Associational Susceptibility of a Native Shrub, Atriplex canescens, Mediated by an Invasive Annual Forb, Brassica tournefortii, and Invasive Stinkbug, Bagrada hilaris

Indirect interactions have increasingly been recognized as important forces influencing population dynamics and structuring communities. Associational susceptibility is a form of indirect effect in which a focal plant experiences greater herbivore damage due to neighboring plant identity or diversity. These interactions remain poorly understood in the context of invasion ecology, though they may be responsible for huge impacts of invasive species on native communities. This dissertation investigates the potential mechanisms and consequences of associational susceptibility of a native perennial shrub, Atriplex canescens, driven by an invasive annual forb, Brassica tournefortii, and an invasive herbivorous stinkbug, Bagrada hilaris. In Chapter 1, a potential associational effect is experimentally demonstrated and a phenologically-driven trait is identified as a potential mechanism for this interaction. In Chapter 2, relative host plant quality is explored for its role in mediating the numerical response of the shared herbivore, and the herbivore’s damage impact on A. canescens. In Chapter 3, neighbor density, herbivore presence and herbivore density were manipulated to identify their impacts on spillover timing, extent, and fitness consequences for A. canescens. Overall, potential mechanisms of A. canescens associational susceptibility to Br. tournefortii and Ba. hilaris identified include: Ba. hilaris accumulation on Br. tournefortii followed by Br. tournefortii senescence and depletion, triggering Ba. hilaris alternative host-seeking. Associational susceptibility of A. canescens could not be re-created under experimental conditions, but further study is required to ascertain whether this interaction is due to experimental limitations or ecological implausibility