Quantifying non-native plant impacts : Centaurea stoebe L. (spotted knapweed) and Bromus tectorum L. (downy brome) in sagebrush-grasslands of the greater Yellowstone ecosystem

Abstract

Non-native plant species (NIS) are considered a significant threat to individual species, communities, and ecosystems; thus, NIS management is mandated in most natural areas (i.e. non-crop). Controlling NIS in natural areas should, ideally, not only reduce target NIS abundance, but also benefit broader management objectives such as conserving native species and improving wildlife habitat. In this context, the benefits of NIS control must be weighed against the impacts of NIS themselves. This dissertation examines ecological impacts of NIS through a synthesis of previous experimental research and field-based manipulative experiments. In a review and synthesis of experimental research, we found that NIS caused significant impacts in approximately half of all experiments. Negative impacts were most frequent on community structure, followed in descending order by individual species, ecosystem properties, and ecosystem processes. Contrary to common assumptions, NIS typically caused impacts by modifying the abiotic environment rather than outcompeting native species for resources. NIS impacts were also examined through experiments conducted in sagebrush-grasslands of the GYE. The first study compared plant community impacts of Centaurea stoebe L. and common herbicide treatment for C. stoebe. The broadleaf herbicide, picloram, was highly effective at reducing C. stoebe, but also significantly reduced native forb cover and significantly increased non-native grass cover. Native forb cover increased with manual removal of C. stoebe, suggesting C. stoebe had been suppressing native forbs. However, there was an equivalent increase with no treatment. In these communities, C. stoebe appears to have a negligible effect on native forb and grass cover and richness. The final study examined plant community impacts of the non-native annual grass, Bromus tectorum L, and relationships between impacts and NIS abundance. In a four-year field experiment, we did not detect significant impacts of B. tectorum on native plant cover and richness. Environmental factors, particularly climate variability, are likely more important determinants of current vegetation patterns in these communities rather than the presence of B. tectorum. Overall, the research shows NIS often do not have significant detectable impacts on native species and communities, and that the negative non-target effects of herbicide treatments may outweigh the benefits of NIS control.