Abstract
The role of grazers in determining vegetation community compositions and structuring plant communities is well recognised in grassy systems. The role of browsers in affecting savanna woody plant communities is less clear. We used three long-term exclosures in the Kruger National Park to determine the effect of browsers on species compositions and population structures of woody communities. Species assemblages, plant traits relating to browsing and soil nutrients were compared inside and outside of the exclosures. Our results showed that browsers directly impact plant species distributions, densities and population structures by actively selecting for species with traits which make them desirable to browsers. Species with high leaf nitrogen, low total phenolic content and low acid detergent lignin appeared to be favoured by herbivores and therefore tend to be rare outside of the exclosures. This study also suggested that browsers have important indirect effects on savanna functioning, as the reduction of woody cover can result in less litter of lower quality, which in turn can result in lower soil fertility. However, the magnitude of browser effects appeared to depend on inherent soil fertility and climate.Conservation implications: Browsers were shown to have significant impacts on plant communities. They have noticeable effects on local species diversity and population structure, as well as soil nutrients. These impacts are shown to be related to the underlying geology and climate. The effects of browsers on woody communities were shown to be greater in low rainfall, fertile areas compared to high rainfall, infertile soils.
Highlights
The role of herbivory in structuring plant communities and determining community compositions is well recognised
A better understanding of how browsers and woody plants interact at the community level and how these interactions affect savanna dynamics will be invaluable in managing browser populations
Acid detergent lignin and total phenolics were most influential along Principle components analyses (PCA) axis 1 (24% of total variation), whilst leaf nitrogen, leaf carbon–nitrogen ratio and stem density were most influential along PCA axis 2 (20% of total variation)
Summary
The role of herbivory in structuring plant communities and determining community compositions is well recognised (see reviews by Augustine & McNaughton 1998; Huntly 1991). Native ungulate browsers have been found to have major impacts on shrub dynamics in East African savannas (Augustine & McNaughton 2004). Different feeding behaviours and food preferences of browser species have major impacts on South African (Owen-Smith & Cooper 1987) and West African savanna woody communities (Jachmann & Croes 1991). Coetzee et al (2008), Gadd (2002) and Helm and Witkowski (2012) looked at browser interactions with Sclerocarya birrea, whilst Fornara and Du Toit (2007) documented the response of Acacia nigrescens to ungulate browsing These two species are important browse species in southern African savannas. A better understanding of how browsers and woody plants interact at the community level and how these interactions affect savanna dynamics will be invaluable in managing browser populations
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