Grazing ungulates, plant biomass concentrations, and nutrient cycling

The evolutionary ecologist S.J. McNaughton is well-known as a strong advocate of the grazing optimization hypothesis.  Essentially, this hypothesis is summarized as follows:

…grazing benefits many grasses and other plants in grassland ecosystems… moderate grazing promotes the productivity of many grasslands above the levels that prevail in the absence of grazing. {footnote} Grasses and Grazers, Science and Management. McNaughton, S.J. 1993. Ecological Applications, 3(1) pp. 17-201

Through the years, McNaughton’s research has been empirically thorough and statistically rigorous.  His general methodological approach has also been simple and straightforward.  Through the use of fencing exclosures in the still wild grassland ecosystem of Serengeti National Park, he has been able to measure and compare the effects of grazing on vegetation, soil, water, and nutrient cycling.  The African Serengeti is home to 3 million head of over 25 species of herbivorous ungulates, and is one of the last remaining wild grassland ecosystems in the world.  Therefore, McNaughton’s research provides key insights into the natural functioning of grassland ecosystems in the absence of massive human interventions like fencing and domestic livestock.

In one of his research papers, McNaughton explores the idea that

…gregariousness in grazing animals may increase foraging efficiency by modifying vegetation structure to increase food yield per bite to the individual grazer in a herd. {footnote} Grazing Lawns: Animals in herds, plant form, and coevolution.  McNaughton, S.J. 1984.  The American Naturalist, Vol. 124. No. 6 pp. 863-886.2

In this research, he compares biomass concentration data from exclosures and the natural surrounding grasslands.  His measurements reveal that biomass concentration “was consistently higher outside exclosures (0.44 mg/cc) than inside them (.34 mg/cc)”.  And “the maximum biomass concentration of grazed vegetation was achieved only when the canopy height was arrested at very short statures, i.e., in grazing lawns.” 3

These research results have implications for both land managers and ecologists:

…low plant biomass concentrations, even if total standing crop is high, can result in forage consumption rates insufficient to meet herbivore energy and nutritional requirements, leading to declining herbivore condition amid high plant biomasses.  Below a bite size of about 0.3 g, a cow-sized animal is food limited…cow bite size will fall below that level at plant biomass concentrations below 0.8 mg/cc. 4

As noted above, what is required to maintain high biomass concentrations are “large dense animal aggregations [that] create and maintain vegetation of high biomass concentration and quality…”  Moreover, “individual grazers obtain a foraging advantage by membership in a herd because of the greater forage yield per bite from grazing lawns compared with lightly grazed vegetation.” 5

In this case, biomass concentration was measured as the weight per volume of green forage (milligrams per cubic centimeter).  McNaughton’s research demonstrates that biomass concentration could in fact be used as a key measure of grazing efficiency as a function of plant productivity; as a rule of thumb, biomass concentrations at 0.8 mg/cc and above could be considered optimal for superior animal performance.

Other similar research conducted by McNaughton demonstrates the importance that large animal herds have for nutrient cycling.  Through the course of 20 years of observation, he noticed that animals tend to concentrate in some areas and not in others.  To test the hypothesis that soil nutrients are higher at areas of animal concentration, McNaughton extracted soil cores at these sites and paired them with similar sites where animal concentrations are not common.  In terms of Nitrogen (N) results were noteworthy:

…the net N mineralization rate in soils supporting dense resident animal populations was over twice that of areas where animals are uncommon…[and grazing] leads to increased leaf N concentration and therefore to litter of greater decomposability.  In addition, urination enriches soil with N from urea, leading to a burst of organic matter mineralization that produces greater available mineral N in the soil than is added as urea.  {footnote} Promotion of the Cycling of Diet-Enhancing Nutrients by African Grazers. McNaughton, S.J. Banyikwa, F.F. McNaughton, M.M. 1997. Science Vol 278.6

Similarly noteworthy were the results for sodium (Na) concentrations.

Standing stocks of extractable Na concentrations were universally, and substantially, higher in soils of animal concentration areas…[and] grazing increased the Na supply from Serengeti soils by an order of magnitude. 7

Sodium is an essential element for animals but required at best in very small concentrations by plants.

Also, “Serengeti grazers tangibly accelerate the mineralization of two minerals of considerable importance in animal nutrition.”  This means that “habitat deterioration is not an inescapable consequence of increased density of organisms.”  On the contrary, in some cases it would seem that dense animal herds are in fact responsible for maintaining resilient and productive grassland ecosystems.

Other research by McNaughton provides further insight into the nature of biological organisms as the lynch pin for nutrient cycling in grassland ecosystems.  McNaughton has observed that “freshly deposited dung was more likely to be adjacent to other fresh dung with an active dung beetle fauna than to older dung.” 8 McNaughton, S.J. 1985. Ecology of a Grazing Ecosystem: The Serengeti. Ecological Monographs, 55(3) 1985, pp. 259-2949  While the mechanisms for this are not well understood, it seems that grazing ungulates deliberately deposit their dung in areas where it will be quickly recycled by localized dung beetle populations.

Finally, McNaughton recognizes the role of grazing ungulates as nutrient recyclers in their own right:

…a major contributor to the stimulatory effect of grazing on growth of the Serengeti grasslands likely is nutrient recycling through dung and urine, emphasizing the importance of large grazing mammals in the dynamics of grassland ecosystems.9

In summary, ecosystems that in which grazers, grasses, and other biota have coevolved are enmeshed in a complex and interrelated web of symbiosis and mutualism; understanding the dynamics of these relationships will help land managers to more effectively mimic the natural processes at work in these ecosystems.

1Grasses and Grazers, Science and Management. McNaughton, S.J. 1993. Ecological Applications, 3(1) pp. 17-202.

2Grazing Lawns: Animals in herds, plant form, and coevolution.  McNaughton, S.J. 1984.  The American Naturalist, Vol. 124. No. 6 pp. 863-886.

3Ibid.

4Ibid.

5Ibid.

6Promotion of the Cycling of Diet-Enhancing Nutrients by African Grazers. McNaughton, S.J. Banyikwa, F.F. McNaughton, M.M. 1997. Science Vol 278.

7Ibid.

8McNaughton, S.J. 1985. Ecology of a Grazing Ecosystem: The Serengeti. Ecological Monographs, 55(3) 1985, pp. 259-294

9Ibid.


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