Article: Ecology of a Grazing Ecosystem: The Serengeti
Authors: McNaughton, S.J.
Journal: Ecological Monographs, 55(3) 1985, pp. 259-294
As noted, the relationships between grazers, grasses, and other biota are complex and highly connected. The research of Samuel J. McNaughton has provided a treasure trove of empirical insights, both observational and statistical, into these relationships. This post explores the relationships between plant productivity and grazing as described by McNaughton.
Using his traditional approach of observation and data collection, McNaughton presents a rigorous statistical analysis of a variety of factors in the paper cited above. The research presented was
…designed to obtain information on vegetation dynamics, the nature of plant-herbivore relations, and ecosystem processes in Earth’s last, vast unmanaged grazing ecosystem [the Serengeti] constituting a natural ecological unit.
A principle part of his analysis was evaluating the factors that measurably affect ecosystem productivity. Naturally, productivity had a strong relationship to annual precipitation:
Productivity of control plots was linearly related to annual rainfall….this relationship explained [approximately] 48% of the variance; inclusion of hilltop and lowland stands from the regional sites reduce the correlation substantially (r = .416) and there were three evident outlier stands at high rainfalls.
McNaughton also presents evidence consistent with his hypothesis that grazing can increase grassland productivity:
The stimulation of aboveground productivity due to grazing was maximum at intermediate grazing intensities… for midslope and flatland grasslands…
The following figure, taken directly from the referenced article, illustrates nicely the ability of grazing animals to maximize productivity at intermediate grazing intensities:
Also noted is the fact that grazing intensity and plant compensatory mechanisms may be the driving forces determining productivity:
Due to the partial ability of grazing to override the rainfall dependence of plant productivity, [annual primary productivity] was controlled more by mean annual grazing intensity than by annual rainfall [and]…grazers tended to override rather than merely amplify patterns of intrinsic vegetation productivity.
Compensatory growth in response to herbivory was also measured and “…on average, only [approximately] 60% of the forage consumed by herbivores was replaced by compensatory plant growth within the same year.”
Although covered in a previous post, it is worthwhile to reiterate some of the mechanisms responsible for these rather impressive compensatory responses:
Reductions in plant competition in grasslands maintained in a short grazing lawn, and competitive release due to canopy opening in taller vegetation, also may be important. By maintaining an open canopy, conserving soil moisture, and recycling nutrients that would become immobilized in senescent plant tissues, grazing may alleviate the intensity of both intraplant and interplant competition.
Yet plant compensatory growth responses to grazing induced defoliation do not necessarily indicate symbiosis or mutualism:
… compensatory growth of the grasslands did not compensate completely for removal by herbivores, and the grasses have evolved levels of silicification, an antiherbivore defense, more pronounced than have been recorded in any other ecosystem. Thus it is improper to conclude that grazing is strictly advantageous to the plants. Highly interactive organisms can be interdependent, as are a parasite with a reduced virulence and a host with increased resistance, without being mutualistic.
In nature, grazing is a self-limiting phenomenon driven by successional dynamics and changes in the species composition of the plant community:
The invasion of heavily grazed grasslands by other species that are more grazing tolerant or avoidant (due to low palatability) than previous species, commonly referred to as a consequence of “overgrazing”, indicates that there are limits to which a flora con tolerate defoliation and other grazing effects. That consequence of grazing represents a natural negative feedback at the community level that will tend to restore a moderate level of grazing in the system.
Land managers observe similar changes when land deterioration and species composition change as a result of poor management decisions, which often force destocking. So while grazing at optimum levels can in fact increase overall productivity, ecosystem constraints will limit grazing in both managed and unmanaged grasslands.