Plant traits that mediate mutualistic interactions as a mode of defense are pervasive, have originated independently many times within angiosperms, and are highly variable across taxa. My dissertation research examines the evolutionary ecology of two common plant traits that mediate defense mutualisms in plants: extrafloral nectaries (EFNs), plant organs that secrete small volumes of nectar, thereby attracting predacious arthropods to leaves, and (2) leaf domatia, small structures on the undersides of leaves that provide housing for predacious or fungivorous mites. Because traits like EFNs and domatia influence multiple trophic levels, their evolution can have strong impacts on community dynamics relative to other plant characters. Nonetheless, studies that directly link the ecological effects of these traits with their evolutionary dynamics are rare.