Article: Ecology of a Grazing Ecosystem: The Serengeti
Authors: McNaughton, S.J.
Journal: Ecological Monographs, 55(3) 1985, pp. 259-294
McNaughton’s research also sheds light on the proportions of utilization and the effects of multiple ungulate species harvesting biomass from the same resource base. As we have noted repeatedly on this blog, utilization is a key factor that determines the spatial geometry, nutrient cycles, and successional trajectory of grassland ecosystems. In light of what McNaughton’s research has shown us about natural cycles in unmanaged grazing ecosystems, the question arises: What utilization patterns are observed in nature?
McNaughton provides utilization estimates through a variety of statistical tools:
…estimating consumption by comparing peak biomass inside fences and terminal biomasses outside fences would have led to the conclusion that 92% of aboveground production was consumed…the annual average consumption as a proportion of actual primary production in Serengeti grasslands exceeded .5 at 19 of the 28 study sites; some exceeded that value by a substantial margin. The most lightly grazed Serengeti site, with 17% of aboveground primary productivity consumed by herbivores, was well above the values from most terrestrial ecosystems. The maximum proportion consumed was .94.
And:
Similar estimates for the Serengeti region would indicate that consumption averaged 92% of aboveground production. A more realistic estimate is the ratio of consumption to actual net productivity, which averaged .66 with a median of .71.
Clearly there is significant variability in utilization patterns, with extraordinarily high values at some spots and relatively low values at others. The average and mean may mask this variability. Deriving a utilization rule of thumb from this data is not necessarily recommended. However, what is apparent is that animals are selecting for high utilization in some areas and lower utilization in others. Animal behavior in response to a changing environment is a strong factor that drives uneven utilization on the landscape. This will be addressed in a subsequent post. Grazing ungulates in the African Serengeti also prefer green forage; migratory patterns on a vast, unfenced landscape allow animals to graze green forage almost exclusively: “green forage made up a relatively constant proportion of consumption.
80% over a broad range of green forage on offer.” In modern-day ranching operations, hay is usually shipped in from irrigated pastures or distant biomes to simulate similar feeding behavior.
One must remember that in ecological terms, the Serengeti is a multi-dimensional system. Grazing herds are not monotypic; that is, a variety of grazing ungulates coexist on the landscape, each one seeking advantage through different feeding strategies. McNaughton notes this in his observation of the synergies between zebra (an equine species) and wildebeest (a ruminant species):
The appearance of the sward changed substantially during the grazing succession. Prior to wildebeest passage, the grassland had little apparent structural differentiation. After they passed, ungrazed patches stood out as tall clumps. That structure facilitated zebra foraging in a way heretofore undescribed, as I observed zebras walking from tall patch to tall patch to feed. Grazing by zebras had a leveling effect on the vegetation, and the regrowth following their passage produced a structurally homogenous grazing lawn of high biomass concentration.
These interactions also shed light on how utilization occurs over time and space in a multi-species environment. As McNaughton observed:
During their passage through the stand, wildebeest consumed 156 g/m2, 76% of the initial standing crop, in a few hours. The zebra herd subsequently consumed 21 g/m2, or 44% of the remaining forage. Overall, the two grazers consumed 87% of the initial standing crop in a 4-d period. To the extent that there is excess soil moisture, such stands will regrow after grazing.
The following table, taken directly from the referenced article, displays the extent to which multi-species environments are subjected to different grazing pressures in the course of a year.
A study of this table reveals seasonal changes in grazing preferences for each species. It also shows that nature has filled various niches with the preferential feeding behavior of different species, thereby allowing for a more complete trophic food web and full-spectrum utilization of available biomass resources. McNaughton’s observations force us to reevaluate our own approach to land management, raising some difficult questions in the process. Can we simulate natural ecosystem processes through the use of multi-species herds? Does multi-species grazing in a ranching environment result in a more effective utilization of available biomass resources? Will multi-species grazing increase the nutrient density of the grassland, creating a dense and nutrient rich “grazing lawn”? And can human ingenuity be used to apply similar grazing patterns that are based on nature’s model?