Understanding the consequences of invasive plant species for native rangeland communities in the context of global climate change

Invasive species wreak havoc on nearly every ecosystem on the planet, including grasslands, and are considered currently to be one of the most substantial threats to ecosystems worldwide. Invasive species outcompete native species for resources with far-reaching direct and indirect consequences for the ecosystems they invade. Importantly, invasion is expected to change under global climate change, with global change factors like drought creating complex interactions with invasion, yielding a dire need to understand the widespread consequences of invasion for ecosystem sustainability. My research addresses this, specifically for two invasive grass species in the northern mixed-grass prairies of North America and on a global scale through a meta-analysis of grasslands. Grasslands, which are essential for ecosystem services like providing usable forage for livestock, are one of the most widespread biomes in the world, covering nearly 40% of the land area of the Earth. My dissertation assessed 1) how multi-year, multi-intensity drought and grazing altered two invasive brome grass species, 2) how metrics of stability associated with invasion gradients of two brome grass species, 3) the relationships between multiple trophic levels and gradients of invasion of two brome grass species, and 4) the responses of native and introduced plant species in grasslands to drought. My dissertation utilized diverse methods to address how invasion will impact ecosystem sustainability. Under a multi-site, long-term, manipulative field experiment, invasive annual bromes decreased in biomass under multiple years of drought, while summer grazing alone did not alter production of these species. Importantly, however, post-drought during recovery, annual bromes increased in biomass, particularly so in the plots which were heavily grazed during the drought (Chapter II). Using an observational field study, I found that invasive brome species tend to destabilize native plant communities, especially functional groups important for forage production, suggesting that invasion has consequences for predictability of yearly forage availability on rangelands (Chapter III). Similarly, in another observational field study, I found plant and insect communities and functional groups, and to a lesser extent, soil microbes, differed with invasion abundance, but relationships between invasive bromes and rangeland communities differed based on the particular invasive species (Chapter IV). Last, using a meta-analysis, I found that under drought, introduced plant species in grasslands tend to fare worse than native plants, but this advantage of native species over introduced species was negated when other global change variables co-occurred (Chapter V). In all, my dissertation shows that invasion destabilizes native communities, has consequences across trophic levels, and alters competition between native and invasive species. This work has implications for ecological theory related to invasions, as well as broad applications for management of our critically important grassland ecosystems