Food web structure of three guilds of natural enemies: predators, parasitoids and pathogens of aphids

Authors

  • F. J. F. Van Veen,

    Corresponding author
    1. NERC Centre for Population Biology and
    2. Division of Biology, Imperial College London, Silwood Park Campus, Ascot, Berks, SL5 7PY, UK; and
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  • C. B. Müller,

    1. Division of Biology, Imperial College London, Silwood Park Campus, Ascot, Berks, SL5 7PY, UK; and
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    • Present address: Institute of Environmental Sciences, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland.

  • J. K. Pell,

    1. Plant and Invertebrate Ecology Division, Rothamsted Research, Harpenden, Herts, AL5 2JQ, UK
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  • H. C. J. Godfray

    1. NERC Centre for Population Biology and
    2. Division of Biology, Imperial College London, Silwood Park Campus, Ascot, Berks, SL5 7PY, UK; and
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    • Present address: Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK.


*Correspondence author. E-mail: f.vanveen@imperial.ac.uk

Summary

  • 1Most communities of insect herbivores are unlikely to be structured by resource competition, but they may be structured by apparent competition mediated by shared natural enemies.
  • 2The potential of three guilds of natural enemies (parasitoids, fungal entomopathogens and predators) to influence aphid community structure through indirect interactions is assessed. Based on the biology, we predicted that the scope for apparent competition would be greatest for the predator and least for the parasitoid guilds.
  • 3Separate fully quantitative food webs were constructed for 3 years for the parasitoid guild, 2 years for the pathogen guild and for a single year for the predator guild. The webs were analysed using standard food web statistics designed for binary data, and using information-theory-based metrics that make use of the full quantitative data.
  • 4A total of 29 aphid, 24 parasitoid, five entomopathogenic fungi and 13 aphid specialist predator species were recorded in the study. Aphid density varied among years, and two species of aphid were particularly common in different years. Omitting these species, aphid diversity was similar among years.
  • 5The parasitoid web showed the lowest connectance while standard food web statistics suggested the pathogen and predator webs had similar levels of connectance. However, when a measure based on quantitative data was used the pathogen web was intermediate between the other two guilds.
  • 6There is evidence that a single aphid species had a particularly large effect on the structure of the pathogen food web.
  • 7The predator and pathogen webs were not compartmentalized, and the vast majority of parasitoids were connected in a single large compartment.
  • 8It was concluded that indirect effects are most likely to be mediated by predators, a prediction supported by the available experimental evidence.

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