1 The level of functional redundancy in natural communities is likely to modulate how ecosystem stability is affected by local species extinction. Thus, extinction should have no effect if all species have similar functions but a major effect if each carries different functions 2 We provide a probabilistic framework that, from any distribution of species number across functional groups, generates specific predictions of how functional groups are lost when species become randomly extinct within a given community. In particular, we predict how many species can go extinct before a community loses its first functional group, a useful index for conservation purposes. 3 We demonstrate that the probability of a whole functional group becoming extinct from a given community increases with the number of recognized functional groups (functional richness) but decreases with species richness and functional evenness (the distribution of species across functional groups). 4 Application of this framework to one published data set for a South American plant community suggested that, if local extinction is random, 75% of the species could be lost before the disappearance of the first functional group. 5 However. if redundancy is to be used to determine conservation priorities, the definition of functional groups must be carefully reviewed.891118125