Invasive species are a growing international conservation concern. In Canada, the federal agency responsible for national parks, Parks Canada, has recently identified non-native species as important threats to the integrity and richness of Canadian national parks. To test for potential effects of non-native species on native richness, we examined the relationship between native and non-native/invasive species for 3 taxa – plants, mammals, and birds – in 42 Canadian national parks. We found positive relationships between native and non-native/invasive species richness for plants and birds and a negative relationship between native and non-native/invasive mammal species richness. Consistent with most large-scale studies, our results suggest that for birds and plants, regions that are good for native species are also good for non-native species. However, we provide one of the few examples of a negative relationship between native and non-native/invasive species. In mammals we show that there are more non-natives present in species-poor communities. This may indicate that non-native mammal species are having a negative impact on native species, that non-native mammal species are more able to invade richness-poor communities, or that a third factor such as fire, habitat degradation, or climate change has a negative effect on native species while having a positive effect on non-native species. In any case, it is evident that the pattern present between native and non-native mammals differs from those in plants and birds and could be representative of a negative effect associated with non-native introductions.