An Allee effect at the front of a plant invasion: Spartina in a Pacific estuary

doi:10.1111/j.0022-0477.2004.00873.x

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Wiley InterScience

Journal of Ecology

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Volume 92 Issue 2, Pages 321 - 327

Published Online: 23 Mar 2004

An Official Journal of the British Ecological Society

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An Allee effect at the front of a plant invasion: Spartina in a Pacific estuary

Heather G. Davis*¶, Caz M. Taylor†, Janie C. Civille‡ and Donald R. Strong§¶

*Population Biology Graduate Group, 2320 Storer Hall, University of California, Davis, California 95616, USA, †Ecology Graduate Group, 2132 Wickson Hall, University of California, Davis, California 95616, USA, ‡Ecology Graduate Group, 2320 Storer Hall, University of California, Davis, California 95616, USA, §Section of Evolution and Ecology, 2320 Storer Hall, University of California, Davis, California 95616, USA, and ¶Bodega Marine Laboratory, Box 247, Bodega Bay, CA 94923–0247, USA

Correspondence: Heather G. Davis (tel. +1 530 754 6456; fax +1 530 752 1449; e-mail hgdavis@ucdavis.edu).

KEYWORDS

Allee effect • density • invasive species • reproduction • Spartina alterniflora

Summary

1
Spartina alterniflora sets very little viable seed at the leading edges of an invasion in Willapa Bay, Washington, USA, where it was introduced c. 100 years ago. This largely outbreeding, rhizomatous grass recruits into previously unoccupied areas at low density, so young plants initially grow isolated from one another but eventually coalesce to form continuous meadows.
2
Isolated recruits set approximately one-tenth the seed of meadow plants at five sites, spread over the 230 km² of Willapa Bay mudflats, and this seed germinated at only one-third the rate observed in meadow plants.
3
The consistent patterns suggested that the low seed set in the isolated plants was largely due to the demographic effects of density. Differences between sites in the incidence and amount of seed set and germination rate indicated, however, that there was some environmental influence.
4
These data imply that plants in newly invaded, low-density areas produce little viable seed until rhizomatous growth brings them into close contact. This Allee effect could substantially reduce the rate of invasion.