Does cheatgrass invasion contribute to rarity? Small mammal abundance, habitat use, and movement patterns

Background/Question/Methods Cheatgrass (Bromus tectorum) is an invasive grass that has proliferated throughout the Great Basin, altering vegetation structure and decreasing native plant diversity. The drastic changes in vegetation brought about by cheatgrass invasion are expected to alter small mammal communities because of structural and compositional changes in the plant community. The objectives of this study were: 1) to determine the effects of cheatgrass on small mammal abundance and community composition and 2) to quantify movement patterns and habitat use of Great Basin pocket mice (Perognathus parvus), western harvest mice (Reithrodontomys megalotis) and deer mice (Peromyscus maniculatus), as possible mechanisms to explain patterns of small mammal abundance and community composition. Trapping and tracking were conducted in 2008 in the Okanagan valley of British Columbia at the northern reaches of the Great Basin ecosystem on 12 sites that spanned a gradient of cheatgrass abundance. Habitat use and movement patterns of mice were delineated using fluorescent powder. Linear regression was used to determine the relationship between small mammal population size and cheatgrass abundance. Manly's alpha was selected as a habitat preference index for small mammals. Results/Conclusions Great Basin pocket mice were the most abundant species. The average population size per 1-hectare site was 20.3 ± 4.9 Great Basin pocket mice, 1.8 ± 0.7 deer mice and 0.7 ± 0.3 western harvest mice. The gradient of cheatgrass invasion ranged from 1 to 18 percent cover, and cheatgrass abundance was not correlated with the population size of any species of small mammal. Great Basin pocket mouse abundance was correlated with bare ground (r2=0.083, p< 0.001). Great Basin pocket mice (n=71) preferred bare ground, cheatgrass and biological soil crusts, but avoided forbs and perennial grasses. Western harvest mice (n=5) selected shrubs and bare ground while avoiding perennial grasses, annual grasses, and biological soil crusts. Deer mice (n=13) selected shrubs and bare ground, and avoided perennial grasses, annual grasses, biological soil crusts and litter. Bare ground may have been selected because it facilitates movement, whereas perennial grasses, which were avoided, may hinder small mammal movement through grasslands.