Demographic Responses of Bromus Tectorum and Seedlings of Agropyron Spicatum to Grazing by Small Mammals: Occurrence and Severity of Grazing

Abstract

(1) The impact of grazing by small mammals, Microtus montanus and Peromyscus maniculatus (Cricetidae), upon the demographies of the alien annual Bromus tectorum and seedlings of the native perennial Agropyron spicatum was investigated for two years in a meadow-steppe community in eastern Washington, U.S.A. (2) Cricetid populations were highest between late winter and early spring; the M. montanus population increased more than two-fold between the first and the second year of the study. (3) Most grasses (c. 60%) emergent from winter until early spring were grazed at the time of the peak in the M. montanus population. (4) Most (c. 80%) of the grazed individuals had more than 75% of above-ground vegetation removed. Plants from autumnand winter-emergent cohorts were grazed more severely than spring-emergent plants. (5) Survival of B. tectorum was not influenced by grazing in either year, whereas A. spicatum cohorts grazed in late winter had significantly higher mortality than cohorts of the same age that were simultaneously protected from grazing. (6) For B. tectorum, biomass and seed production declined significantly as the severity of grazing increased. For A. spicatum, there was no significant relationship between grazing severity and biomass.