Grazing optimization and nutrient cycling: potential impact of large herbivores in a savanna system
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Authors: C. de Mazancourt, M. Loreau, and L. Abbadie
Date: 1999
Journal: Ecological Applications
Volume: 9
Number: 3
Pages: 784-797
Summary of Methods: Estimated field data was used in the development of a model to test the prediction that herbivory can result in grazing optimization. The 35 years of field data was collected on the savanna of Lamto, located 200 km north of Abidjan, Ivory Coast (5° 02' W, 6° 13' N). Multiple previous studies were utilized as comparison sources for references and data acquisition for developing the model. The model includes a nitrogen cycle diagram divided into two separate categories with a total of seven compartments. The parameters of the model cover a wide range of nitrogen inputs and losses (atmospheric, mineral, organic etc.) and plant decomposition rates along with herbivory consumption. Using these parameters, multiple compartmental flux equations were developed explaining the correlation and simple function between nitrogen losses and plant-herbivore interactions.
Article Summary / Main Points: Short-term grazing optimization could occur if the percentage of N lost from the system by herbivore ingestion is less than 24%, whereas long-term grazing optimization requires less than 19% N loss. Primary production reaches zero when herbivore consumption is 98% of primary production. Grazing optimization is most likely to occur in systems with large losses of nitrogen during recycling of plant detritus. Herbivores help decrease nitrogen loss by fires as they conserve nitrogen through the reduction of above ground biomass. Herbivory helps increase plant nitrogen content by keeping plants maintained in an immature, nitrogen-rich state. Spatial heterogeneity encouraged by plant-herbivore interactions can lead to selection for plant palatability.
Vegetation Types: Southern Mixed Prairie Tallgrass Prairie
MLRA Ecoregions: Not Applicable
Agrovoc Control Words: Grazing Rangelands Primary Production
Article Review Type: Refereed
Article Type: Experimental Research
Keywords: grazing optimization, herbivores, primary production, herbivory, savanna, nitrogen cycle, nitrogen budget, nutrient cycling, grazing model, plant pathway, herbivore pathway, grazing equilibrium
Annotation: This study helps illustrate the potential impact of herbivory on the nitrogen cycle with the goal of implementing grazing optimization. Interestingly, herbivores conserve and keep nitrogen within the system by increasing the turn-over rate, even though nitrogen deposition from herbivores is generally patchy, highly concentrated and exhibits high losses from volatilization of ammonia. Furthermore, this study demonstrates that herbivores consistently increase primary production in grazing system despite the removal of 60-90% of above ground biomass. Several diagrams were generated illustrating the relationship between herbivore consumption, production and nitrogen lost through the 'herbivore pathway'. Additional diagrams were produced demonstrating the effects of time on grazing optimization in relationship to grass biomass and primary production.
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