Abstract
Many populations introduced into a novel environment fail to establish. One underlying process is the Allee effect, i.e., the difficulty of individuals to survive and reproduce when rare, and the consequently low or negative population growth. Although observations showing a positive relation between initial population size and establishment probability suggest that the Allee effect could be widespread in biological invasions, experimental tests are scarce. Here, we used a biological control program against Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae) in the United States to manipulate initial population size of the introduced parasitoid Aphelinus asychis Walker (Hymenoptera: Aphelinidae) originating from France. For eight populations and three generations after introduction, we studied spatial distribution and spread, density, mate-finding, and population growth. Dispersal was lower in small populations during the first generation. Smaller initial population size nonetheless resulted in lower density during the three generations studied. The proportion of mated females and the population sex ratio were not affected by initial population size or population density. Net reproductive rate decreased with density within each generation, suggesting negative density-dependence. But for a given density, net reproductive rate was smaller in populations initiated with few individuals than in populations initiated with many individuals. Hence, our results demonstrate a demographic Allee effect. Mate-finding is excluded as an underlying mechanism, and other component Allee effects may have been overwhelmed by negative density-dependence in reproduction. Impact of generalist predators could provide one potential explanation for the relationship between initial population size and net reproductive rate. However, the continuing effect of initial population size on population growth suggests genetic processes may have been involved in the observed demographic Allee effect.
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Acknowledgments
This research benefited from US Department of Agriculture competitive Grant 93-37302-9432 to K.R.H. We thank Aaron Chavez, Matthew Harbey, Nathalie Ramualde and Richard Turcotte for their technical assistance, Abdel Berrada and his colleagues at the Southwestern Colorado Research Center for facilities and cooperation in conducting this research, Walter and Heidi Henes for their hospitality, and Robert Hammon for help with finding field sites and making contacts. We are grateful to Philippe Aubry for the spatial analyses. We also thank Franck Courchamp for initial encouragements to write a manuscript for this special issue, and Andrew Liebhold and Takehiko Yamanaka for their patience when the manuscript was loitering on the way.
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Fauvergue, X., Hopper, K.R. French wasps in the New World: experimental biological control introductions reveal a demographic Allee effect. Popul Ecol 51, 385–397 (2009). https://doi.org/10.1007/s10144-009-0147-3
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DOI: https://doi.org/10.1007/s10144-009-0147-3