Mutualisms are interspecific interactions in which the fitnesses of each partner benefit from the association. Mutualisms are ubiquitous and ecologically important in nature and, as a result, a wealth of theoretical and empirical research has been conducted to better understand the interactions between mutualistic partners. However, mutualists are commonly associated with a broader community of interacting organisms that may antagonize, destabilize, or limit the fitness of these partners. In addition to the consistent community-level pressures, organisms must ensure their own fitness through proper sex ratio allocation even if mating patches are isolated and the potential numbers of reproductive individuals are routinely low. To empirically evaluate the dynamics of mutualist partners over geographic space and throughout time, the influence of community-level antagonists on mutualism fitness, and the processes of sex ratio adjustment to ensure successful reproduction I studied a community comprised of figs (genus Ficus), their pollinating fig wasp mutualists (genus Pegoscapus), non-pollinating fig wasp antagonists (multiple genera), and nematode associates of fig wasps (genus Parasitodiplogaster). In this dissertation, I use repeated sampling excursions, field experiments, and dissection efforts to investigate the dynamics of nematode infection in relation to their wasp hosts, the range of host specificity and community context of nematode infection, and the sex ratio adjustment of adult nematodes. Through these efforts, I found that Parasitodiplogaster nematodes are extremely common associates of pollinating and non-pollinating fig wasps in multiple New World Ficus communities. Next, observational and dissection data suggest that nematode infection may limit non-pollinators significantly more than pollinating fig wasps, implying a facultative mutualism between nematodes and pollinating wasps. Finally, analyses of adult nematode mating groups show a female-biased sex ratio consistent with local mate competition theory and may suggest a novel density-dependent nematode sex determination mechanism in which nematodes use the presence of co-infectives to determine their sex. Taken together, the findings of this dissertation further our knowledge of Parasitodiplogaster nematode biology, the effects of community-level antagonism on interspecific interactions, sex allocation theory, and the ecology and evolution of mutualistic associations in general.