Heterospecific pollination by an invasive congener threatens the native American bittersweet, Celastrus scandens

Abstract

Invasive plants have the potential to interfere with native species’ reproductive success through a number of mechanisms, including heterospecific pollination and hybridization. This study investigated reproductive interactions between a native North American woody vine (American bittersweet,Celastrus scandens) and an introduced congener (oriental bittersweet,C.orbiculatus). The decline ofC.scandensin the eastern portion of its range is coincident with the introduction and spread ofC.orbiculatus, and the two species are known to hybridize. The relationship between proximity and floral production of conspecific and heterospecific males on fertilization and hybridization rates was measured at a field site in northwestern Indiana, USA where both species occur and reproduce. We found that the invasive vine had an extreme advantage in both male and female floral production, producing nearly 200 times more flowers per staminate plant and 65 times more flowers per pistillate plant than the native. Using nuclear microsatellite DNA markers we found that hybridization rates were asymmetric; 39% of theC.scandensseeds tested were hybrids, compared to only 1.6% ofC.orbiculatusseeds. The asymmetric hybridization rates were likely not solely due to greater abundance ofC.orbiculatuspollen because experimental hand crosses revealed thatC.scandenshad a higher rate (41%) of heterospecific fertilization thanC.orbiculatus(2.4%). We previously reported that few hybrids were observed in the wild, and hybrids had greatly reduced fecundity. Thus, in our system, the threat posed by heterospecific pollen is not replacement by hybrids or introgression, but rather asymmetric reproductive interference. Reproductive interference extended to distances as great as 100 meters, thus, efforts to conserve the native species must reduce its exposure toC.orbiculatusover a relatively large spatial scale.