The Joint Effects of Herbivory and Competition Upon Plant Communities

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This proposal by Dr. Norma Fowler of the University of Texas, Austin is to examine the combined influences of grazing and interplant competition on plant community structure. The effects of grazing upon plants are not limited to the effects of defoliation, but include the effects of trampling, and urine and dung deposition. Investigation of these secondary effects will not be done. Initially, the investigation will concentrate upon the effects of defoliation. However, each of the field experiments will include both a (fenced) clipping treatment and an (unfenced) grazing treatment as well as a (fenced) control; differences between the clipping and grazing treatments may be due to these secondary effects, or perhaps to differentiate between mechanical clipping and the action of an animal's jaw. An absence of differences between the clipping and grazing treatments will support the null hypothesis that in this system the effects of grazing are largely limited to the direct effects of tissue removal. A clipping treatment will be included in these experiments, not only to test this hypothesis, but also because the effects of grazing are also a function of animal preferences for plants of different species, sizes, physiological and developmental states, surrounding vegetation, and so on. An apparent interaction effect between competition and defoliation can arise from the effects of preference, when none is actually present. Estimates of preference will be made and the results used to indicate appropriate additional experimental treatments. This work has much intrinsic, ecological interests, but its immediate practical value is in its importance for understanding plant community maintenance, especially in over-grazing situations in nutrient poor prairies, savannas, etc.