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The bulldozer herbivore: how animals benefit from elephant modifying an African savanna

  • Kohi, E.
Publication Date
Jan 01, 2013
Wageningen University and Researchcenter Publications
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Herbivore-vegetation interactions are important structuring forces in savanna that modify the availability and quality of forage resources. Elephant for example, are known for their ability to change the vegetation structure through toppling trees, uprooting, snapping, debarking and breaking branches. Controlling the number of elephant is a common response of wildlife managers who think that the increase of elephant will further destroy the habitat and hence cause loss of biodiversity. However, our knowledge on how elephant feeding habits affect other large herbivore species in habit use is limited. Therefore, the question in this thesis is: What is the impact of elephant feeding habits on species diversity of large herbivores in African savanna? To answer this question, it is important to understand the responses of trees when impacted by elephant. What proportion of the browsed biomass is made available after a tree is pushed over or snapped by elephant? How is the forage quality affected? Is the seasonal and intensity of browsing affecting forage availability? Field experiments and field surveys were used to investigate the tree’s response and herbivore species responses to elephant impact. These experiments were (1) simulation of timing and intensity of browsing (hand defoliation) and (2) manipulation of vegetation i.e. simulated pushed over trees, uprooted trees (tree removal) and snapped trees (tree cut at the stem). The field survey involved measuring impacted trees by elephant. Leaf biomass and quality of pushed over, snapped and uprooted trees were measured. The defoliation experiment was conducted in the roan antelope enclosure in Kruger National Park South Africa, and the vegetation manipulation experiment was conducted in the Umbabati Private Nature Reserve, South Africa. The results indicate that elephant foraging habits change the distribution of forage, increasing the forage availability at lower feeding heights, which means that accessibility of forage to medium and small herbivores increases. Elephant browsing also improved forage quality and availability in the dry season, which is very important to browsing animals. A high intensity of browsing by elephant in the wet season increased the dry season forage, because the amount of new regrowth (leave compensation) is proportional to the amount of leaves that was removed. Elephant therefore initiate inter and intra-species facilitation processes. Inter-species facilitation occurs when other herbivores species utilize the regrowth stimulated by elephant, whereas intra-species facilitation occurs when the browse resource is exploited by other elephants. The herbivore responses to elephant browsing clearly indicated that facilitation effects occur, especially for certain guilds when selecting their habitat. For example, small predation-sensitive herbivore species (steenbok, impala and common duiker) preferred completely opened up areas, whereas large herbivores were less affected in their habitat preference by elephant impact. Greater kudu selected pushed over and control plots and rarely visited opened up areas. These differential response of herbivores species to elephant impact resulted in a high species richness of large mammals in elephant impacted areas. In conclusion, elephant feeding habits play a major role in structuring the herbivore assemblage/community through modifying the vegetation. Resource heterogeneity increased under the influence of elephant feeding, in particular through increasing the accessibility of leaf biomass at lower feeding heights, increasing green leaf availability in the dry season, and improving the nutrient content in re-growth. Moreover, habitat selection, especially that of small herbivores was positively influenced by elephant impact. With these findings, this study contributes to a better understanding of the role of elephant feeding habits and its cascading effects to other herbivore species.

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