The introduction of new predators and pathogens has caused numerous well-documented extinctions of long-term resident species, particularly in spatially restricted environments such as islands and lakes. However, there are surprisingly few instances in which extinctions of resident species can be attributed to competition from new species. This suggests either that competition-driven extinctions take longer to occur than those caused by predation or that biological invasions are much more likely to threaten species through intertrophic than through intratrophic interactions. The likely threat of introduced species to resident controphics (species in the same trophic level) can be assessed with the help of existing biodiversity and extinction data sets and of two recent theories: (1) the fluctuating resource availability hypothesis, developed to account for changes in the invasibility of communities, and (2) the unified neutral theory, proposed to account for patterns of biodiversity at the community and metacommunity levels. Taken together, theory and data suggest that, compared to intertrophic interactions and habitat loss, competition from introduced species is not likely to be a common cause of extinctions of long-term resident species at global, metacommunity, and even most community levels.