A test for Allee effects in the self-incompatible wasp-pollinated milkweed Gomphocarpus physocarpus
Version of Record online: 21 MAY 2009
© 2009 The Authors. Journal compilation © 2009 Ecological Society of Australia
Volume 34, Issue 6, pages 688–697, September 2009
How to Cite
COOMBS, G., PETER, C. I. and JOHNSON, S. D. (2009), A test for Allee effects in the self-incompatible wasp-pollinated milkweed Gomphocarpus physocarpus. Austral Ecology, 34: 688–697. doi: 10.1111/j.1442-9993.2009.01976.x
- Issue online: 28 AUG 2009
- Version of Record online: 21 MAY 2009
- Accepted for publication October 2008.
- Allee effect;
- Gomphocarpus physocarpus;
- pollen transfer efficiency;
- pollinarium reconfiguration;
- wasp pollination
It has been suggested that plants that are good colonizers will generally have either an ability to self-fertilize or a generalist pollination system. This prediction is based on the idea that these reproductive traits should confer resistance to Allee effects in founder populations and was tested using Gomphocarpus physocarpus (Asclepiadoideae: Apocynaceae), a species native to South Africa that is invasive in other parts of the world. We found no significant relationships between the size of G. physocarpus populations and various measures of pollination success (pollen deposition, pollen removal and pollen transfer efficiency) and fruit set. A breeding system experiment showed that plants in a South African population are genetically self-incompatible and thus obligate outcrossers. Outcrossing is further enhanced by mechanical reconfiguration of removed pollinaria before the pollinia can be deposited. Self-pollination is reduced when such reconfiguration exceeds the average duration of pollinator visits to a plant. Observations suggest that a wide variety of wasp species in the genera Belonogaster and Polistes (Vespidae) are the primary pollinators. We conclude that efficient pollination of plants in small founding populations, resulting from their generalist wasp-pollination system, contributes in part to the colonizing success of G. physocarpus. The presence of similar wasps in other parts of the world has evidently facilitated the expansion of the range of this milkweed.