Relationships among leaf damage, natural enemy release, and abundance in exotic and native prairie plants
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The Enemy Release hypothesis holds that exotic plants may have an advantage over native plants because their specialized natural enemies are absent. We tested this hypothesis by measuring leaf damage and plant abundance for naturally-occurring plants in prairies, and by removing natural enemies in an enemy exclusion experiment. We classified plants as invasive exotic, noninvasive exotic, or native, to determine if their degree of invasiveness influenced their relationships with natural enemies. Our field surveys showed that invasive exotic plants generally had significantly lower levels of foliar damage than native species while there was no consistent pattern for noninvasive exotics compared to natives. The relationship between damage and abundance was different for exotic and native plants: foliar damage decreased with increasing abundance for exotic plants while the trend was positive for native plants. While these results from the field surveys supported the Enemy Release Hypothesis, the enemy exclusion experiment did not. There was no relationship between a species’ status as exotic or native and its degree of release from herbivory. Pastinaca sativa, the invasive exotic in this experiment, experienced gains in leaf area and vegetative biomass when treated with pesticides, indicating substantial herbivore pressure in the introduced range. These results show that foliar damage may not accurately predict the amount of herbivore pressure that plants actually experience, and that the Enemy Release hypothesis is not sufficient to explain the invasiveness of P. sativa in prairies.
KeywordsEnemy release hypothesis Invasive plant Pastinaca sativa Enemy exclusion experiment Noninvasive exotic plant
We thank Jim Reinartz and Stefan Schnitzer for advice and helpful comments and Kayvon Ali for assistance in the field and laboratory. The Wisconsin Department of Natural Resources granted permission to use many of the sites for this experiment, and in particular we are grateful to Matt Zine for his assistance at Young Prairie. Mike Fort assisted with plant identification. Two anonymous reviewers provided comments that improved the manuscript. This research was supported by a Ruth Walker Grant-in-Aid and an Advanced Opportunity Fellowship, both from the University of Wisconsin-Milwaukee, to ECV.
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