4 October 2010 | By James Rodger

It’s an old idea that plants that are specialised for particular pollinators should become invasive less often than those which can be pollinated by a broad range of animal species. Most plant species will not encounter their original pollinators when introduced elsewhere so to reproduce in their novel range they will have to recruit novel pollinators. Although this is not a great challenge for the many species pollinated by a broad range of animal taxa in their native range, pollination specialists, it would seem, are unlikely to recruit novel pollinators and their invasion should be prevented by failure to reproduce. This has been shown to hold for species like figs, where pollinator distribution is quite restricted. C·I·B PhD student James Rodger, with supervisors and co-authors C·I·B core team member Steve Johnson and former C·I·B postdoc Mark van Kleunen, shed new light on this concept in a recent study that was published as the cover story in the May 2010 issue of The International Journal of Plant Sciences.

The authors investigated the pollination of the specialised plant Lilium formosanum (Formosa lily) in its invasive range in KwaZulu-Natal, South Africa. They found that reproduction of this species is partially dependent on pollinators although it is self-compatible and can set some seed by autonomous self-pollination. Flowers are pale coloured, scented at night and contain nectar at the base of a long floral tube, features typical of plants specialised for pollination by hawkmoths. The primary pollinator in Kwazulu-Natal is indeed a hawkmoth, the cosmopolitan Agrius convolvuli. Most intriguing is that A. convolvuli is indigenous across most of Africa and the warmer parts of Europe and Asia, including Taiwan. Agrius convolvuli therefore probably pollinates L. formosanum in its native range too. Upon introduction to South Africa, L. formosanum would then have found its original pollinator already present, so in this case specialised pollination would not have hindered invasion.

The prediction that specialised pollination systems should hinder plant invasions thus needs to be refined. Specialised pollination can be expected to impede invasion only when the pollinator species and functional groups onto which plants are specialised have narrow distributions. This improvement in understanding contributes to a longstanding and elusive goal of invasion biology- to be able to prevent invasions by predicting whether species will become invasive before they are introduced.

Specialised plants and pollinator species
Specialised plants need not necessarily find the exact same pollinator species in the novel range as pollinators are often functionally equivalent from the plant’s point of view. Nicotiana glauca is specialised for hummingbird pollination in Argentina but has recruited sunbirds to pollinate it in South Africa (photo E). Hummingbirds and sunbirds both belong to a functional group of specialist nectar feeding bird pollinators. Similar examples exist for other pollinator functional groups. Stapelia gigantea, native to South Africa where it is pollinated by carrion flies (photo A), is invasive in Venezuela by virtue of pollination by indigenous carrion flies (photo B, taken in Cerro Seroche National Park, Venezuela); Gomphocarpus physocarpus is pollinated by paper wasps (Vespidae) in its native range in South Africa (photo C) and in Australia where it is invasive (Photo D) and the buzz-pollinated Senna didymobotrya from West Africa is pollinated by carpenter bees (Xylocopa, Apidae) in South Africa (photo F). Photo credits: A, C, & F by SD Johnson, B by I. Herrera, D by M. Ward and E by S. Geerts.

Front Cover: International Journal of Plant Sciences, Vol 171Read the paper Rodger, J. G., van Kleunen, M., and Johnson, S. D., Does specialized pollination impede plant invasions? International Journal of Plant Sciences 171 (4), 382 (2010)

Contact the author: James Rodger