Community-level net spillover of natural enemies from managed to natural forest

Authors

  • Carol M. Frost,

    Corresponding author
    1. School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140 New Zealand
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  • Raphael K. Didham,

    1. School of Animal Biology, The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009 Australia
    2. CSIRO Ecosystem Sciences, Centre for Environment and Life Sciences, Underwood Avenue, Floreat, Western Australia 6014 Australia
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  • Tatyana A. Rand,

    1. USDA-ARS Northern Plains Agricultural Research Laboratory, Sidney, Montana 59270 USA
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  • Guadalupe Peralta,

    1. School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140 New Zealand
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  • Jason M. Tylianakis

    1. School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140 New Zealand
    2. Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot, Berkshire SL5 7PY United Kingdom
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  • Corresponding Editor: W. E. Snyder.

Abstract

Edge effects in fragmented natural habitats may be exacerbated by intensive land use in the surrounding landscape. Given that most managed systems have higher primary productivity than adjacent natural systems, theory suggests that bottom-up subsidized consumers are likely to spill over from managed to natural habitats. Furthermore, the magnitude of spillover is likely to differ between generalist and specialist consumers, because of differences in their ability to use the full spectrum of resources. However, it is unknown whether there is indeed asymmetrical spillover of consumers between managed and natural habitats, and whether this is related to resource abundance or the trophic specialization of the consumer. We used flight intercept traps to measure spillover of generalist predators (Vespula wasps, Vespidae) and more specialist predators (106 species of parasitoids, Ichneumonidae and Braconidae) across habitat edges between native New Zealand forest and exotic plantation forest over a summer season. We found net spillover of both generalist and specialist predators from plantation to native forest, and that this was greater for generalists. To test whether natural enemy spillover from managed habitats was related to prey (caterpillar) abundance (i.e., whether it was bottom-up productivity driven, due to increased primary productivity), we conducted a large-scale herbivore reduction experiment at half of our plantation sites, by helicopter spraying caterpillar-specific insecticide over 2.5 ha per site. We monitored bidirectional natural enemy spillover and found that herbivore reduction reduced generalist but not specialist predator spillover. Trophic generalists may benefit disproportionately from high resource productivity in a habitat, and their cross-habitat spillover effects on natural food webs may be an important source of consumer pressure in mosaic landscapes.

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