Introduced species and their missing parasites

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Damage caused by introduced species results from the high population densities and large body sizes that they attain in their new location1,2,3,4. Escape from the effects of natural enemies is a frequent explanation given for the success of introduced species5,6. Because some parasites can reduce host density7,8,9,10,11,12,13 and decrease body size14, an invader that leaves parasites behind and encounters few new parasites can experience a demographic release and become a pest4,15. To test whether introduced species are less parasitized, we have compared the parasites of exotic species in their native and introduced ranges, using 26 host species of molluscs, crustaceans, fishes, birds, mammals, amphibians and reptiles. Here we report that the number of parasite species found in native populations is twice that found in exotic populations. In addition, introduced populations are less heavily parasitized (in terms of percentage infected) than are native populations. Reduced parasitization of introduced species has several causes, including reduced probability of the introduction of parasites with exotic species (or early extinction after host establishment), absence of other required hosts in the new location, and the host-specific limitations of native parasites adapting to new hosts.

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Figure 1: Parasite release experienced by introduced species.
Figure 2: Parasitism in introduced and native populations.


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This work was conducted as part of the Diseases and Conservation Biology Working Group supported by the National Center for Ecological Analysis and Synthesis, a centre funded by the National Science Foundation (NSF), the University of California, and the Santa Barbara campus. We thank S. Altizer, S. Gaines, P. Hudson, H. McCallum, A. W. Miller, C. Mitchell and A. Power for discussion and comments; A. Dove and G. Ruiz for providing data; and L. Mababa for data collection. This research was supported by NSF through the NIH/NSF Ecology of Infectious Disease Program, and by the National Sea Grant College Program, National Oceanic and Atmospheric Administration (NOAA), US Department of Commerce through the California Sea Grant College System, and in part by the California State Resources Agency. The views expressed herein are those of the authors and do not necessarily reflect the views of NOAA or any of its subagencies. The US Government is authorized to reproduce and distribute for governmental purposes.

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Correspondence to Mark E. Torchin.

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Torchin, M., Lafferty, K., Dobson, A. et al. Introduced species and their missing parasites. Nature 421, 628–630 (2003) doi:10.1038/nature01346

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