Spatial redistribution of nutrients by large herbivores and dung beetles in a savanna ecosystem

Veldhuis MP, Gommer MI, Olff H and Berg MP

Publication   Laymen’s summary   Nederlandse samenvatting

1.   Territorial or resting behaviour of large herbivores can cause strong local deposits of dung, in different places than where they graze. Additionally, dung beetles and other macrodetritivores can subsequently affect local nutrient budgets through post-depositional re-dispersion of dung and accompanying nutrients. Such horizontal displacement of nutrients by animals has strong implications for savanna ecosystem functioning, but remains poorly studied as it is notoriously difficult to accurately map these flows and incredibly time-consuming.
2.   In an African savanna, with alternating patches of lawn, bunch grasses and trees/shrubs, we undertook such effort and studied nutrient aggregation and redistribution by different large herbivore functional groups and dung beetles for a full growing season. We used movable cages to quantify herbivore consumption rates and measured nutrient return through biweekly dung counts. Furthermore, we estimated the offtake of nitrogen (N) and phosphorus (P) by the dominant megagrazer (white rhinoceros) to middens (dung deposition hotspots). Last, we experimentally measured the removal amount and movement paths of telocoprid dung beetles to quantify their nutrient redistribution effects.
3.   Our estimates suggest white rhinoceros to cause a large export of nutrients from grazing areas towards middens resulting in negative nutrient budgets for both lawn and bunch grassland types. Mesograzers (50–600 kg) realized a net nitrogen input towards high forage quality lawn vegetation at the expense of lower quality bunch grasslands. Browsers caused a net flow from trees/shrubs towards grassland patches.
4.   Interestingly, while the magnitude of our estimated flows of N consumption and return by large herbivores were rather similar, the P returns were about half of what has been consumed. This is in agreement with ecological stoichiometry theory that predicts that large herbivores should recycle more N than P, due to their relatively high P demand. Furthermore, dung-rolling beetles had a small, but significant, directed movement from lawn to bunch grassland vegetation.
5.   Synthesis. We conclude that within-ecosystem nutrient redistributions by animals are important and approximately of the same order of magnitude as regional atmospheric nutrient in and outputs (e.g. fire emissions, atmospheric N deposition, biological N fixation), and hence are important for understanding savanna ecosystem functioning.

Graphical abstract

Figure: Overview of the horizontal nutrient flows between vegetation types and dung deposition points for four different functional groups of animals. 1) Browsing herbivores consume nutrients from woody species and deposit urine and dung below trees or redistribute them to grasslands. 2) Grazers consume nutrients from grasses and redistribute them below woody species or within the grass layer through dung and urine deposition. Within grasslands they have been shown to redistribute nutrients from nutrient-poor areas towards nutrient-rich areas. 3) Megagrazers, such as white rhinoceros and hippopotamus, consume grasses and are known to deposit their dung at specific deposition points (middens, rivers). 4) Dung beetles can redistribute dung after herbivores deposited the dung, possibly resulting in flows between the vegetation types.