The degree to which plant communities are vulnerable to invasion by alien species has often been assessed using the relationship between native and alien plant species richness (NAR). Variation in the direction and strength of the NAR tends to be negative for small plot sizes and study extents, but positive for large plots and extents. This invasion paradox has been attributed to different processes driving species richness at different spatial scales. However, the focus on plot size has drawn attention away from other factors influencing the NAR, in part because the influence of other factors may be obscured by or interact with plot size. Here, we test whether variation in the NAR can be explained by covariates linked to community susceptibility to invasion and whether these interact with plot size using a quantitative meta-analysis drawn from 87 field studies that examined 161 NARs. While plot size explained most variation, the NAR was less positive in grassland habitats and in the Australasian region. Other covariates did not show strong relationships with the NAR even after accounting for interactions with plot size. Instead, much of the unexplained variation is associated with article or author specific differences, suggesting the NAR depends strongly on how different authors choose their study system or study design.