Habitat fragmentation and invasive species often contribute to the decline of native taxa. Since the penetration of non-native species into natural habitat may be facilitated by habitat fragmentation, it is important to examine how these two factors interact. Previous research documented that, in contrast to most other arthropod taxa, spiders increased in density and morphospecies richness with decreasing fragment area and increasing fragment age (time since insularization) in urban habitat fragments in San Diego County, California, USA. We tested whether a specific mechanism, an increase in non-native species with fragmentation, is responsible for this pattern. We found that both native and non-native taxa contributed to the pattern. Abundance of native spiders per pitfall trap sample increased significantly with decreasing fragment size (i.e. a negative density–area relationship) and abundance of non-natives increased significantly with increasing fragment age. The proportion of non-native individuals also increased significantly with age. One non-native species, Oecobius navus, comprised the majority of non-native individuals (82.2%) and a significant proportion of total individuals (25.1%). Richness of spider families per sample (family density) increased with fragment age due to an increase in the occurrence of non-natives in older fragments, however, native family richness did not vary with age or area. Due to increasing dominance by non-native and some native families, family evenness declined with decreasing fragment size and increasing fragment age. Native and non-native abundance covaried positively arguing against strong negative interactions between the two groups. O. navus had a strong positive association with another common non-native arthropod, the Argentine ant (Linepitheme humile), suggesting a possible direct interaction. In contrast, abundance of native spiders was negatively correlated with Argentine ant abundance. We hypothesize that fragmentation in this semiarid habitat increases productivity in smaller and older fragments enhancing the density of both native and non-native taxa.