Reproductive ecology of the bird-pollinated Nicotiana glauca across native and introduced ranges with contrasting pollination environments

Abstract

Introduced plants become decoupled from their usual pollinators and by relying on resident pollinator species or other reproductive strategies become established in new areas, spread and, eventually, invade. Here, using breeding system experiments, we studied the reproductive biology of the bird-pollinated South American species Nicotiana glauca across native and non-native areas, both inside and outside the range of flower-visiting birds. In the native range, where the species is visited by hummingbirds, open cross-pollinated flowers set as many seeds and almost as many fruits as open pollination controls, suggesting that pollinators make a major contribution to reproductive success. In South Africa, the flowers were pollinated by sunbirds which, although less efficiently, also contributed to N. glauca reproduction, replacing hummingbirds. In contrast, in Mallorca, where nectar feeding birds are absent, fruit production in open cross-pollinated flowers was near zero, and significantly lower than in open pollination controls, suggesting that reproduction is almost entirely by autonomous self-pollination. Hand-pollination experiments showed that the species is self-compatible throughout its range. Pollinator exclusion experiment showed that N. glauca relies on self-pollination only in pollinator poor areas, where plants have a much higher capacity for autonomous self-pollination than elsewhere. A reduction in anther-stigma distance does not seem to account the higher self-pollination capacity in the non-native environment without pollinators. Despite probable evolutionary adaptation, and flexibility in pollinator association and mating system, seed production in the introduced range was somewhat pollen limited, suggesting that the mismatch between N. glauca and its novel pollination environment might retard invasion.