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

Author
item FROST, CAROL - Canterbury Christchurch College
item DIDHAM, RAPHAEL - University Of Western Australia
item Rand, Tatyana
item PERALTA, GUADALUPE - Canterbury Christchurch College
item TYLIANAKIS, JASON - Canterbury Christchurch College

Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/2/2014
Publication Date: 1/15/2015
Publication URL: http://handle.nal.usda.gov/10113/60319
Citation: Frost, C., Didham, R.K., Rand, T.A., Peralta, G., Tylianakis, J.M. 2015. Community-level net spillover of natural enemies from managed to natural forest. Ecology. 96(1):193-202. DOI: 10.1890/14-0696.1.

Interpretive Summary: The effects of edges, or boundaries between different habitats, may become more severe as surrounding habitats become managed more intensively. Given that most managed systems, such as crops fields, have higher productivity than adjacent natural systems, such as forests or grassland, theory suggests that predators and other natural enemies are likely to spill over from managed to natural habitats. Furthermore, the magnitude of spillover is likely to differ between natural enemies that have a broad diet , generalists, compared with those that eat fewer different prey species. However, it is unknown how commonly managed-to-natural spillover occurs, and there is little understanding of how this influences ecosystem functioning in nearby natural habitats. We used flight intercept traps to measure spillover of generalist predators (Vespula wasps, Vespidae) and more specialist predators (106 species of parasitoid wasps, Ichneumonidae and Braconidae) across habitat edges between native New Zealand forest and exotic pine plantation forest over a summer season. We found net spillover of both generalist predators and parasitoids from plantation to native forest, and that the ratio of the number of individuals moving from plantation to individuals moving from native was greater for generalist predatory wasps. To test whether natural enemy spillover from managed habitats was related to prey (caterpillar) numbers, we used insecticides to reduce caterpillar densities at half of our sites. We monitored bi-directional natural enemy spillover before and after the insecticide treatments. We found a reduction in generalist predator but not parasitoid spillover. The effects of cross-habitat spillover of generalist predators into natural food webs deserve more attention in mosaic landscapes containing conservation land.

Technical 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 productivity than adjacent natural systems, theory suggests that subsidised 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 resource use. However, it is unknown how commonly managed-to-natural spillover occurs, and there is little understanding of how this influences ecosystem functioning in adjacent natural habitats. 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 predators and parasitoids from plantation to native forest, and that the ratio of the number of individuals moving from plantation to individuals moving from native was greater for generalists. To test whether natural enemy spillover from managed habitats was related to prey (caterpillar) abundance, we conducted a large-scale herbivore reduction experiment at half of our sites. We monitored bi-directional natural enemy spillover before and after the herbivore reduction. We found a reduction in generalist predator but not parasitoid spillover. The effects of cross-habitat spillover of trophic generalists into natural food webs deserve more attention in mosaic landscapes containing conservation land.