Non-invasive naturalized alien plants were not more pollen-limited than invasive aliens and natives in a common garden

1. Many invasive alien plants are, in contrast to many non-invasive ones, self-compatible and autofertile and thus do not fully depend on mates and pollinators for seed production. Nevertheless, they often still depend on pollinators for maximal reproduction. Indeed, it has been shown that many invasive plants can attract pollinators in the non-native range and may attract more of them than non-invasive plants do. This suggests that many introduced alien plants may have failed to become invasive because they are pollen-limited. 2. We used an experimental approach to test whether non-invasive alien species suffer higher pollen limitation and have lower autofertility than invasive aliens and natives. In a common garden, we assessed the degree of pollen limitation and autofertility of eight confamilial (or congeneric) triplets of native, invasive and non-invasive (but naturalized) alien plant species (totalling 24 species). Our breeding-system experiment included three treatments: pollen supplementation, open pollination and pollinator exclusion. We report three measures of pollen limitation and autofertility (fruit set, seed production per fruit and indices of pollen limitation and autofertility based on seed production per flower). 3. We found that the three plant groups had low degrees of pollen limitation and were almost all autofertile to some degree. Moreover, our phylogenetically informed analyses showed that the three plant groups did not differ significantly in their degrees of pollen limitation and autofertility. 4. Our results support previous findings that alien plants are able to attract pollinators in non-native regions. Nevertheless, because invasive and non-invasive naturalized alien species did not differ in their degrees of pollen limitation, our results suggest that pollen limitation may not play a major role in the spread of alien plants, once they have become naturalized.