Oecologia

, Volume 136, Issue 4, pp 551–557 | Cite as

Density-dependent seed set in the Haleakala silversword: evidence for an Allee effect

Population Ecology

Abstract

Plant species may be subject to Allee effects if individuals experience a reduction in pollination services when populations are small or sparse. I examined temporal variation in reproductive success of the monocarpic Haleakala silversword (Argyroxiphium sandwicense subsp. macrocephalum) over five years, to determine if plants flowering out of synchrony with most of the population (i.e., in low flowering years) exhibited lower percent seed set than synchronously-flowering plants (i.e., those flowering in high flowering years). Through two pollination experiments conducted over multiple years, I also measured pollen limitation and self-incompatibility in this species. The number of flowering plants varied greatly among years, as did reproductive success. Percent seed set was significantly correlated with the number of plants flowering annually, such that plants flowering in high flowering years (1997 and 2001) exhibited significantly higher percent seed set than did plants flowering in low flowering years (1998–2000). In the 3-year pollen limitation study, plants flowering asynchronously were pollen-limited, whereas plants flowering synchronously were not. This species is strongly self-incompatible. Results of this study demonstrate that the Haleakala silversword experiences reduced reproductive success in low flowering years, and suggest that this Allee effect is pollinator-mediated. Allee effects in plants are an understudied yet potentially important force with implications for the population dynamics and conservation of rare species.

Keywords

Argyroxiphium Monocarpy Pollen limitation Self-incompatibility Synchronous flowering 

Notes

Acknowledgements

I thank Clare Aslan for assistance in the field and laboratory, and Regis Ferriere for inspiring discussion. Much thanks to Lloyd Loope and Ron Nagata for logistical support at Haleakala National Park. Rob Robichaux, Judie Bronstein, Lucinda McDade, Molly Hunter, Nick Waser and an anonymous reviewer provided valuable feedback on earlier drafts of this manuscript. This project was funded by the National Science Foundation (Graduate Research Fellowship), the National Parks Conservation Association, and the Department of EEB at the University of Arizona. This manuscript was submitted to the University of Arizona in partial fulfillment of requirements for the degree of Doctor of Philosophy.

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Copyright information

© Springer-Verlag 2003

Authors and Affiliations

  1. 1.Department of Ecology and Evolutionary Biology, Biological Sciences West 310University of ArizonaTucsonUSA

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