Because most plants require pollinator visits for seed production, the ability of an introduced plant species to establish pollinator relationships in a new ecosystem may have a central role in determining its success or failure as an invader. We investigated the pollination ecology of three milkweed species – Asclepias curassavica, Gomphocarpus fruticosus, and G. physocarpus – in their invaded range in southeast Queensland, Australia. The complex floral morphology of milkweeds has often been interpreted as a general trend towards specialized pollination requirements. Based on this interpretation, invasion by milkweeds contradicts the expectation that plant species with specialized pollination systems are less likely to become invasive than those with more generalized pollination requirements. However, observations of flower visitors in natural populations of the three study species revealed that their pollination systems are essentially specialized at the taxonomic level of the order, but generalized at the species level. Specifically, pollinators of the two Gomphocarpus species included various species of Hymenoptera (particularly vespid wasps), while pollinators of A. curassavica were primarily Lepidoptera (particularly nymphalid butterflies). Pollinators of all three species are rewarded with copious amounts of highly concentrated nectar. It is likely that successful invasion by these three milkweed species is attributable, at least in part, to their generalized pollinator requirements. The results of this study are discussed in terms of how data from the native range may be useful in predicting the pollination success of species in a new environment.
