Establishment and dominance of an introduced herbivore has limited impact on native host-parasitoid food webs |
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Authors: | Laura L Timms Steven C Walker Sandy M Smith |
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Institution: | (1) Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, ON, M5S 3B3, Canada;(2) D?partement de Sciences Biologiques, Universit? de Montr?al, P.O. Box 6128, Station Centre-Ville, Montreal, QC, H3C 3J7, Canada;(3) Department of Natural Resource Sciences, McGill University, Macdonald Campus, 21,111 Lakeshore Road, Ste-Anne-de-Bellevue, Montreal, QC, H9X 3V9, Canada |
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Abstract: | The gypsy moth is considered one of the most harmful invasive forest insects in North America. It has been suggested that
gypsy moth may indirectly impact native caterpillar communities via shared parasitoids. However, the impact of gypsy moth
on forest insect food webs in general remains unstudied. Here we assess such potential impacts by surveying forest insect
food webs in Ontario, Canada. We systematically collected caterpillars using burlap bands at sites with and without histories
of gypsy moth outbreak, and then reared these caterpillars until potential parasitoid emergence. This procedure allowed us
to generate quantitative food webs describing caterpillar-parasitoid interactions. We estimated the degree of parasitoid sharing
between gypsy moth and native caterpillars. We also statistically modeled the effect of gypsy moth outbreak history and current
gypsy moth abundance on standard indices of quantitative food web structure and the diversity of parasitoid communities. Rates
of gypsy moth parasitism were very low and gypsy moth shared very few parasitoids with native caterpillars, suggesting limited
potential for indirect interactions. We did not detect any significant effects of gypsy moth on either food web structure
or parasitoid diversity, and the small amount of parasitoid sharing strongly implies that this lack of significance is not
merely due to low statistical power. Our study suggests that gypsy moth has limited impact on native host-parasitoid food
webs, at least for species that use burlap bands. Our results emphasize that extrapolations of theoretical and experimental
conclusions on the impacts of invasive species should be tested in natural settings. |
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