Michiel Veldhuis tells us more about his recent paper on the role of large herbivores and dung beetles in the redistribution of nutrients in the African savanna. You can also see some videos taken by the authors, available below and also on the Journal of Ecology YouTube channel.
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 herbivores 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 findings to the ecosystem. We therefore measured the consumption of grasses and the nutrients inside these areas for a full year to estimate the amount of nutrients extracted from the vegetation by herbivores. At the same time we measured the amount of dung that was produced by different herbivore 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 plants available for nutrients increase or decrease over time. We did this for three distinct vegetation types in a 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.
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 nutrients from woody patches to grasslands as grazers moved from grasslands to woody patches, balancing each other’s effect.
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).
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 distances (>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 there. 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.
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.
View more videos taken by the authors on the Journal of Ecology YouTube channel.
Michiel Veldhuis, University of Groningen, The Netherlands