The impact of shrub encroachment on savanna bird diversity from local to regional scale

Authors

  • Clélia Sirami,

    Corresponding author
    1. Climate Change and BioAdaptation Division, South African National Biodiversity Institute, P/Bag X7, Claremont 7735, Cape Town, South Africa
    2. Botany Department, University of Cape Town, Private Bag, Rondebosch 7701, South Africa
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  • Colleen Seymour,

    1. Applied Biodiversity Research Division, South African National Biodiversity Institute, P/Bag X7, Claremont 7735, Cape Town, South Africa
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  • Guy Midgley,

    1. Climate Change and BioAdaptation Division, South African National Biodiversity Institute, P/Bag X7, Claremont 7735, Cape Town, South Africa
    2. Botany Department, University of Cape Town, Private Bag, Rondebosch 7701, South Africa
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  • Phoebe Barnard

    1. Climate Change and BioAdaptation Division, South African National Biodiversity Institute, P/Bag X7, Claremont 7735, Cape Town, South Africa
    2. DST-NRF Centre of Excellence at the Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Private Bag, Rondebosch 7701, South Africa
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*Clélia Sirami, Climate Change and BioAdaptation Division, South African National Biodiversity Institute, P/Bag X7, Claremont 7735, Cape Town, South Africa.
E-mail: cleliasirami@hotmail.com

Abstract

Aim  Evidence is accumulating of a general increase in woody cover of many savanna regions of the world. Little is known about the consequences of this widespread and fundamental ecosystem structural shift on biodiversity.

Location  South Africa.

Methods  We assessed the potential response of bird species to shrub encroachment in a South African savanna by censusing bird species in five habitats along a gradient of increasing shrub cover, from grassland/open woodland to shrubland dominated by various shrub species. We also explored historical bird species population trends across southern Africa during the second half of the 20th century to determine if any quantifiable shifts had occurred that support an ongoing impact of shrub encroachment at the regional scale.

Results  At the local scale, species richness peaked at intermediate levels of shrub cover. Bird species composition showed high turnover along the gradient, suggesting that widespread shrub encroachment is likely to lead to the loss of certain species with a concomitant decline in bird species richness at the landscape scale. Finally, savanna bird species responded to changes in vegetation structure rather than vegetation species composition: bird assemblages were very similar in shrublands dominated by Acacia mellifera and those dominated by Tarchonanthus camphoratus.

Main conclusions  Shrub encroachment might have a bigger impact on bird diversity in grassland than in open woodland, regardless of the shrub species. Species recorded in our study area were associated with historical population changes at the scale of southern Africa suggesting that shrub encroachment could be one of the main drivers of bird population dynamics in southern African savannas. If current trends continue, the persistence of several southern African bird species associated with open savanna might be jeopardized regionally.

Ancillary