Invasive, nonnative plant species have become one of the most pressing rangeland management issues. In the western United States (the 17 US states from North Dakota, south to Texas, and west to the Pacific coast), 51 million hectares of rangeland arenow dominated by invasive plants considered to be noxious weeds. In over two-thirds of western rangelands, nonnative annual grasses account for 50-85% of vascular plant cover. Invasive plants have large negative impacts on the prevalence and diversity ofnative species, and many decrease livestock production through decreases in forage quantity and/or quality (Table 1). Invasive species on US rangelands have an estimated annual cost of US$2 billion due to lost production and costs of control efforts. There are also hidden costs associated with invasive species in the form of degraded ecosystem services—key functions provided by ecosystems that benefit humans (e.g., water provisioning, flood control, erosion control, carbon storage, nutrient supply, climate regulation). In some cases, invasive species change ecosystem processes in ways that are self-reinforcing, making the system more suitable for the invader than for the previous inhabitants, in what is known as a positive feedback loop. The combination of degraded ecosystem properties and positive feedbacks can make invasive plant control and rangeland restoration much more challenging because in these cases, it is not sufficient to simply remove the invaders. The ecosystem impacts of invasive species can persist long after the plants have been removed, and when this occurs, the system can remain vulnerable to reinvasion until the ecosystem effects are mitigated or reversed. We review the ecosystem impacts of the current major rangeland invaders in the western United States, discuss the potential for these ecosystem changes to further promote invasion through positive feedbacks, and suggest strategies to address persistent ecosystem effects in order to enhance invasive plant control and restoration of native (or otherwise desirable) plant communities.
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