Species richness maintains mutualisms: Mutualistic communities of species that benefit each other are ubiquitous in ecosystems and are important for ecosystem functioning. However, the relationship between the persistence of mutualisms and species richness has remained unclear. Vidal et al. used a synthetic mutualism in brewer's yeast to experimentally test whether species richness buffers mutualistic communities against exploitation by species that do not provide benefits in return. They showed that richer mutualist communities survive exploitation more often than pairwise mutualisms and that higher species richness and functional redundancy allow mutualist communities to persist in the presence of exploiters. These results provide experimental support for the hypothesis that species richness is necessary for the function and maintenance of mutualistic communities. Science , this issue p. 346