The influence of wildlife and livestock herbivore assemblages on grazing lawn flora

Abstract

Large swathes of savannas in Africa have witnessed the replacement of indigenous herbivore assemblages by domestic livestock, the consequences of which remain poorly understood for savanna plant communities. Grazing lawns, which are important floristic features of African savannas, are characterised by intensely grazed, short-grass patches with grass species uniquely adapted to tolerate high grazing intensity. In turn, lawn-grass ecosystems provide a nutrient-rich forage resource to large mammal grazers. However, heavy grazing sometimes results in woody expansion, which threatens to diminish the productivity of lawn-grass ecosystems. The aim of this study was to determine the consequences of replacing wild herbivore assemblages by domestic livestock for grazing lawn flora. The first part of this study set out to determine the prevalence of grazing lawns created by livestock, and then to contrast species composition, structure and function of grazing lawn flora between wildlife and livestock herbivore areas. Transects were surveyed in wildlife and livestock areas in and around Orpen, Kruger National Park, South Africa. Grass and forb species composition, growth form, cover and height were recorded. So too was tree density and tree height. Thereafter, the effect of wildlife vs. livestock areas on these was tested. Livestock areas were characterised by denser and taller tree communities, suggesting that browser absence facilitates tree establishment and growth. Grass and forb species composition differed between the two herbivore communities. Livestock areas uniformly possessed short grass, whereas wildlife areas were heterogeneous, possessing both short and tall grass patches. Furthermore, the strong positive effect of tree density on grass height in wildlife, but not in livestock areas, suggests predator-avoidance behaviour as a distinctive feature of wildlife systems. In summary, replacement of wildlife by livestock (at higher densities) results in very different ecosystem function, driven by differences in the functional composition of the two herbivore assemblage types, and also by contrasting management practices. The second part of this study set out to determine the role of wildlife browsers in maintaining the open physiognomy of lawn-grass ecosystems, to better understand why heavily grazed wildlife areas tend to experience woody suppression while heavily grazed livestock areas mostly experience woody expansion. The effect of grass height on browser species presence was tested in a short- and tall-grass mosaic in Satara, Kruger National Park. In the same mosaic, an exclosure experiment was set-up to test the effect of grass height habitat (short vs. tall) and herbivory (exclosed vs. non-exclosed) on seedling survival and sapling growth. The effect of grazing intensity and herbivory were also tested on long-term growth rates of woody plants in Hluhluwe-iMfolozi Park (HiP). Preference for short grass habitats scaled negatively with interspecific herbivore body mass, indicating that predator avoidance might be a key factor driving smaller bodied herbivores to utilize grazing lawns. At intraspecific level, impala was the only species to significantly select short- over tall-grass as preferred foraging sites, while large browsers, elephant and giraffe, preferred tall grass. The exclosure experiments revealed that seedling survival and sapling growth are indeed limited by herbivory in short grass, though evidence for this was stronger in the long-term exclosure experiment in HiP than at Satara. Combined, these results support the hypothesis that browsers, specifically impala and small-bodied mixed feeders elsewhere, are necessary for maintaining grazing lawn open physiognomy. Overall, this study demonstrated that the replacement of wildlife by domestic livestock (at higher densities) has important consequences for grazing lawn flora, and savanna plant communities in general, by altering the structure, function and species composition of savanna grass, forb and woody plant communities.