Spatial Heterogeneity of Nitrogen Processes in a Modern Savanna Ecosystem: The Relative Influences of Termites, Megaherbivores, and Acacia Trees

In semi-arid landscapes within Africa termites are one of the most ubiquitous modifiers of habitats, and have been described as both ecosystem engineers and keystone species. However, few data exist on; 1) the magnitude and mechanisms for community-wide effects of termites, and 2) how these effects are mediated by interactions with other keystone groups – namely megaherbivores. Within our study system in East Africa termites mounds are essentially islands of fertility (hotspots) that are responsible for a large fraction of ecosystem-level spatial heterogeneity; they differ in soil composition, support a unique flora, have higher invertebrate densities, and experience elevated levels of grazing by megaherbivores. Matrix soils are clay-rich vertisols, with low plant-available N, and support two dominant plant types; an Acacia tree species, and C4 grasses.

Away from termite mound ‘hotspots' N2-fixation by Acacia trees is an important input of N into savanna ecosystems. We used stable isotopic analyses of plant tissues to investigate how termite mounds, and the preferential use of mounds by megaherbivores, generate spatial heterogeneity in N processes. We sampled Acacia, C4 grass and Aspelia sp. (non-fixing herb) foliage on and off termite mounds inside and outside of large herbivore exclosures at the Mpala Research Center in Kenya. The abundance of 15N in Acacia foliage, and therefore the amount of N not derived from N2-fixation, was significantly higher near termite mounds, and megaherbivore presence had no effect. Likewise, C4 grasses and Aspelia had higher d15N values on vs. off mounds (again no herbivore effect), indicating that Acacia trees are a source of biologically fixed N2 away from termite ‘hotspots'. Our results show that termite mounds exert strong spatial controls on landscape-scale N cycling in forested savannas. Megaherbivore use of mound ‘hotspots' did not enhance these patterns, indicating that termites can independently generate ecosystem-level heterogeneity.