Document: PET-3-48-1

Effects of grazing on the spatial heterogeneity of vegetation.

ADLER, P.B.*, D.A.RAFF and W.K.LAUENROTH

Colorado State University, Fort Collins, CO 80523 ¹

Abstract:
When does grazing increase rather than decrease the spatial heterogeneity of vegetation? The influence of grazing on habitat diversity makes this question of interest to conservation management. Our review of the literature suggests that, at a given scale, grazing's effect on heterogeneity depends on the interaction between the spatial patterns of grazing and spatial patterns of vegetation. Three interactions are possible: 1) Grazing increases spatial heterogeneity of vegetation at scales at which grazing pressure is 'patchy' relative to vegetation. 2) Grazing decreases heterogeneity when grazing pressure is distributed randomly relative to vegetation. 3) When the patterns of grazing and vegetation match, heterogeneity usually but not always decreases. While the first interaction, which produces increases in heterogeneity, is intuitively obvious, interactions causing decreases in heterogeneity with grazing are more interesting. In particular, can grazing have different effects on species composition in different ecosystems, but a consistent effect on spatial pattern? We explored this question with a simple cell-based simulation model of competition between a competitively dominant plant species and a superior disperser. Random grazing (Interaction 2) and selective grazing on the competitive dominant or the disperser alone (Interaction 3), caused subtle decreases in the spatial heterogeneity of both species, reproducing the pattern we inferred from the literature. Simulations incorporating patchy disturbance produced the same pattern. However, in simulations with underlying environmental heterogeneity the spatial effect of grazing depended on which plant species were grazed.

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This abstract is being presented at: 3:15 PM in session:
Oral Session #51: Disturbance Ecology: Harvesting, Grazing and Roads.