1. Species pairs that form mutualistic associations are also components of broader organismal community networks. These network-level associations have shaped the evolution of individual mutualisms through interspecific interactions ranging from secondarily mutualistic to intensely antagonistic. Our understanding of this complex context remains limited because characterizing the impacts of species interacting with focal mutualists is often difficult. How is the fitness of mutualists impacted by the co-occurring interactive network of community associates? 2. We investigate this context using a model network comprised of a fig and fig wasp mutualist, eight non-pollinating fig wasp (NPFW) antagonists/commensals, and a nematode previously believed to be associated only with the pollinator wasp mutualist. 3. Through repeated sampling and field observations we characterize the ecological roles of these mutualist-associated organisms to identify key antagonists. We then we investigated how potential nematode infection of NPFWs could impact wasp survival across key life stages and, in turn, inferred how this influences the fitness of the fig-pollinator mutualists. 4. Unexpectedly, we found all F. petiolaris associated NPFWs to be targets for nematode infection, with infection-levels sometimes exceeding that of pollinators. Experimental data collected for the most abundant NPFW species suggests that nematode infection significantly reduces their longevity. Further, comparisons of nematode loads for emerging and successfully arriving NPFWs suggest that infection severely limits their dispersal ability. 5. Through these observations we conclude that this infection could impact NPFWs more severely than either mutualistic partner, suggesting a novel role of density-dependent facultative mutualism between figs, pollinator wasps, and the nematode. This antagonist-mediated suppression of other network antagonists may present an ecologically common mechanism through which antagonists can present net benefits for mutualism fitness. 

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