Cheatgrass invasion of sagebrush steppe : impacts of vegetation structure on small mammals
Bachen, Daniel Allen
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Nonnative plants can affect habitat quality for native animals directly, through changes in resources like cover or food, and indirectly, through changes in access to resources or predation risk. Understanding these effects is crucial to develop management techniques and maintain ecosystem processes. In sagebrush steppe, brome grasses such as cheatgrass (Bromus tectorum) can invade and form dense stands, increasing the depth and persistence of litter, as well as the density of standing vegetation. These structural changes alter abundance and composition of the small mammal community. We used a series of experiments to explore whether changes in vegetation structure associated with the invasion of cheatgrass would alter foraging and predation risk of small mammals, to better understand mechanisms driving documented population- and community-level effects. In the first experiment, we placed a measured amount of grain at stations with either increased litter or stem density, and examined how much grain was removed nightly. We found that adding litter reduced the amount of grain removed in 2 of our 3 study areas. In the second experiment, we timed animals fleeing a simulated predator through various depths of litter or densities of stems. We found that dense stems impeded movement more than litter. In the third experiment, we recorded animals moving through native sagebrush steppe and cheatgrass monocultures, and analyzed these recordings to detect differences in the volume of noise created, especially for frequencies detected by common predators. We found that animals moving through cheatgrass made more noise at high frequencies, compared to native sagebrush steppe. Based on these experiments, cheatgrass monocultures may reduce habitat quality for small mammals by decreasing foraging efficiency and increasing vulnerability to predators. Mitigation strategies should focus on reducing the density of standing vegetation where predation is a limiting factor and litter depth where small mammals are food-limited.