Rangeland degradation around water-points under different management systems

Smet, Michiel (2004-03)

Thesis (MScConEcol )--University of Stellenbosch, 2004.

Thesis

ENGLISH ABSTRACT: Over 70% of South Africa is too arid for crop farming and is used for commercial livestock ranching, communal livestock ranching, or game ranching. These management systems differ from each other in a number of aspects, e.g. herbivore species, stocking rate, grazing regime, and management structure. The main question addressed in this study is: to what extent do the three management systems affect rangeland condition? Following the equilibrium paradigm of vegetation dynamics, communal livestock ranches are expected to have a greater detrimental effect on rangeland condition than other management systems because stocking densities and, consequently, herbivore impacts, are usually far higher than under the other two management systems. However, recent nonequilibrium theories argue that vegetation dynamics in arid systems are mainly driven by rare and stochastic rainfall events. Biotic factors, such as grazing and herbivore diversity, are predicted to have little effect on rangeland condition. This leads to the prediction that herbivore impacts on arid ecosystems are not density-dependent and, consequently, the high stocking densities commonly recorded on communal livestock ranches will not cause more rangeland degradation than other management systems. Arid and semi-arid rangelands are characterised by high inherent spatial and temporal variation in vegetation and soil parameters. Hence, differentiating between the effects of grazing management systems and natural variability caused by abiotic factors, such as rainfall, is difficult. This problem can be circumvented by examining gradients of grazing intensity radiating from water-points (= piospheres). I examined changes in vegetation and soil parameters along 500 m-Iong grazing gradients on ranches in the semi-arid Northem Cape province, South Africa, with the three abovementioned management systems. Analysis of the vegetation on these ranches demonstrated the existence of grazing gradients around the water-points. Two characteristic zones described in other studies were identified, namely the sacrifice zone (almost complete denudation of the vegetation close to the water-point) and the dense shrub zone (distal to the sacrifice zone). Communal livestock ranching had a greater negative effect on rangeland vegetation than the two other ranching systems, particularly with regard to annual grass species diversity, percentage of bare soil, and shrub density. Within 100 m of the water-points, soil quality was negatively affected by commercial cattle ranching. Contrastingly, there were no significant effects of the other two management systems on soil quality in the piosphere. The feeding of supplemental forage and nutrients may explain the greater impact of commercial cattle farming on soil quality. The results of this study are consistent with the predictions of the equilibrium theory of vegetation dynamics because grazing impacts were density-dependent in these semi-arid rangelands. However, the results presented here also indicate that forage supplementation alters the spatial and temporal distribution of livestock in a manner that may have greater negative impacts on soil quality than density of livestock per se.

AFRIKAANSE OPSOMMING: Meer as 70% van Suid-Afrika is te droog vir gewasverbouing en word gebruik vir kommersiële veeboerdery, gemeenskaplike veeboerdery of wildboerdery. Die bestuur van dié verskillende boerderytipes verskil wesentlik van mekaar in verskeie opsigte, ondermeer herbivoorspesies, veeaanskaffingstempo, weidingstipe en bestuurstruktuur. Die belangrikste vraag wat aangespreek word in die studie is die volgende: Tot watter mate affekteer die drie verskillende boerderybestuurstipes die toestand van weivelde? Gesien teen die agtergrond van die ekwilibriumparadigma van plantedinamika, word verwag dat gemeenskaplike veeboerdery 'n groter impak op die toestand van weiveld sal hê as ander boerderytipes, omdat veedigtheid en gevolglik herbivoor-impak gewoonlik veel hoër is as met die ander twee boerderytipes. Onlangse nie-ekwilibria-teorië voer egter aan dat plantedinamika in ariede gebiede gedryf word deur seldsame en buitengewone reënvalgebeure. Biotiese faktore, soos weiding en herbivoor diversiteit sal na verwagting min effek hê op die toestand van weiveld. Hieruit volg die voorspelling dat herbivoorimpak op ariede ekosisteme nie afhanklik is van digtheid nie, en dat hoë veedigtheid algemeen aangeteken vir gemeenskaplike boerderye, nie rede sal wees vir groter ver1iese in weiveld nie. Ariede en semi-ariede weivelde word gekenmerk deur 'n hoë ruimtelike en tydelike veranderinge in plant- en grondparameters. Gevolglik is dit moeilik om te onderskei tussen die effekte van weidingsbestuur en natuur1ike veranderings wat veroorsaak word deur nie-biotiese faktore, soos reënval. Dié probleem kan oorbrug word deur gradiënte van weidingsintensiteit, wat vanaf waterpunte uitstraal, te ondersoek. Vervolgens het ek binne al drie bg. boerderybestuursisteme veranderinge in plant- en grondparameters langs SOOm-lange weidingsgradiënte ondersoek op plase in die semiariede Noord-Kaap provinsie van Suid-Afrika. Plantegroei-analise het getoon dat 'n weidingsgradiënt rondom waterpunte voorkom. Twee kenmerkende sones is in ander studies geïdentifiseer, nl., 'n opofferingsone (waar amper alle plantegroei naby waterpunte uitgetrap is) en die digte, struik-sone (distaal to die opofferingsone). Gemeenskapsveeboerdery het 'n groter negatiewe effek op plaasgewasse as die ander twee boerderytipes, veral waar dit betrekking het op grasspesie-diversiteit, persentasie grond sonder plantbedekking en struikdigtheid. Binne 100m vanaf waterpunte, is grondkwaliteit merkbaar negatief geaffekteer deur kommersiële veeboerdery. Daarteenoor, was daar geen merkbare effek op die grondkwaliteit naby waterpunte van die ander twee boerderytipes nie. Aanvullingsvoer kan moontlik die rede wees vir die waargenome, groter impak van kommersiële veeboerdery op grondkwaliteit. Die resultate van die studie strook met voorspellings van die ekwilibria-teorie van plante-dinamika, daarin dat weidingsimpak digtheidsafhanklik is in hierdie semiariede plaasgebiede. Die resultate toon ook egter, dat voedingsaanvullings die ruimtelike en tydelike verspreiding van vee in so 'n wyse verander, dat dit moontlik 'n veel groter negatiewe uitwerking op grondkwaliteit het as veedighteid per se.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/50074
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