Abstract
Invasive annual grasses are a growing threat to ecosystems globally, disrupting system processes through nutrient and fire cycle alteration as well as reduced cover and diversity of native plant communities. One such ecosystem, the sagebrush steppe of the North American west, has been reduced to approximately 50% of its historic range, primarily due to annual grass expansion. Land managers have used multiple tools to address this expansion, including herbicides; however, most common herbicides used to remove invasive annual grasses such as cheatgrass (Bromus tectorum) are non-selective, meaning they also impact non-target plants. Indaziflam is a relatively new, pre-emergent herbicide that effectively impedes the growth of plant seedlings, thereby strongly impacting invasive annual grasses. It has recently been proposed for widespread use in wildlands to reduce cheatgrass cover and ultimately benefit sagebrush systems; and yet, little is known about the indirect impacts of indaziflam on wildlife. Sagebrush wildlife species often have specific habitat requirements that may be impacted by substantial shifts in plant communities via indaziflam; as such, it is crucial to develop base knowledge of those potential changes to conduct informed and intentional herbicide application.Greater sage-grouse (Centrocercus urophasianus; hereafter, sage-grouse) are an indicator species for the sagebrush steppe, and their life history is closely tied to sagebrush (Artemesia tridentata spp.) and other plant food resources. In particular, sage-grouse rely on a subset of broad-leaved plants for food, often referred to as preferred forbs, during brood rearing. This group of plants includes perennials and annuals, both of which are subject to impacts from indaziflam application. Changes in the plant community may therefore drive shifts in use of habitat by grouse. In Chapter 1, we evaluated the impact of indaziflam application on habitat use by sage-grouse as mediated by changes in the vegetation community, including preferred forbs. To do this, we established indaziflam-treated and control areas within units across the sagebrush steppe at Rinker Rock Creek Ranch in central Idaho. Indaziflam-treated areas underwent application in 2019 or 2020, at a rate of 369.4 ml/ha with 468.2L/ha of water as a carrier. Within these areas, we placed 50-m transects (n = 48) based on ecological site type, along which we conducted vegetation cover surveys in June of 2023 and 2024 (years three and four after treatment, respectively). To assess habitat use, we completed surveys for sage-grouse pellets every other week from June to August. We found that Bromus spp. cover and annual forb cover and richness were lower on indaziflam-treated transects, whereas bare ground was greater. Perennial forb and total (perennial + annual) plant responses to treatment varied by year in several cases, exhibiting no difference in 2023 while increasing on treated transects in 2024, a relatively low productivity year. Greater sage-grouse habitat use doubled on indaziflam-treated transects compared to untreated transects. Our results suggest that reducing annual grass cover via indaziflam may have benefits for sage-grouse in areas with an established perennial plant base.
Small mammals within the sagebrush steppe, and across systems, are important seed dispersers, bioindicators, and prey. Given that small mammals consume plants and associated arthropods, they are likely to be indirectly impacted by shifts in the plant community, and their ease of capture allows them to serve as model organisms for how indaziflam may reshape dynamics between generalist vs. specialist species. Western deer mice (Peromyscus maniculatus), as habitat and diet generalists, can inhabit a variety of ecosystems and consume arthropods, plants, and even carrion. Contrastingly, Columbia Plateau pocket mice (Perognathus parvus) tend to fill a specialist role, living exclusively in sandy soils of the sagebrush steppe or juniper scrubland where they subsist on a primarily granivorous diet. In Chapter 2, we assessed how indaziflam application impacted population dynamics of both species, including survival, abundance, and fecundity. We collected data within indaziflam-treated and control areas at Rinker Rock Creek Ranch, this time via live-trapping of small mammals (n = 14 trapping grids) and quadrat-based microhabitat surveys at each trap location. As expected, we found that indaziflam-treated trapping grids had lower annual forb cover and richness, Bromus spp. cover, and total forage for pocket mice on average; bare ground, however, was greater. Deer mice were more abundant in herbicide-treated grids, while apparent survival and fecundity did not differ between treated and control grids. Specifically, deer mouse abundance was positively associated with bare ground, and negatively associated with annual forb cover and richness. Pocket mouse abundance was increased with indaziflam treatment, though the effect was marginal, and was negatively associated with annual forb cover and richness. These results suggest that though indaziflam treatment will impact species differently depending on their ecological role, predictions based on assumed roles as habitat generalists vs. specialists may lead to incorrect conclusions regarding responses to habitat management. Overall, our results emphasize the value of understanding the relationships between desirable wildlife and specific plants impacted by herbicide treatment while also considering the significant benefits of annual grass reduction for the system as a whole.