We live in an era of global change. Human activities have significantly reshaped ecosystems, driving biodiversity loss and altering fundamental ecological processes. Invasive species are a major component of anthropogenic change, altering the composition, structure, and function of ecosystems worldwide. The impacts of invasion are particularly severe on islands due to their geographic isolation, small size, and high endemism. Ungulates, such as feral pigs (Sus scrofa), are among the most destructive invaders globally, and have been found to disproportionately impact island systems. Hawaiʻi Island serves as an ideal model system to investigate the ecological mechanisms driving invasion and the effects of non-native plant encroachment. This dissertation investigates the interaction of feral pig activity and non-native plant invasions within Kohala, in an invaded but relatively intact, montane forest-peatland complex at risk of further encroachment. Across four chapters, I explore how feral pigs drive ecosystem change through direct disturbance and trophic interactions. Chapter 1 uses field surveys to map understory vegetation composition and link feral pig activity and habitat characteristics to patterns of invasion. Chapter 2 investigates how feral pig disturbance facilitates non-native plant establishment, leveraging plant functional traits to understand species’ responses. Chapter 3 explores functional and phylogenetic diversity through the lens of niche theory to assess community assembly processes. Chapter 4 reconstructs feral pig trophic dynamics using stable isotopes to understand their dietary ecology and functional role in their introduced range. Together, these studies advance our understanding of the mechanisms behind the spread of non-native species, the interactions among ungulates and invasive plants, and the unique challenges to island biota in the face of global change.