UC Riverside

An ongoing critical challenge in invasion ecology is the ability to predict invaderresponses to environmental change and their associated impacts on the native resident communities, especially in historically uninvaded systems such as North American drylands. Biological invasions occur over distinct stages such as introduction (i.e.,, species introduced into novel ranges), establishment (i.e., species establishing self-sustaining populations), and spread (i.e., species dispersing and expanding their ranges), and different factors may mediate invasion dynamics within each of these stages. Thus, I designed my dissertation to focus on identifying the drivers and impacts of invasive species establishment and spread, as well as investigating effective strategies to manage biological invasions in temporally variable environments. My dissertation uses a combination of observational, experimental, and modeling-based approaches to elucidate the mechanisms that allow invaders to establish, spread and persist in dryland systems. Chapter 1– spread – assessed the role of environmental niche shifts in facilitating the spread of a winter annual invasive plant, Brassica tournefortii (sahara mustard) in North America. Through a biogeographic approach, I discovered that despite having a relatively unchanged and consistent environmental niche (including climate and soil conditions), Brassica tournefortii still has a significant amount of available and unused habitat in its introduced North American range. These findings indicate that the invader may persist in spreading throughout North America and highlight potential regions where preventative measures can be taken. Chapter 2 – impact – leveraged a long-term dataset to understand how attributes of an invasion regime influence community synchrony and stability in a stabilized sand dune system. I found that both boom frequency and the dominant magnitude of invader boom are strong predictors of community synchrony and stability, becoming more asynchronous and less stable as the intensity of invasion increases. Further, the underlying invasion level mediates the dynamics that a system experiences. These findings move beyond basic invader abundance-native diversity metrics and shed light on additional factors that may be contributing to invader impacts in these historically uninvaded environments. Chapter 3 – management – addressed the role of seasonality during herbicide applications for the control of an emerging invasive winter annual forb, Oncosiphon pilulifer (stinknet). I found that seasonal herbicide efficacy is contingent on the density of invasive species stored within the soil seed bank, highlighting the importance of incorporating above and below-ground metrics of community composition when evaluating strategies for invasive plant management. For instance, in areas that have a long history of invasion (i.e., saturated invader seed bank), using pre-emergent herbicides provides higher control compared to post-emergents. These findings allowed me to make recommendations to land practitioners on when to apply herbicides to limit the spread of invasive species. Overall, the results from my dissertation provide insight into the mechanisms that allow invasive species to spread and persist within historically uninvaded landscapes.