Generalised pollination systems for three invasive milkweeds in Australia

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 specialised pollination requirements. Based on this interpretation, invasion by milkweeds contradicts the expectation than plant species with specialised pollination systems are less likely to become invasive that those with more generalised pollination requirements. However, observations of flower visitors in natural populations of the three study species revealed that their pollination systems are essentially specialised at the taxonomic level of the order, but generalised 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 generalised pollinator requirements. The results of this study are discussed in terms of how data from the native range may be useful in predicting pollination success of species in a new environment.