French wasps in the New World: experimental biological control introductions reveal a demographic Allee effect |
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Authors: | Xavier Fauvergue Keith Roderick Hopper |
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Institution: | (1) UMR 1301 INRA, CNRS, UNSA “Interactions Biotiques et Santé Végétale”, 400 Routes des Chappes, BP 167, 06903 Sophia-Antipolis Cedex, France;(2) Beneficial Insect Introductions Research Unit, USDA-ARS, 501 South Chapel Street, Newark, DE 19713, USA |
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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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Keywords: | Biological invasion Bottleneck Competition Mating Propagule pressure Sex pheromone |
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