Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Introduced species and their missing parasites


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.

Your institute does not have access to this article

Relevant articles

Open Access articles citing this article.

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

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


  1. Vitousek, P. M. Biological invasions and ecosystem processes: Towards an integration of population biology and ecosystem studies. Oikos 57, 7–13 (1990)

    Article  Google Scholar 

  2. Wilcove, D. S., Rothstein, D., Dubow, J., Phillips, A. & Losos, E. Quantifying threats to imperiled species in the United States. Bioscience 48, 607–615 (1998)

    Article  Google Scholar 

  3. Ruiz, G. M., Fofonoff, P., Hines, A. H. & Grosholz, E. D. Non-indigenous species as stressors in estuarine and marine communities: Assessing invasion impacts and interactions. Limnol. Oceanogr. 44, 950–972 (1999)

    ADS  Article  Google Scholar 

  4. Torchin, M. E., Lafferty, K. D. & Kuris, A. M. Parasites and marine invasions. Parasitology 124, S137–S151 (2002)

    Article  Google Scholar 

  5. Keane, R. M. & Crawley, M. J. Exotic plant invasions and the enemy release hypothesis. Trends Ecol. Evol. 17, 164–170 (2002)

    Article  Google Scholar 

  6. Shea, K. & Chesson, P. Community ecology theory as a framework for biological invasions. Trends Ecol. Evol. 17, 170–176 (2002)

    Article  Google Scholar 

  7. Crofton, H. D. A model of host–parasite relationships. Parasitology 63, 343–364 (1971)

    CAS  Article  Google Scholar 

  8. Anderson, R. M. & May, R. M. Regulation and stability of host–parasite population interactions I. Regulatory processes. J. Anim. Ecol. 47, 219–247 (1978)

    Article  Google Scholar 

  9. May, R. M. & Anderson, R. M. Regulation and stability of host–parasite population interactions II. Destabilizing processes. J. Anim. Ecol. 47, 249–267 (1978)

    Article  Google Scholar 

  10. Scott, M. E. Regulation of mouse colony abundance by Heligmosomoides polygyrus (Nematoda). Parasitology 95, 111–129 (1987)

    Article  Google Scholar 

  11. Gulland, F. M. D. The role of nematode parasites in Soay sheep Ovis aries L. mortality during a population crash. Parasitology 105, 493–503 (1992)

    Article  Google Scholar 

  12. Kuris, A. M. & Lafferty, K. D. Modeling crustacean fisheries: Effects of parasites on management strategies. Can. J. Fish. Aquat. Sci. 49, 327–336 (1992)

    Article  Google Scholar 

  13. Hudson, P. J., Dobson, A. P. & Newborn, D. Prevention of population cycles by parasite removal. Science 282, 2256–2258 (1998)

    ADS  CAS  Article  Google Scholar 

  14. Torchin, M. E., Lafferty, K. D. & Kuris, A. M. Release from parasites as natural enemies: Increased performance of a globally introduced marine crab. Biol. Invas. 3, 333–345 (2001)

    Article  Google Scholar 

  15. Dobson, A. P. & May, R. M. in Ecology of Biological Invasions of North America and Hawaii (eds Mooney, H. A. & Drake, J. A.) 58–76 (Springer, New York, 1986)

    Book  Google Scholar 

  16. Shimura, S. & Ito, J. Two new species of marine cercariae from the Japanese intertidal gastropod Batillaria cumingii (Crosse). Jpn. J. Parasitol. 29, 369–375 (1980)

    Google Scholar 

  17. Rybakov, A. V. & Lukomskaya, O. G. On the life cycle of Acanthoparyphium macracanthum sp.n. (Trematoda, Echinostomatidae). 22, 224–229 (1988)

  18. Harada, M. & Suguri, S. Surveys on cercariae in brackish water snails in Kagawa Prefecture, Shikoku, Japan. Jpn. J. Parasitol. 38, 388–391 (1989)

    Google Scholar 

  19. Chapman, F. M. Handbook of Birds from Eastern North America (Dover, New York, 1966)

    Google Scholar 

  20. Hair, J. D. & Forrester, D. J. The helminth parasites of the starling (Sturnus vulgaris L.): A checklist and analysis. Am. Midl. Nat. 83, 555–564 (1970)

    Article  Google Scholar 

  21. Mitchell, C. E. & Power, A. G. Release of invasive plants from fungal and viral pathogens. Nature 421, 625–627 (2003)

    ADS  CAS  Article  Google Scholar 

  22. Gregory, R. D. & Blackburn, T. M. Parasite prevalence and host sample size. Parasitol. Today 7, 316–318 (1991)

    CAS  Article  Google Scholar 

Download references


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.

Author information

Authors and Affiliations


Corresponding author

Correspondence to Mark E. Torchin.

Ethics declarations

Competing interests

The authors declare that they have no competing financial interests.

Supplementary information


Examples of parasites and introduced species and Supplementary Table I: Parasite data collection for 26 host species (DOC 101 kb)

Rights and permissions

Reprints and Permissions

About this article

Cite this article

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

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI:

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing