Sterility and lack of pollinator services explain reproductive failure in non-invasive ornamental plants

Aim Mutualisms, including pollination, have been hypothesized to influence establishment and success of introduced species. However, lack of pollinators has rarely been demonstrated as a barrier to plant invasion. We examined barriers to seed set in nine introduced non-invasive tropical ornamental plants which do not regularly produce seeds in Hawai'i. Location Hawai'i. Methods We tested for pollen viability using 1% thiazolyl blue tetrazolium bromide (MTT) stain. We also supplementally hand-pollinated flowers and examined subsequent fruit set. Results We found that four species had non-viable pollen, as measured by pollen staining, and failed to produce seeds. By contrast, the milkweed Calotropis gigantea, though invasive elsewhere, produced seeds only when hand-pollinated, indicating that natural seed set is prevented by lack of legitimate pollen deposition. The remaining four species apparently had viable pollen, but did not set seed when hand-pollinated. Seed set in these species may be limited by lack of compatible mates in Hawai'i. Main conclusions Lack of seed production due to pollen sterility or lack of compatible pollen can function as an early barrier to invasion success in ornamental plants, helping to explain why some ornamental species fail to invade. Additionally, we document one of the first concrete examples of a lack of pollinator services inhibiting invasion success in a species outside of the genus Ficus. Understanding the nature of early barriers to invasion can help us evaluate the future invasion risks posed by these species in Hawai'i and elsewhere.