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University of the Western Cape (2006)

Granivores as ecosystem regulators of woody plant increasers in semi-arid Savannas of the Lowveld, South Africa.

Petersen, Leif Michael

Titre : Granivores as ecosystem regulators of woody plant increasers in semi-arid Savannas of the Lowveld, South Africa.

Auteur : Petersen, Leif Michael

Université de soutenance : University of the Western Cape

Grade : Magister Scientiae (Biodiversity and Conservation Biology) - MSc 2006

Résumé partiel
In recent years, a global trend of increasing woody vegetation densities in semi-arid savanna habitats has been recorded, commonly described in South Africa as ‘bush encroachment’. The shrubs and trees that do this (Increasers) have wrought significant economic and ecological impacts upon carrying capacities of large areas of savannas. This occurs as suitable grazing areas are incrementally engulfed in shrubs and trees establishing new equilibria, from open savannas (essentially grasslands with scattered trees) into closed woodlands (treelands with scattered grasses). In many places such increasers have reached proportions where economically viable livestock properties can no longer be grazed (Child 1995). Factors such as megaherbivore overabundance, overgrazing, atmospheric CO2 enrichment and incorrect fire management regimes are commonly blamed for change in woody vegetation densities in South African savannas. Additional to these factors, small mammals have been pinpointed as ecosystem regulators in semi-arid Australia (Noble 1997) and USA (Weltzin et al. 1997). In both cases, small mammal species were largely eradicated & increases in woody vegetation densities subsequently occurred. Granivorous small mammals are abundant in South African semi-arid savannas and have high dietary nutritive demands which would encourage them to consume energy rich tree and shrub seeds. Similar to the work of Noble (1997) and Weltzin et al. (1997) this study is an investigation into the role of small mammal granivores in South African savannas as potential ‘regulators’ of increasers in the ecosystem. Research was conducted in a semi-arid Lowveld savanna across three management areas ; 1 : A large herbivore exclosure (the Southern African Wildlife College - SAWC) ; 2 : A ‘natural’ area managed as a conservation zone with a wide variety of indigenous South African wildlife (Kempiana) and ; 3 : A communally managed grazing area utilised by domestic cattle and goats (Welverdiend). Grass biomass was measured for each study site using a disk pasture meter (Bransby & Tainton 1977). Highest levels of grass biomass (effectively small mammal understorey cover) were found at the SAWC megaherbivore exclosure (5,744 tonnes/ha), followed by Kempiana (3,360 tonnes/ha) and Welverdiend (466 tonnes/ha). Within the three management areas 4,300 live trapping nights were conducted between August and December 2000 to find resident small mammals. In total 106 unique small mammals were captured of which 70% were from the SAWC, 21% trapped in Kempiana and 9% were from Welverdiend. In all, 11 spp of small mammals were captured. All were represented at the SAWC, eight from Kempiana and three from Welverdiend. Both abundance and diversity of small mammals appeared related to site habitat structure and grass biomass.

Mots clés : Ecosystem management ; South Africa. Biological diversity ; South Africa.


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