Termites and the structures they create are integral components of African savannahs, particularly with respect to nutrient cycling. However, the routes by which termite mounds influence nutrient cycling and hence plant and animal communities have not been fully understood. These effects are of particular interest in light of the strong spatial patterning they create in many otherwise highly uniform savanna systems. Here, we use stable isotope data and the 15N natural abundance method to estimate the indirect effects of termites on nitrogen fixation rates in clay-rich vertisol soil (“black cotton soil”) savannahs in north central Kenya.
Results/Conclusions We found that that subterranean termites in these common East African communities strongly influence a key ecosystem process, nitrogen fixation by a monodominant Acacia tree species. Further, we document that the spatial extent of these effects combine with the spacing of termite mounds to create a regular spatial pattern in nitrogen fixation rates, creating marked habitat heterogeneity in an otherwise uniform landscape. We estimate that Acacias derive less than 50% of their nitrogen from fixation near to mounds, and greater than 80% when far from mounds. Strikingly, these effects occur well beyond the typical radius of discernable mound effects above ground (~5-10 meters). At the scale of the entire landscape, our models predict that 73% of total N in Acacia is derived from atmospheric fixation; if no termite mounds were present, the total N from fixation would rise to 89 %. We contrast these strong termite-associated effects on nitrogen processes with the much weaker effects of the far more apparent large herbivores in the same system, highlighting the importance of cryptic drivers of community and ecosystem structure.
