Year: 2011
Type: dissertation
Abstract: Herbivores are important drivers and have a longstanding history in shaping our terrestrial \nenvironments. However, during the past decades, changes in woody vegetation in savanna and \nforest systems have been observed in southern Africa. Subsequently, concerns have been raised \nabout the loss of (tall) trees in areas with elephant. The relative effects of browsing herbivores on \nvegetation and the potential browsing interaction with other herbivore species remain unclear and \nwere examined using vegetation transects and exclosure experiments in savanna woodland and \nSand Forest. \nRainfall, fire and elephant were important savanna determinants. Especially rainfall positively \naffected woody densities, which were negatively affected by a longer exposure time to elephant, \nbut not to elephant densities itself. In general, within South Africa’s savannas, tree height classes \nwere absent from the population demography. Different height classes were likely to be impacted \nby different drivers. For example, seedling and sapling densities were greater with longer fire \nreturn periods and increased rainfall. The Sand Forest exclosure experiments showed that forest \nregeneration was impacted by nyala and both elephant and nyala, as the absence of both species \nincreased tree densities. Both species combined, and individually, also affected tree species \nassemblages. In contrast, short term elephant access to a savanna area did not affect tree densities \nor species assemblages. In both savanna and Sand Forest elephant displaced mesoherbivores, and \nin Sand Forest both elephant and mesoherbivores displaced their smaller counterparts. The \npresence of competitive displacement also affected recruitment (i.e. seedlings and/or saplings) of \nwoody vegetation both in Sand Forest and savanna. \nThus, elephant and mesoherbivores exert direct and indirect (i.e. competitive displacement \nproviding a window for recruitment) impact on vegetation. Active management of the herbivore \nspecies assemblage affects both vegetation and other herbivores, which effects potentially \ncascade into lower trophic levels, jeopardising biodiversity and ecosystem processes. Therefore, \nthe full herbivore assemblage present and their combined and individual browsing effects need to \nbe considered when setting management goals to conserve habitats and biodiversity across all \ntrophic levels. In addition some contrasting results between Sand Forest and savanna emphasise \nthe need for caution when extrapolating results from different areas and ecosystems.
Author: D. D. Georgette Lagendijk
Language: English
Subfield: Ecology
Field: Environmental Science
Domain: Physical Sciences
SDG: Life on land
Open Access status: green
OpenAlex ID: W1482274517