Alien pollen grains interfere with the reproductive success of native congener

The effects of invasive species on native species comprise important conservation issues. Determining the mechanisms by which invasives exclude natives is indispensable to efficiently control their impact, but most invasives remain poorly studied. The purpose of this study was to elucidate potentially important but neglected mechanisms, reproductive interference, in wild Taraxacum systems, in which invasive Taraxacum officinale has displaced its native congener T. japonicum in Japan. Hand-pollination of mixed pollen grains significantly reduced the native seed-set compared to conspecific-only pollination. Moreover, natives with a high ratio of invasive pollen on their stigmas suffered severe seed-set reduction, and the proportion of invasive pollen on native stigmas increased as frequencies of the alien neighbor increased. These results, combined with those of previous studies, revealed that depositing invasive pollen on native stigmas contributes to the observed alien-frequency-dependent reduction of native seed-set, and strongly suggest that reproductive interference was the primary cause of displacement in the Taraxacum systems.