Pollination and dispersal of the noxious vine Passiflora mollissima
Thesis DisciplineEnvironmental Sciences
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Banana passionfruit (Passiflora mollissima) is a noxious weedy vine that is problematic in coastal regions throughout New Zealand. I investigated the pollination and dispersal mutualisms that facilitate its spread, in the Marlborough Sounds. Flower observations revealed that visits were almost exclusively from introduced honey bees and bumble bees, indicating an invasive mutualism. I investigated the pollination system of banana passionfruit by comparing fruit set, fruit size, seed set, germination success and the effects of inbreeding depression on seedlings when grown in competition, between hand-selfed, handcrossed, bagged and open flowers. Fruit set was significantly reduced when pollinators were excluded (3.0%) indicating reliance on pollinators for reproduction. While banana passionfruit is partially selfcompatible, fruit set was significantly reduced in selfed flowers (17.5%) compared to crossed flowers (29.5%), and we found significant pollen limitation (natural [18.0%] was significantly lower than crossed). A significant degree of inbreeding depression was apparent in germination success but not in fruit size, seed set or seedling growth and survival. Surprisingly, open flowers had significantly lower germination success (10.1%) than the other three treatments (crossed = 16.5%, selfed = 15.0%, bagged = 25.3%). Overall, natural flowers produce more seedlings per flower (1.7) than bagged flowers (0.9), but less than hand-selfed (3.0) and crossed (5.3) flowers. To investigate dependence on dispersal, germination success was compared between hand-cleaned seeds, fleshy seeds and intact fruits in the field and glasshouse. Seeds germinated readily in all treatments, with no significant difference between cleaned and fleshy seeds, so seeds do not require frugivore handling to germinate. In the field, tagged fruits were removed quickly after ripening. Significantly fewer fruit were wholly removed from off-road locations than locations on road edges, but removal rate was high in both cases (93% and 70%) indicating dispersal by both humans and wild animals. I found no evidence of dispersal by birds. Infra-red camera evidence revealed possums and rats consuming fruits in the field. In addition, 2818 seeds were extracted from 1.5 kg of pig excrement collected from Te Weu Weu Bay in February 2006. Seeds readily germinated following extraction from pig and possum droppings with a final germination success, in the glasshouse, of 45.6% and 53% respectively. None of the few intact seeds excreted by rats germinated. Therefore, together with direct weed control, limiting the spread of banana passionfruit through the control of pigs and possums may improve management success and reduce the spread of banana passionfruit.