Biodiversity is regulated by a wide array of processes; among the most widely recognized drivers at broad spatial scales are environmental filtering and negative biotic interactions. In contrast, although ecologists have increasingly recognized the importance of positive biotic interactions—such as facilitation and mutualism—their influence on regional biodiversity patterns remains controversial, largely due to limited empirical evidence. Here, we examine the role of positive interactions in shaping diversity at the metacommunity level. Using extensive freshwater fish community data from North America, we focus on nest association—a form of reproductive facilitation between host and beneficiary species—as a model system to test whether and how the geographic range of host species (i.e., metapopulation occupancy) regulates regional biodiversity. We found that greater host occupancy increased local species richness (α diversity) but decreased β diversity, indicating that positive interactions can promote biotic homogenization. As a result, γ diversity remained unchanged because opposing patterns in α and β diversity offset one another at the regional scale. These patterns were supported by strong local associations between host and beneficiary species, demonstrating that positive interactions influence both local richness and spatial variation in community structure. Together, our results provide robust empirical evidence that positive biotic interactions can have far-reaching effects on biodiversity across spatial scales and represent an underappreciated yet important component of metacommunity dynamics with implications for large-scale biodiversity conservation.