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United States Department of Agriculture

Agricultural Research Service

Research Project: ECOLOGY AND MANAGEMENT OF GRASSHOPPERS AND OTHER INSECT PESTS IN THE NORTHERN GREAT PLAINS

Location: Pest Management Research Unit

Title: Landscape complexity differentially benefits generalist fourth, over specialized third, trophic level natural enemies

Authors
item RAND, TATYANA
item Van-Veen, Frank J. -
item Tscharntke, Teja -

Submitted to: Ecography
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 4, 2011
Publication Date: February 1, 2012
Repository URL: http://handle.nal.usda.gov/10113/56480
Citation: Rand, T.A., Van-Veen, F.F., Tscharntke, T. 2011. Landscape complexity differentially benefits generalist fourth, over specialized third, trophic level natural enemies. Ecography. doi: 10.1111/j.1600-0587.2011.07016.x. 35(2): 97-104.

Interpretive Summary: The differential loss of higher trophic levels (eg. predators relative to herbivore and plants) in the face of the loss of natural habitat can result in the disruption of important ecosystem services such as pollination and biological control. Landscape- level conservation biological control aims to mitigate these negative impacts by conserving or restoring natural habitats within agricultural landscapes. Natural enemies that feed on crop pests have been shown to benefit from increasing landscape complexity, likely due to the presence of key resources, such as alternative food, in extra-field habitats. However, any benefits of increasing complexity may be counteracted if this also enhances enemies at the fourth trophic level (i.e. enhances enemies of the enemies of crop pests). We investigated the effects of landscape complexity on the primary and secondary parasitoid wasp communities of aphids in wheat and an abundant extra-field weed, stinging nettle. We found that primary parasitoid (3rd trophic level) communities attacking the cereal aphid, Sitobion avenae, had little overlap with the communities attacking the nettle aphid, Microlophium carnosum, while secondary parasitoids (4th trophic level) showed high levels of species overlap across these two aphids hosts (73% shared species), more strongly linking crop and non-crop habitats. In cereals, parasitoid diversity was not related to landscape complexity for either primary or secondary communities. Rates of primary parasitism were exceedingly low (1-6%), and positively related to landscape complexity late in the season. Secondary parasitism rates were high (37-94%) and also increased significantly with increasing landscape complexity. Overall, our results suggest that extra-field habitats and landscape complexity can differentially benefit fourth over third trophic level natural enemies. In situations where the fourth trophic level exerts significant impacts on the third, this would be predicted to disrupt biological control of crop pests. More generally our results underscore the need to take a more holistic food-web approach to studies of conservation biological control.

Technical Abstract: The differential loss of higher trophic levels in the face of natural habitat loss can result in the disruption of important ecosystem services such as pollination and biological control. Landscape- level conservation biological control aims to mitigate these negative impacts by conserving or restoring natural habitats within agricultural landscapes. Natural enemies of herbivores have been shown to benefit from increasing landscape complexity, likely due to the presence of key resources, such as alternative hosts, in extra-field habitats. However, any benefits of increasing complexity may be counteracted if this also enhances enemies at the fourth trophic level. Remarkably, studies of the influence of agricultural landscape structure on interactions at the fourth trophic-level are lacking. We investigated the effects of landscape complexity on the primary and secondary parasitoid communities of aphids in wheat and an abundant extra-field weed, stinging nettle. We found that primary parasitoid communities (3rd trophic level) attacking the cereal aphid, Sitobion avenae, had little overlap with the communities attacking the nettle aphid, Microlophium carnosum, while secondary parasitoids (4th trophic level) showed high levels of species overlap across these two aphids hosts (73% shared species), resulting in significantly higher linkage density and lower specialization for secondary than primary parasitoid webs. In cereals, parasitoid diversity was not related to landscape complexity for either primary or secondary communities. Rates of primary parasitism were exceedingly low (1-6%), and positively related to landscape complexity late in the season. Secondary parasitism rates were high (37-94%) and also increased significantly with increasing landscape complexity. Overall, our results suggest that extra-field habitats and landscape complexity can differentially benefit fourth over third trophic level natural enemies. In situations where the fourth trophic level exerts significant impacts on the third, this would be predicted to disrupt biological control of crop pests. More generally our results underscore the need to take a more holistic food-web approach to studies of conservation biological control.

Last Modified: 9/10/2014