Abstract: What is happening belowground and how does it drive what we see aboveground? I am interested in understanding mutualist influence on plant diversity, biogeography, and ecology. The vast majority of plant species rely on mutualisms for nutrition, defense, and even survival. However, these mutualists are often still underappreciated as important determinants of plant community composition. Here, I will share work focusing on how plant-associated mutualists – from mycorrhizal fungi to plant pollinators – shape global plant biogeography. I will discuss how dispersal limitation of mutualists mediates plant colonization on both island systems and mainlands. I will also discuss how plant introductions and human land use have altered these biogeographical patterns and interactions. Together, this work establishes the important, but historically overlooked role of mutualisms in mediating island biogeography and biogeographic patterns of plant diversity on Earth. Ultimately, this research contributes to improved conservation of natural systems and restoration of degraded ones.