Invasive annual grass relationships to environment and grazing vary with species identity

Authors: DENNIS, RACHAEL - University Of Oregon; CASE, MADELON - Us Geological Survey (USGS); Davies, Kirk; Boyd, Chad; PENKAUSKAS, CALVIN - University Of Oregon; HALLET, LAUREN - University Of Oregon

Submitted to: Biological Invasions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/29/2025
Publication Date: 12/29/2025
Citation: Dennis, R., Case, M.F., Davies, K.W., Boyd, C.S., Penkauskas, C., Hallet, L.M. 2025. Invasive annual grass relationships to environment and grazing vary with species identity. Biological Invasions. (2026)28:18. https://doi.org/10.1007/s10530-025-03719-w.
DOI: https://doi.org/10.1007/s10530-025-03719-w

Interpretive Summary: Exotic annual grass invasion is a major threat to the sagebrush steppe of the western United States. Understanding how different species of annual grasses respond to grazing intensity and environmental factors is needed to inform management. Species varied in their response to grazing intensity and environmental factors, suggesting that species-specific environmental responses may contribute to the overall expansion of annual grasses into new environments. Furthermore, high grazing intensity may enhance the expansion of some species while mitigating others. Management and assessments of the risk of annual grass invasion can likely be improved by recognizing species-specific responses. These results are of interest to land managers, invasive plant managers, livestock producers, and other scientists.

Technical Abstract: Annual grass invasion is a major threat to the sagebrush steppe of the Intermountain West, United States. While environmental drivers such as wildfire, elevation, and climate are well-known to shape annual grass invasion at broad scales, there is often high local heterogeneity in annual grass dynamics. Here we conducted an extensive field survey of burned and unburned grazed public lands across the Northern Great Basin to understand how environmental factors combine with two potentially key drivers of local heterogeneity – species identity and grazing intensity – to shape annual grass presence and abundance. We sampled plant species cover as well as soil texture, cattle dung, and topographic features in plots stratified at the pasture level by fire history and within pastures by aspect and distance to water (to capture a grazing intensity gradient, 100 to 500 m from cattle water sources). The most common annual grass species, cheatgrass, occurred ubiquitously despite fire history, elevation, or climate. The occurrence probabilities of three other species – Japanese brome, medusahead, and ventenata – decreased with elevation, aligning with conventional wisdom about ecosystem resistance to invasion increasing with elevation. However, all three exhibited a counterintuitive pattern in which their likelihood of occurrence at higher elevation sites increased with precipitation. Further, these species often diverged from cheatgrass (and one another) in regional and local responses. In contrast to cheatgrass, Japanese brome had higher abundances at cooler sites regionally and was affected by cattle grazing locally, decreasing with multiple indices of higher local grazing intensity. Medusahead had an opposite response to local grazing gradients, with higher cover closer to water sources where grazing tends to be heaviest. Both Japanese brome and medusahead had higher cover in burned plots, but cheatgrass and ventenata – perhaps because the first is ubiquitous and the second is in its early stage of invasion – responded more weakly to fire history. Taken together, our results show that species-specific environmental responses may contribute to the overall expansion of annual grasses into new environments, that high local grazing intensity may enhance the expansion of some species while mitigating others, and that environmental drivers may be most predictive of spread mid-invasion, before a species has become ubiquitous.