Invasional meltdown: pollination of the invasive liana Passiflora tripartita var. mollissima (Passifloraceae) in New Zealand

Banana passionfruit (Passiflora tripartita var. mollissima) is an invasive vine in New Zealand where it lacks its natural hummingbird pollinator. We investigated the mating system and reproductive traits that facilitate its spread in the Marlborough Sounds. Flower observations revealed that visitors were almost exclusively introduced honeybees and bumblebees, indicating an invasive mutualism. We investigated the pollination system of banana passionfruit by comparing fruit set, fruit size, seed set, and germination success between hand-selfed, hand-crossed, bagged and open flowers, and inbreeding depression in seedlings grown in competition. Fruit set was reduced by 83% when pollinators were excluded (3.0% fruit set, compared with 18.0% for unmanipulated flowers) indicating reliance on pollinators for reproduction. While banana passionfruit is partially self-compatible, fruit set was significantly reduced in hand-selfed flowers (17.5%) compared with crossed flowers (29.5%), and we found significant pollen limitation (hand-crossed vs unmanipulated, Pollen Limitation Index = 0.39). There was no significant inbreeding depression found in fruit size, seeds per fruit, germination success, seedling growth or seedling survival. Combining these data showed that natural unmanipulated flowers produce more seedlings per flower (1.7) than bagged flowers (0.9), but fewer than hand-selfed (3.0) and hand-crossed (5.3) flowers. Thus, reproduction in Passiflora tripartita var. mollissima is facilitated by an (imperfect) new association with exotic bees.