Invasive species represent a critical threat to ecosystems and ecological communities. Understanding the mechanisms behind their invasion is important for understanding why they invade and the consequences of their invasions. Furthermore, invasive species, like all macroscopic organisms, harbor symbiotic microbes that constitute their microbiomes. Symbionts have been implicated in invasion success of several species, and they also represent an opportunity to learn about community assembly. In this dissertation, I explore the causes and consequences of plant invasion, and the microbiomes that invasive species harbor. In Chapter 1, I explore how two invasive, congeneric beachgrasses (Ammophila arenaria and A. breviligulata) differently alter plant succession in Pacific Northwest, USA dunes. The newer invader, Ammophila breviligulata, occupied a wider distribution across dunes than the established invader, A. arenaria, allowing A. breviligulata to persist longer through successional time. In Chapter 2, I characterize the above- and belowground symbionts of these two Ammophila species and the native Elymus mollis. I found that symbiont communities aboveground are most influenced by host species and geographic distance from one another, while those belowground are most influenced by environmental filtering. In Chapter 3, I conducted two experiments to test the mechanisms by which A. breviligulata invades dunes dominated by A. arenaria. I found that A. breviligulata dominates by tolerating competition from A. arenaria, and that there is some evidence that A. arenaria might limit itself via negative plant-soil feedbacks. Finally, in Chapter 4, I explore the relationship between herbivory and symbiont communities using the invasive wetland forb Lythrum salicaria. I found positive associations between herbivory and symbiont communities indicative of facilitation.