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Shrinking sable antelope numbers in Kruger National Park: what is suppressing population recovery?

Authors

  • N. Owen-Smith,

    Corresponding author
    • Centre for African Ecology, School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Wits, South Africa
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  • G. J. Chirima,

    1. Centre for African Ecology, School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Wits, South Africa
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  • V. Macandza,

    1. Centre for African Ecology, School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Wits, South Africa
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  • E. Le Roux

    1. Centre for African Ecology, School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Wits, South Africa
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Correspondence

Norman Owen-Smith, School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Wits 2050, South Africa. Tel: +27-11-717-6454; Fax: +27-11-717-6494

Email: norman.owen-smith@wits.ac.za

Abstract

Sable antelope numbers in the Kruger National Park have declined substantially since the mid-1980s and have shown little recovery despite improved rainfall conditions. We used aerial survey records to investigate how changes in herd numbers, herd sizes, calf proportions and consequent changes in the distribution range of breeding herds contributed to this situation. Both herd sizes and herd numbers decreased in the drier northern half of the park, coupled with low calf proportions, especially during and following a severe drought, while declines in these measures became evident later in the wetter southern half. Herd extirpations led to a 25% contraction in the local distribution range of sable antelope, although some of these herds had only become established during the high rainfall period in the late 1970s. Local population trends were not related to increases in zebra numbers potentially attracting more predators. Compensatory density dependence in the population growth rate was no longer evident after 1986 during the period of the population decline. The population trajectory modelled from rainfall relationships indicated that the depressed population growth rate relative to rainfall apparent after 1986 has persisted, implicating either enduring habitat degradation or continuing high predation pressure. The pattern of the population decline and subsequent lack of recovery raises the possibility that an Allee effect could be operating, mediated through the consequences of reduced herd sizes for exposure to predation. Our findings suggest how delays in responding to declining population numbers could prejudice population recovery well before the overall population size drops to levels threatening population viability.

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