Hempson, Gareth

Hendricks, Howard

Illius, Andrew

Vetter, Susanne

English

University of Kentucky

University of Kentucky UKnowledge International Grassland Congress Proceedings 21st International Grassland Congress / 8th International Rangeland Congress A Test of the Key Resource Hypothesis in the Richtersveld, South Africa Gareth Hempson University of Edinburgh, UK Andrew Illius University of Edinburgh, UK Howard Hendricks South African National Parks, South Africa Susanne Vetter Rhodes University, South Africa Follow this and additional works at: https://uknowledge.uky.edu/igc  Part of the Plant Sciences Commons, and the Soil Science Commons This document is available at https://uknowledge.uky.edu/igc/21/4-1/18 The 21st International Grassland Congress / 8th International Rangeland Congress took place in Hohhot, China from June 29 through July 5, 2008. Proceedings edited by Organizing Committee of 2008 IGC/IRC Conference Published by Guangdong People's Publishing House This Event is brought to you for free and open access by the Plant and Soil Sciences at UKnowledge. It has been accepted for inclusion in International Grassland Congress Proceedings by an authorized administrator of UKnowledge. For more information, please contact UKnowledge@lsv.uky.edu. 瞯 ]576　 瞯 　 Multifunctional Grasslands in a Changing World 　 Volume Ⅰ 　Grasslands/Rangelands Resources and Ecology ——— Indicators for Sustainable Use and Conservation of Grasslands/Rangelands ResourcesA test of the key resource hypothesis in the Richtersveld , South AfricaGareth Hempson1 , A ndrew I llius 1 , How ard Hendricks2 , Susanne Vetter 31 School o f Biological Sciences , University o f Edinburgh , The K ing摧s Buildings , West Mains Road , Edinburgh , EH9 3JY ,UK , E‐mail :ghempson ＠ gmail .com , 2 South A f rican National Parks , P . O . Box 787 , Pretoria 0001 , South A f rica ,3 Department o f Botany , Rhodes University , G rahamstown 6140 , South A f ricaKey words : key resource model , herbivory , density‐dependence , Succulent Karoo biomeIntroduction The key resource model predicts that herbivore numbers in seasonally‐variable grazing systems are determined byaccess to a critical subset of forage resources , and that herbivore population dynamics are essentially uncoupled from resourcesaccessible during the growing season ( Illius & O摧Connor ２０００) . This contrasts with the view that nonequilibrial dynamics areubiquitous in variable environments , with weak or no coupling between herbivore and resource dynamics ( Ellis & Swift １９８８) .We tested predictions of the key resource model in the Richtersveld National Park ( RNP) , South Africa , where herders movetheir herds in response to local resource availability ( Hendricks et al . ２００５ ) . Herd dynamics , local densities and habitat usewere examined to test whether herbivores are : (１ ) limited by density‐dependent competition for the dry season forage resource ;and (２) coupled to their forage resource in the dry season , but not in the wet season .Materials and methods Regular censuses of the mixed goat and sheep herds in the RNP were initiated in July １９９５ . These wereused to determine goat population grow th rate , calculated as ln ( Nt ＋ １ / Nt ) , during the period November １９９９ — October ２００１ ,for which weekly records of herd positions were also kept . Local animal densities were calculated for １５ regions within whichthe daily foraging paths from individual stockposts were likely to overlap . These regions were classified into three habitattypes : riparian , plains and mountains . The climate is arid and erratic , but three‐month summed rainfall over the study periodwere twice as high on average for a wet season month (２０ .４ ± ２２ .９ mm ; April‐October) as for a dry season month ( １１ .７ ±１９ .２ mm ; October‐April) . The effects of season and local animal density on goat population grow th rates were examined , aswere seasonal habitat preferences .　 Figure 1 Goat population grow th rate versus local animaldensity f or the dry and wet season . The solid linerep resents the best f it f or the dry season ( y ＝ ‐0 .016x ＋0 .116 , R2 ＝ 0 .25 , p ＝ 0 .085) .Results and discussion Goat herd grow th rate and local animaldensity were not correlated in the wet season ( Figure １ ; n ＝１４ , R２ ＝ ０ .０５ , p ＝ ０ .４１９ ) , but were negatively correlated inthe dry season ( n ＝ １３ , R２ ＝ ０ .２５ , p ＝ ０ .０８５ ) . Mean goatpopulation grow th was positive in the wet season [０ .１６５ ± ０ .２３ ( s . e .) ] and negative in the dry season (‐０ .０６２ ± ０ .１８ ) .During the wet season , animal numbers were split fairly evenlybetween riparian ( ５５％ ) and non‐riparian ( ４５％ ; plains ＝３５％ , mountains ＝ １０％ ) habitats . However , the riparian zonewas preferred (７９％ ) to non‐riparian habitats ( ２１％ ; plains ＝１７％ , mountains ＝ ３％ ) in the dry season .Conclusions The density‐dependent decline of herd grow th ratein the dry season suggests resource limitation at this time .This is not apparent in the wet season . Hendricks et al .(２００５ ) and our discussions with the herders indicate thatmovements away from the riparian zone are in order to accessthe flush of annual forage species on the plains after rainevents . In contrast , movements back to the riparian zone aredue to the need for daily access to drinking water , rather thanforage resource limitation . The riparian zone thus appears toform a key resource in the RNP , and there is some degree ofsupport for the first two predictions of the key resource model :(１) density‐dependent limitation may occur in the dry season ,likely due to coupling with the forage resource , while (２) the population appears to be relatively uncoupled from the wet seasonresource .ReferencesEllis , J .E . , Swif t , D .M . , ( １９８８ ) . Stability of African pastoral ecosystems : alternate paradigms and implications fordevelopment . Journal o f Range Management , ４１ , ４５０‐４５９ .Hendricks , H .H . , Clark , B . , Midgley , J .J . , Novellie , P .A . , ( ２００５) . Movement response patterns of livestock to rainfallvariability in the Richtersveld National Park . A f rican Journal o f Range and Forage Science , ２２ (２) , １７‐１２５ .Illius , A .W . , O摧Connor , T .C . , (２０００) . Resource heterogeneity and ungulate population dynamics . Oikos , ８９ , ２８３‐２９４ .Q) _. ■ Dry season ro □ ,_ □ Wet season ..c::: 0.5 _. □ ::: □ 0 □ □ ,_ 0) ■ C □ 0 _. 0.0 -~ ro ■ :::J ■ 0.. ■ ■ 0 .!:--0.5 ro 0 (9 0 5 10 15 20 25 30 No. animals per square kilometre 

A Test of the Key Resource Hypothesis in the Richtersveld, South Africa

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A Test of the Key Resource Hypothesis in the Richtersveld, South Africa

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