Effects of Range Treatment with 2,4-D on Rodent Populations

Abstract

The treatment of perennial forb and shrub-grass ranges with 2,4-D in western Colorado usually produced an increase in grass cover and a decrease in the cover of most forbs and shrubs. Recovery time of herbicide-sensitive species varied. Dandelion reestablished dominance within 6 years following treatment of a perennial forb range; big sagebrush showed little sign of recovery 5 years after treatment. The density and litter size of the deer mouse (Peromyscus maniculatus) was little affected by the 2,4-D treatment. The density of northern pocket gophers (Thomomys talpoides) and least chipmunks (Eutamias minimus) was reduced. Montane voles (Microtus montanus) increased in abundance following treatment of a perennial forb range. With the reestablishment of forb dominance, pocket gopher and vole populations returned to pre-treatment levels. Density changes of pocket gophers on treated ranges were due primarily to changes in the availability of food, those of chipmunks to both food and cover, and those of voles to cover changes. During the past two decades herbicides have been widely used on rangelands of the western United States in attempts to improve range condition. Progress has been made in determining the effects of range treatment with 2,4-dichlorophenoxyacetic acid (2,4-D) on rodent populations in western Colorado. Keith et al. (1959) found that treatment reduced the density of northern pocket gophers and that grass comprised a larger part of their diet on treated areas. Johnson (1964) reported that changes in the diets of deer mice and least chipmunks were correlated with increases in grass and decreases in forb production following treatment. Montane voles continued to subsist on the leaves and stems of forbs and shrubs although these plants were less abundant following treatment. Hansen and Ward (1966) observed that pocket gopher populations, initially reduced as a result of treatment, recovered with the reestablishment of perennial forbs on the treated range. Tietjen et al. (1967) observed similar results. They also found that captive pocket gophers sustained no direct toxic 1 This study was supported by the National Science Foundation (G-6152). 2 Present address: Department of Biological Sciences, University of Idaho, Moscow. effects from ingestion of native plants treated with 2,4-D. Treatment seemed to have little effect on the abundance of deer mice or meadow mice (Microtus sp.) inhabiting the herblands they studied. We have studied deer mice, least chipmunk, and montane vole populations inbiting shrub-grass and perennial forb anges in western Colorado, from 1959 through 1962 in an effort to determine the effects of habitat alteration with 2,4-D on the density and reproduction of these rodents. We wish to thank T. A. Vaughan, R. A. Ryder, the late L. D. Hibbs, P. D. Leiby, R. A. Perry, F. D. Steams and, particularly, G. D. Bear for field and laboratory assist-