Simulating cheatgrass (Bromus tectorum) invasion decreases access to food resources for small mammals in sagebrush steppe
Invasions by nonnative plants can alter the abundance of native animals, yet we know little about the mechanisms driving these changes. Shifts in vegetation characteristics resulting from nonnative plants can alter availability of food resources, predation risk, and foraging efficiency (both the access to and ability to find food), each providing a potential mechanism for documented changes in animal communities and populations in invaded systems. Cheatgrass (Bromus tectorum) is a nonnative grass that invades sagebrush steppe, resulting in declines in some small mammal populations. We examined whether changes in structural characteristics associated with cheatgrass invasion could alter foraging by small mammals, providing a potential mechanism for documented population declines. We quantified differences in vegetation structure between native and cheatgrass-invaded sagebrush steppe, then experimentally added artificial structure in native areas to simulate these differences. We placed grain at foraging stations and measured the amount removed by small mammals nightly. Adding litter at depths approximating invasion by cheatgrass reduced the average amount of grain removed in 2 of 3 study areas, but increasing stem density did not. Based on this experiment, the deeper litter created by cheatgrass invasion may increase costs to small mammals by decreasing foraging efficiency and access to existing food resources, which may explain population-level declines in small mammals documented in other studies. By isolating and identifying which structural attributes of cheatgrass invasion are most problematic for small mammals, land managers may be able to design treatments to efficiently mitigate impacts and restore invaded ecosystems.
KeywordsForaging behavior Predation risk Habitat manipulation Exotic species Habitat selection Optimal foraging
We thank Dr. Megan Higgs for her assistance throughout this project. Comments from Dr. Higgs and several anonymous reviewers greatly improved this manuscript. We also appreciate financial and logistical support from the Department of Ecology at Montana State University, Montana Fish, Wildlife, and Parks, the Montana Chapter of The Wildlife Society, the Montana Institute on Ecosystems, and the Bureau of Land Management.
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