The Context of Grassland Defoliation

Jeffrey Fehmi

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Published: April 1, 2002

The underlying assumption of Gutman et al. (2002) seems to be that livestock management is the determinant of vegetation structure. With stocking rates typical of the developed world, which may use even less than half of the available forage, this may not be true. Experimental treatments in this study (as well as in most other field-based livestock studies) may have inadvertently combined the direct impact of livestock with an indirect response of voles or other rodents. In the control or lesser-grazed treatments in livestock grazing studies, the increased biomass and vegetation cover greatly enrich rodent habitat and often allow populations to increase (especially voles). The disproportionately high metabolic rates, activity levels, and corresponding appetites of such rodents have the potential to profoundly impact these systems. In addition to the food consumed, other behaviors, such as the aboveground runways (tunnels made from clipped vegetation) constructed by many species of voles, can also destroy substantial amounts of vegetation.

The interacting effects of cattle and voles might explain the inconsistent results from grazing experiments and their derived management practices, including seasonal grazing practices designed to maintain and enhance native perennial grasses. It seems easy to conclude that grazing studies that control only livestock may not answer important questions about the forces that structure plant communities or control the responses of individual species. For instance, one recent study demonstrated that the impact of six months of vole activity was still apparent nearly 3 yr after voles were excluded from the study area (Howe and Brown 2001). Future livestock impact studies need to better document the relationship between the plant community and rodent activity, especially with experiments that directly manipulate rodent numbers.

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Gutman, M., I. Noy-Meir, D. Pluda, N. A. Seligman, S. Rothman, and M. Sternberg. 2002. Biomass partitioning following defoliation of annual and perennial Mediterranean grasses. Conservation Ecology 5(2): 1. [online] URL: http://www.consecol.org/Journal/vol5/iss2/art1

Howe, H. F., and J. S. Brown. 2001. The ghost of granivory past. Ecology Letters 4:371-378.