plot scale was significantly influenced by the loss of grass cover, while colonization at the plot scale was influenced by tree cover increase. Main conclusions: This study represents the first evidence of temporal changes in bird communities owing to shrub encroachment in southern Africa. Despite its short time frame (10years), this study shows dramatic changes in both vegetation structure and bird community composition. This confirms the general concern for southern African bird species associated with open savanna if current trends continue. © 2011 Blackwell Publishing Ltd.