The effects of elephant and mesoherbivores on woody vegetation.
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Date
2011
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Abstract
Herbivores are important drivers and have a longstanding history in shaping our terrestrial
environments. However, during the past decades, changes in woody vegetation in savanna and
forest systems have been observed in southern Africa. Subsequently, concerns have been raised
about the loss of (tall) trees in areas with elephant. The relative effects of browsing herbivores on
vegetation and the potential browsing interaction with other herbivore species remain unclear and
were examined using vegetation transects and exclosure experiments in savanna woodland and
Sand Forest.
Rainfall, fire and elephant were important savanna determinants. Especially rainfall positively
affected woody densities, which were negatively affected by a longer exposure time to elephant,
but not to elephant densities itself. In general, within South Africa’s savannas, tree height classes
were absent from the population demography. Different height classes were likely to be impacted
by different drivers. For example, seedling and sapling densities were greater with longer fire
return periods and increased rainfall. The Sand Forest exclosure experiments showed that forest
regeneration was impacted by nyala and both elephant and nyala, as the absence of both species
increased tree densities. Both species combined, and individually, also affected tree species
assemblages. In contrast, short term elephant access to a savanna area did not affect tree densities
or species assemblages. In both savanna and Sand Forest elephant displaced mesoherbivores, and
in Sand Forest both elephant and mesoherbivores displaced their smaller counterparts. The
presence of competitive displacement also affected recruitment (i.e. seedlings and/or saplings) of
woody vegetation both in Sand Forest and savanna.
Thus, elephant and mesoherbivores exert direct and indirect (i.e. competitive displacement
providing a window for recruitment) impact on vegetation. Active management of the herbivore
species assemblage affects both vegetation and other herbivores, which effects potentially
cascade into lower trophic levels, jeopardising biodiversity and ecosystem processes. Therefore,
the full herbivore assemblage present and their combined and individual browsing effects need to
be considered when setting management goals to conserve habitats and biodiversity across all
trophic levels. In addition some contrasting results between Sand Forest and savanna emphasise
the need for caution when extrapolating results from different areas and ecosystems.
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2011.
Keywords
Woody plants., Elephants., Vegetation dynamics., Herbivores., Theses--Environmental biology.