Ecologists have long recognized that mutualisms are central features of biotic communities, but we are only just beginning to understand how mutualisms impact biotic networks and, by extension, community structure. Mutualisms can affect the full spectrum of species interactions—other mutualisms, parasitisms, competitions, and trophic interactions—experienced by either partner. Yet mutualists are well known to vary in partner quality, and the effects of mutualisms on communities have thus seemed difficult to predict. Context-dependent mutualisms have their mechanistic basis in variable organismal traits, which suggests that studying these traits will be key to predicting mutualism’s community-wide effects. New methodologies from genomics to metabolomics are transforming the ways that we can measure organismal traits to gain mechanistic insight. This session will explore the power of mutualisms to architect their associated biotic networks through a trait-based lens. Talks in this session will address these issues using diverse mutualisms—from plant-microbe to coral-dinoflagellate—and will also address how understanding the role of mutualists as architects is central to solving applied problems—from the successful recovery of communities after fire to the adaptive responses of communities to ongoing climate change.