Impacts of Black-Tailed Jackrabbits at Peak Population Densities on Sagebrush-Steppe Vegetation

Abstract

In the northern Great Basin, populations of black-tailed jackrabbits (Lepus californicus) are cyclic, reaching high densities at approximately 10-year intervals. This project examined impacts of jackrabbits during a peak in their cycle on sagebrush-steppe vegetation in southeastern Idaho. Total vascular plant cover was significantly lower on plots open to jackrabbit herbivory than on exclosure plots, but in no case was cover of a specific species significantly reduced on open plots. The most severe impacts were on shrubs during winter; most aboveground tissues of both winterfat (Ceratoides lanata) and green rabbitbrush (Chrysothamnus viscidiflorus) plants were completely eaten by spring. However, these impacts were largely ameliorated by compensatory growth during the following growing season, and there was no difference in total biomass for either species between the open and protected plots by July. New growth of winterfat plants that had been browsed the previous winter was significantly greater than that of protected plants. Thus, although the cumulative effects of herbivory reduced total plant cover, no single species was irreparably impacted. Over a year, jackrabbits feeding pressure on nearly all of the important species in these communities; therefore, these hares do not appear to apply differential grazing pressure that would alter the course of vegetation development on northern Great Basin rangelands. The black-tailed jackrabbit (Lepus californicus) is an important native herbivore on rangelands in the western United States (Vorhies and Taylor 1933, Dunn et al. 1982). In northern portions of the species' range, populations are cyclic, reaching high densities at approximately 10-year intervals (Gross et al. 1974, Johnson and Peak 1984). Numerous studies of food preferences and seasonal dietary trends for black-tailed jackrabbits provide a reasonably complete picture of feeding behavior (see Johnson and Anderson 1984), but the effects that black-tailed jackrabbits have on native plant populations are not completely understood. It is generally assumed that peak population densities will have severe impacts on range ecosystems (Vallentine 1971, MacCracken and Hansen 1984), but speculation about such impacts has been based largely upon casual observation. Vorhies and Taylor (1933) argued, Under all but the most conservative stocking with cattle the tendency of grazing by jackrabbits will be to accentuate overgrazing, to eliminate the more palatable grasses and favor their replacement by somewhat less desirable species and by weeds. In contrast, Bond (1945) speculated that jackrabbits would exert a force in favor of succession toward the climax on range that was only moderately deteriorated, whereas on range deteriorated to the point of having more weeds than grasses, their effect would be towards further deterioration. More recently, Rice and Westoby (1978) found that protection from jackrabbits for periods of 5 to 15 years had no consistent effects on vegetation. The impacts of jackrabbit herbivory on long-term vegetation trends are complicated because jackrabbits may suppress or eliminate certain species (Westoby 1974, McKeever and Hubbard 1960) or enhance propagule dispersal of others (Riegel 1941, 1942; Timmons 1942). This project was initiated to assess the impacts of a high populaAuthors are professor and senior research assistant, Department of Biological Sciences, Idaho State University, Pocatello 83209. This paper is a contribution from the Idaho National Engineering Laboratory Ecological Studies Program, supported by the Office of Health and Environmental Research, U.S. Department of Energy. We thank T. Dieffenbach, R. Wilkosz, and D. Pavek for field assistance. Dr. E. Fichter, O.D. Markham, S.J. McNaughton, and R.S. Nowak provided helpful comments on the manuscript. Manuscript accepted 25 July 1985. tion density of black-tailed jackrabbits on sagebrush-steppe vegetation and to examine those impacts in relation to long-term trends in vegetation development. In this paper, we describe the shortterm impacts of a peak in a jackrabbit population on cover, biomass, and leaf length of several important plant species and discuss the implications of these measurements in relation to vegetation