Methods to improve forage quality for mammalian herbivores in nutrient poor savannas
The savanna biome covers one eighth of the earth’s land surface area and contains high levels of biodiversity. Many people inhabit savannas and utilise them for agriculture and livestock farming. Particularly in this biome, biodiversity conservation is under an ever mounting threat by the rapidly growing human population with livestock and their associated requirements for arable land use. The quality of the browse and graze is paramount to the survival of the many free-roaming herbivores, especially in nutrient poor savannas that are characterized by unpalatable grasses and broad-leaved trees with high concentrations of secondary metabolites in their foliage. Grazing lawns, with short, stoloniferous grasses, could provide high-quality food for grazers in these areas, but lawn grasses occur only sporadically in nutrient poor savannas. There is not much known about the effects of adding specific nutrients on the establishment of grazing lawns in nutrient poor savannas and the resultant quality of the forage. Therefore, this thesis aims to improve the understanding of the effect between large free-roaming herbivores in a nutrient poor savanna and improving the quality of the forage.
Often conservation practices lack scientific underpinning, which is the motivation for this thesis. I address the question whether nutrient addition leads to the establishment of grazing lawns in a nutrient poor savanna, coupled with mowing with a tractor and slasher to reduce the woody components which compete with the grasses. Such addition is hypothesized to result in a switch (“jolt”) from tall, nutritionally poor grass to short, highly nutritional grass species. The main hypothesis of this thesis is that a savanna can be forced from a state with low quality forage as food for large free-roaming herbivores, to a state with high-quality forage through management interventions of these systems.
Before understanding the intricacies of evidence-based conservation experiments in forcing a savanna to change its state of food quality, it is important to understand the complexities of managing protected areas and to understand the origins for their establishment. Therefore, I first summarized the motives for establishing protected areas in Southern and Eastern Africa, and the possible consequences for management of these areas today. The findings illustrate that the establishment of protected areas occurred in three periods, and that these periods gave rise to ramifications for management of these areas today. One of the ramifications is to enhance the quality of the vegetation as food for mammalian herbivores.
I then tested whether we could use the addition of fertilizer to establish grazing lawns in a nutrient poor savanna, in order to achieve a switch from a vegetation of tall, nutritionally poor grass species to a short vegetation comprised of (highly) nutritional grass species. The experiments demonstrate that fertilization in nutrient poor savannas can trigger the establishment of grazing lawns, coupled with mowing with a tractor and slasher to reduce the woody components which compete with the grasses. I found that phosphorus supplementation resulted in a shift from tall grasses to short grasses within three years with a higher nutrient concentration in the grass species than without supplementation. When grazed, the biomass of lawn grasses was higher compared to the other grasses when not grazed, demonstrating the role of grazers in lawn grass patch establishment.
I study the effects of increased grazing pressure by mammalian herbivores on maintaining patches of lawn grass and tested whether the application of specific nutrients, nitrogen, phosphorous or in combination with lime, would attract mammalian herbivores to graze frequently. I showed that the addition of N attracts and increases the grazing pressure for a number of herbivore species and our findings suggest that often abundantly present mammalian herbivores with intermediate body mass can maintain grazing lawns. This study illustrates that the distribution of specific nutrients to an area has an effect on the vegetation, causing elevated local grazing pressure resulting in improved quality of forage. I concluded that ongoing local fertilization in nutrient poor savannas is a practical and achievable method of ensuring that herbivores obtain high-quality forage.
Further to obtaining a better understanding of improving the quality of the forage for free-ranging herbivores, I wanted to study how herbivores could obtain additional nutrients or better forage quality from the broad-leaved trees in a nutrient poor savanna. So I deployed game lick blocks with polyethylene glycol (PEG), a polymer, to standard game lick blocks (nutrient supplements) in an attempt to establish if we could broaden the diet of herbivores with higher percentages of browse species and higher amounts of utilisation per browse species that had high levels of condensed tannins. I showed that the addition of PEG to standard game lick blocks supported a change in the dietary choices of herbivore species and that these changes are expressed as a broader diet choice (higher percentage of browse species) and a higher number of browse species utilised with higher concentrations of secondary compounds in their diet. The findings showed that the largest change in diet selection was for Burchell’s zebra (a grazer) and impala (a mixed feeder).
My thesis shows that there is a need for the manipulation of vegetation and animals through evidence-based conservation, in a nutrient poor savanna to help with survival of the herbivore species. I have shown that through the addition of nutrients, and the use of a polymer to combat tannin, in a practical manner will improve forage quality in a nutrient poor savanna. As a consequence, this better quality food that would otherwise not have been utilised, thus helps in alleviating the effect of food shortages during the dry winter season.
My thesis provides scientific understanding of the association between large free-roaming herbivores in a nutrient poor savanna and methods of improving the quality of the forage. In conclusion, this study provides new information on nutrient-plant-herbivore associations that helps evidence-based conservation, forcing a nutrient poor savanna from a state with low quality forage for large free-roaming herbivores, to a state with high-quality forage, through management of these systems. These findings have important consequences for the management of nutrient poor savannas around the world.