Abstract
The enemy release hypothesis (ERH), which has been the theoretical basis for classic biological control, predicts that the success of invaders in the introduced range is due to their release from co-evolved natural enemies (i.e. herbivores, pathogens and predators) left behind in the native range. We tested this prediction by comparing herbivore pressure on native European and introduced North American populations of Hypericum perforatum (St John’s Wort). We found that introduced populations occur at larger densities, are less damaged by insect herbivory and suffer less mortality than populations in the native range. However, overall population size was not significantly different between ranges. Moreover, on average plants were significantly smaller in the introduced range than in the native range. Our survey supports the contention that plants from the introduced range experience less herbivore damage than plants from the native range. While this may lead to denser populations, it does not result in larger plant size in the introduced versus native range as postulated by the ERH.
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Acknowledgements
We are tremendously grateful to Libby Vanwyhe and Jennifer Williams for helping with the field survey, and Daniel Sol and four anonymous reviewers for comments on an earlier draft of the paper. This study was partially funded by NSF grant DB00-98377 to J. L. M. and by DGICYT (REN2000-0361/GLO) to M. V.
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Vilà, M., Maron, J.L. & Marco, L. Evidence for the enemy release hypothesis in Hypericum perforatum. Oecologia 142, 474–479 (2005). https://doi.org/10.1007/s00442-004-1731-z
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DOI: https://doi.org/10.1007/s00442-004-1731-z