Animals move around through the landscape and can therefore disperse materials from one place to another. Among these materials are essential nutrients for plants like nitrogen and phosphorus. For example, large herbivore ingest plant nutrients in one place by eating plant material, but generally excrete them hours or days later at a different place. Although these principles are well known, the actual balances for different areas are hard to quantify as it is practically impossible to follow all these nutrients continuously. Furthermore, herbivore species differ in what they eat and how they move around.
A way to overcome these practical problems is to quantify local nutrient budgets and upscale these finding to the ecosystem. We therefore measured the consumption of grasses and the nutrients inside these grasses for a full year to estimate the amount of nutrients extracted from the vegetation by herbivores. At the same time we counted the amount of dung that was produced by different herbivores species and converted this to nutrients using data from the literature as an estimate of nutrient return. The balance between consumption (nutrient extraction) and excretion (nutrient return) provides an estimate of whether the amount of plant available nutrients increase or decrease over time. We did this for three distinct vegetation types in an South-African savanna ecosystem: short lawn grasses (highly nutritious, preferred by grazers), long bunch grasses (much less nutritious) and woody species (preferred by browsers). From these local balances we could then estimate the nutrient redistributions between the different vegetation types.
We measured biomass consumption using movable cages, where the difference in grass biomass inside and outside the cage is attributed to grazing (left). Dung beetles were collected and identified to species by Matty Berg (right). Photos: Moniek Gommers
Intermediate-sized grazers increase nutrient availability of grazing lawns.
We found that intermediate-sized herbivores (warthog, impala, zebra) moved nutrients from bunch grasslands to grazing lawns, thereby fertilizing their own preferred grazing areas. This confirms the hypothesized positive feedback loop between grazers and grazing lawns. Browsers redistributed similar amounts of nutrient from woody patches to grasslands as grazers moved from grasslands to woody patches, balancing each other’s effect.
Intermediate-sized grazers in Hluhluwe-iMfolozi park. Warthog piglets (left), wildebeest (middle) and zebra (right). Photos: Michiel Veldhuis
White rhinoceros exports nutrient to middens
The megagrazer white rhinoceros defecates in middens (large dung heaps) as part of their territorial behavior, resulting in a continuous export of nutrients from grasslands. This effect overrides the effects of intermediate-sized grazers. White rhino densities are high in the study area and their large body size and specific behavior make their effect on the spatial distribution of nutrients extremely high, with important consequences for ecosystem functioning. A still open question is what happens to all the nutrients in the middens at longer time-scales (decades).
Megagrazer white rhinoceros (left) and a midden (right) in Hluhluwe-iMfolozi park. Photos: Michiel Veldhuis (left) and Ruth Howison (right)
Dung beetles also contribute significantly to nutrient dispersal
Once herbivores have dropped their dung pellets, dung beetles initiate a second phase of nutrient dispersal through rolling dung balls. These can be moved over significant distance (>50m). We therefore executed a small side-experiment to investigate their effect and found that dung beetles prefer to bury their dung balls in taller bunch grass vegetation, probably because the soil is more loose and wet here. This caused a net movement of nutrients from grazing lawns to bunch grasses, the opposite effect of intermediate-size herbivores. However, from our experiment it was not yet possible to obtain reliable flow estimates, so the magnitude of dung beetle nutrient dispersal on the overall nutrient budgets of African savanna ecosystems remains to be seen.
Dung beetles moving balls in Hluhluwe-iMfolozi park. Photos: Michiel Veldhuis (left), Matty Berg (right)
Animals move significant amounts of nutrients and contribute to spatial heterogeneity
This study provides estimates of the spatial redistributions of nutrient by animals in savanna ecosystems and has shown that these flows are significant. Compared to other nutrient flows, like nitrogen deposition, nitrogen fixation or nitrogen emissions through fire, the movement of nutrients by animals is about the same order of magnitude. More importantly, through there nutrient redistribution act they increase the heterogeneity of these incredible ecosystems, contributing to their well-known biodiversity.