The role of termites in South African savanna ecology

Abstract

Termites are ecosystem engineers that influence savannas through the process of decomposition and bioturbation (movement of soil by biota). Firstly, decomposition is the process by which dead plant biomass is recycled and made available again for uptake by other plants. Using a novel large-scale termite suppression experiment, I quantify, for the first time, the relative contribution of microbes, termites, and other invertebrates to the decomposition of wood (native and non-native), dung, and grass in a mesic savanna. I found that termites were responsible for two thirds of the mass loss from dry wood and a third of the mass loss from fresh native wood, dry dung, and dry grass. Microbes were wholly responsible for the difference as there was no evidence of other invertebrates contributing to decomposition, even with fresh wood. Global decomposition studies have demonstrated that decomposition is primarily temperature-driven with rainfall playing a secondary role, although to date, all global decomposition studies have used a single substrate to measure decomposition meaning differences with substrate type may be missed. Here I explored the decomposition of wood, dung and grass along a rainfall gradient (380–650 mm/year) at three savanna sites in South Africa. As the three sites experience a similar temperature range, I effectively controlled for temperature allowing us to explore the effect of rainfall in isolation. I used decomposition bags - with and without termite access - and measured mass loss after set time intervals. I predicted that the absolute decomposition of all three substrates would increase along the rainfall gradient. This assumption held for dung and grass, but I found the reverse for wood. My study highlights the importance of considering multiple substrates as decomposition patterns and dominant agents can vary. I suggest that termites should be recognised alongside microbes and fire as the primary agents of wood, grass, and dry dung turnover in global carbon models. Secondly, through the process of bioturbation, termites profoundly affect soil turnover rates and modify the patterns of water and chemical flux in soils. Termites build two kinds of above-ground biogenic structures: mounds and sheeting. Sheeting is constructed to protect foraging termites against desiccation and predation while they consume the organic material. The characteristics and dynamics of soil sheeting are not yet well understood. In terms of quantity, I found there were approximately two tonnes of soil sheeting per hectare present across both the soil surface and the trees of my mesic savanna (650 mm p.a.) at each survey time. Both horizontal and vertical sheeting soils were significantly enriched in nitrogen relative to the matrix soils. In savannas, primary productivity is strongly limited by nitrogen. These nutrient differences likely give rise to the creation of small nutrient rich patches, on a finer scale than those created by termite mounds, with important consequences for savanna spatial dynamics. Finally, I present the first rigorous and directly measured combined carbon stock and net primary productivity (NPP) dataset from a southern African savanna. This study serves to provide larger system ecological context in which to place termite functioning. My findings serve to highlight the extreme spatial and temporal (annual and inter-annual) heterogeneity present in savannas. In addition, my results are suggestive that greater termite activity gives rise to greater grass production but owing to the small sample size this result needs rigorous testing. I contextualised my biomass and NPP findings within both the published African GEM sites and the other African savanna studies that were assessed using a biomass dynamics approach. The results of this study highlight the need for a greater number of plots per site and greater within plot replication, particularly below-ground, within these plots in future savanna GEM studies. Quantifications such as this are urgently needed to enable ecologists to rigorously evaluate the potential of savannas as carbon sinks.