Indirect effects of phorid fly parasitoids on the mechanisms of interspecific competition among ants |
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Authors: | Lloyd W Morrison |
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Institution: | (1) Department of Zoology and Brackenridge Field Laboratory, University of Texas, Austin, TX 78712, USA e-mail: lmorrison@mail.utexas.edu, Fax: +1-512-4756286, US |
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Abstract: | Indirect effects, which occur when the impact of one species upon another requires the existence of an intermediary species,
are apparently very common and may be of greater magnitude than direct effects. Behaviorally mediated indirect effects occur
when one species affects the behavior of a second, which in turn affects how that species interacts with a third. I studied
behaviorally mediated indirect effects on the mechanisms of competition in two congeneric fire ant species in the presence
and absence of parasitoid phorid flies, which parasitized only one ant species. In observational and experimental field studies,
the presence of native Texas phorid flies in the genus Pseudacteon decreased food retrieval by their host, Solenopsis geminata (F.), by as much as 50%. In the presence of phorid flies, many S. geminata workers assumed a stationary, curled defensive posture and did not forage. Although the phorid parasitoids had a relatively
large effect on exploitative competition, there was no measurable effect on interference competition. Fierce interspecific
aggression was observed between S. geminata and S. invicta Buren, and the presence of phorids had no effect on the outcome of these contests. The indirect effects of Pseudacteon parasitoids on Solenopsis fire ant resource retrieval appear to be larger than the direct effect of mortality. Some aspects of the foraging behavior
of these Solenopsis species may be, in part, evolutionary adaptations to phorid parasitoid pressure. Because of the relatively large indirect
effects, South American Pseudacteon phorids may be promising biocontrol agents of imported fire ants, S. invicta, in the USA. In a laboratory study, a single South American Pseudacteon female was able to significantly decrease food retrieval rates of a North American population of the imported fire ant, S. invicta.
Received: 11 May 1998 / Accepted: 18 April 1999 |
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Keywords: | Direct effects Exploitative competition Indirect effects Interference competition Parasitism |
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