Although some conservationists accept that not all species can be saved, we illustrate the difficulty in deciding which species are dispensable. In this article, we examine the possibility that the integrity of a forest relies on its entire faunal assemblage. In Madagascar, one faunal group, the lemurs, accounts for the greatest biomass and species richness among frugivores. For example, 7 of the 13 sympatric lemur species in Madagascar's eastern rainforests consume primarily fruit. Because of this, we suggest that some tree species may rely heavily on particular lemur taxa for both seed dispersal and germination. In Ranomafana National Park, the diets for four of the day-active lemur frugivores have been documented during annual cycles over a 5-year period. We predicted that, although the fruit of some plant taxa would be exploited by multiple lemur species, the fruit of others would be eaten by one lemur species alone. Analyses reveal that while lemurs overlap in a number of fruit taxa exploited, 46% (16/35) of families and 56% (29/52) of genera are eaten exclusively by one lemur species. We, therefore, predict local changes in forest composition and structure if certain of these lemur species are eliminated from a forest owing to hunting, disease, or habitat disturbance. We also suggest that this result may be of global significance because carbon sequestration by the tropical forests in Madagascar may be reduced as a result of this predicted change in forest composition. Am. J. Primatol. 73:585–602, 2011. © 2011 Wiley-Liss, Inc.