The functional importance of termites across a savanna rainfall gradient

Abstract

Termites are ecosystem engineers with well documented functional roles in African savannas. However, how their importance changes across environmental gradients, such as rainfall and nutrition, is less understood. Using LiDAR data from the Carnegie Airborne Observatory, I mapped the distribution of termite mounds across an entire river catchment in the Kruger National Park, South Africa. Abiotic factors shaped mound densities at broader landscape scales, while local hillslope morphology strongly influenced mound distribution at finer scales. Mound distribution exhibited a clustered pattern throughout the landscape, occurring at higher densities on crests, which are nutrient-poor. However, at fine scales mounds exhibited over-dispersion so that evenly spaced aggregations of termite mounds are embedded within a landscape of varying mound densities. The activity status of mounds also varied across the catchment, with a higher proportion of mounds active in drier sites. A key mechanism accounting for the importance of termite mounds is the high concentration of nutrients they hold. Although these nutrient-hotspots are known to enhance savannah biodiversity and attract herbivores, what is less clear is whether they are as important in nutrient-rich compared to dystrophic savannas. Working across a rainfall gradient, I investigated grass communities on and off termite mounds and along transects away from mounds in order to calculate the spatial influence of termite mounds on grass communities and how this changes with landscape context. Although grass species richness was lower on mounds than in the savanna matrix, the assemblage composition varied significantly, with higher nutrient concentrations in grasses located on mounds; this pattern became more distinct with increasing rainfall. The spatial extent of these nutrient-rich grasses also differed Abstract across the rainfall gradient, with a larger sphere of influence around mounds in the wetter, nutrient-poor areas, due to patterns in soil nutrients. Using termite mound densities estimated from airborne LiDAR, I upscaled field-based results to determine the percentage of the landscape influenced by termite activity, showing that mounds distinctly altered grass communities over ~2% of the landscape. The impact these altered grass communities have on savanna grazing herbivores was then investigated across the rainfall gradient. Grasses on termite mounds were preferentially grazed at all rainfall sites and in both seasons tested. Importantly, however, mound influence varied in time and space. Mounds were more heavily grazed at wetter savanna sites and influenced more of the landscape here during the wet season. However, during the dry season, when mound utilisation was higher, this pattern was reversed, with more of the landscape (19%) influenced at the driest site. Southern slopes of termite mounds exhibited higher grazing pressure and insects were also shown to prefer mound grasses. Finally, through measuring the decomposition rate of four savanna grass species, I show that grass decomposition in African savannas varies significantly along rainfall gradients, with various factors, including termites, becoming influential in different habitats. Importantly, I demonstrate that fire does not always slow decomposition and that it interacts with other factors to influence the process. Overall, termites are important contributors to savanna heterogeneity, with functional importance varying with landscape context.