Abstract: Biological invasions are of interest both because they are causing extinctions worldwide and because they offer unique opportunities to study highly dynamic ecological processes. I examined ant invasions at five natural area sites in the Hawaiian Islands to better understand the factors that influence impacts on arthropod populations, species and communities. Ant invasion usually led to significant changes in overall arthropod community composition, particularly among endemic subsets of the communities. When compositional stability was measured as proportional change in richness, the best predictor of endemic community stability was endemic richness, with richer communities losing proportionately more species than species-poor communities. This relationship suggests that communities that had already lost many endemic species were resistant to further species loss, while more intact communities remained vulnerable to species loss upon ant invasion. Communities also underwent strong but idiosyncratic functional shifts as a result of ant invasion, both in terms of trophic structure and total arthropod biomass. There were no apparent relationships, however, between functional stability and community diversity or density measures. To identify taxa that may consistently be at risk of local extinction as a result of ant invasion, I assessed body size, population density, trophic role and provenance as potential correlates of vulnerability for 300 arthropod species. Provenance was the most important risk factor, with Hawaiian endemic species much more vulnerable than introduced species. Trophic role was also important for rare species, with endemic carnivores by far the most vulnerable group, and low population density increased risk among non-rare species. Despite these trends, it was difficult to predict vulnerability because of relatively high variability in responses both among species within many taxonomic orders and among populations of species occurring at different sites. This variability was exemplified by divergent trends among two families of endemic herbivores in the order Hemiptera. Due to differences in feeding strategies among delphacid planthoppers and mirid bugs, ants exhibited differential patterns of aggression towards these families in laboratory bioassays. This behavioral dichotomy largely determined patterns of survival in the field, but could sometimes be overridden by community compositional factors that resulted in unexpected population-level outcomes.