Alien dandelion reduces the seed-set of a native congener through frequency-dependent and one-sided effects

In conservation biology, increasing numbers of studies have focused on reproductive interference (RI) between a native species and related aliens. However, few studies have examined the frequency dependence of RI, despite of its key importance to invasiveness. Here, we report for the first time frequency-dependent RI in a pair of native and alien dandelions: Taraxacum japonicum and T. officinale, respectively. Taraxacum japonicum has been displaced rapidly by the alien congener T. officinale in Japan and its causal mechanism are still poorly understood. Field observations revealed that the seed-set of natives decreased substantially as the proportion of alien neighbors increased. Subsequently, in a field experiment, the removal of alien flowers only greatly increased the seed-set of natives. We synthesized these results with existing theoretical models of RI and concluded that RI, which is mediated by strong frequency dependence, is presumably responsible for the displacement of T. japonicum by T. officinale.