Most parasites of introduced species were parasites of native hosts that colonized introduced hosts. These parasites reached prevalences similar to parasites that were introduced from the native range of the invading host. For example, introduced populations of the mallard duck, Anas platyrhynchos, were infected with 31 parasite species. Of these, 21 do not occur in the duck’s native range. Additionally, the mean prevalence of the native parasites that colonized introduced populations was 12% compared to 10% for those that derived from the native range of the duck.
Introduced hosts may become sinks for some native parasites if they are insufficiently adapted to the novel hosts to complete their development. Some of these maladapted parasites can cause substantial pathology in their new hosts. As an example, the striped bass, Morone saxatilis, was introduced from the eastern U.S.A. to the San Francisco Bay delta, where it became commonly infected by larval stages of the tapeworm, Lacistorhynchus dollfusi1. The parasite elicits a strong immune response in the host, preventing its further development but also harming the fish. Such maladaptions may diminish over time due to natural selection. This was demonstrated by Sakanari and Moser2 who experimentally infected striped bass from San Francisco Bay as well as naïve bass from native Atlantic populations. They found that fish from naïve native populations had higher intensity infections and elicited a much stronger immune response, resulting in more tissue damage to the fish.
The likelihood that a parasite will encounter and shift to an introduced species may change over time, as will the potential for a parasite to evolve the ability to utilize a novel (introduced) host. This is supported by the positive association between parasite colonization on an invader and time since invasion3-5. Additionally, we surmise that species that invade regions with phylogenetically and ecologically similar species will experience higher rates of parasitization by native parasites than will hosts that became established in areas without similar species. Similarly, the larger the new geographic range of the invader, the more parasites it is likely to encounter6.
Criteria for selecting studies
We generated a list of species for the analysis using a stratified random procedure designed to minimize biases associated with non-random selection. First, we identified lists of invasive species for particular geographic regions7-11. We then conducted a literature search of all species listed in these references using Biological Abstracts (1985-2001, online versions) using genus, species and parasit# or infect# or pathogen# as keywords. We selected host species where this search revealed ten or more citations, or those, which we knew had been parasitologically examined. For taxa that had over ten species which fit these criteria, we randomly selected ten species and continued our search. From this list, we compiled parasitological data from any available source. Several of these had been sufficiently studied to provide prevalence data in both the native and introduced ranges of the invader. We excluded studies focusing on a single parasite species. We also excluded host species under culture from our comparison because these species are introduced intentionally, are managed to reduce parasitism and are typically grown at unnaturally high densities which promotes disease transmission. As a result of our search strategy, for amphibians, reptiles, birds and mammals, the parasitological studies used for comparing native and introduced regions were comprised exclusively of parasitic helminths.
1. Sakanari, J. A. & Moser, M. Lesion induction by the plerocercoid Lacistorhynchus tenuis (Cestoda) and wound healing in the striped bass, Morone saxatilis. Journal of Fish Biology28, 289-296 (1986).
2. Sakanari, J. A. & Moser, M. Adaptation of an introduced host to an indigenous parasite. Journal of Parasitology76, 420-423 (1990).
3. Blaustein, A. R., Kuris, A. M. & Alió, J. J. Pest and parasite species-richness problems. American Naturalist122, 556-566 (1983).
4. Cornell, H. V. & Hawkins, B. A. in Parasitoid Community Ecology (eds. Hawkins, B. A. & Sheehan, W.) 77-89 (Oxford University Press, Oxford, 1994).
5. Guegan, J. F. & Kennedy, C. R. Maximum local helminth parasite community richness in British freshwater fish: A test of the colonization time hypothesis. Parasitology106, 91-100 (1993).
6. Combes, C. Parasitism: the ecology and evolution of intimate interactions (ed. Thompson, J. N.) (University of Chicago Press, Chicago, 2001).
7. CIESM. Atlas of exotic species in the Mediterranean Sea http://www.ciesm.org/atlas/index/html., (2001).
8. Ruiz, G. M., Fofonoff, P., Carlton, J. T., Wonham, M. J. & Hines, A. H. Invasions of coastal marine communities in North America: apparent patterns, processes, and biases. Ann. Rev. Ecol. Syst.31, 481-531 (2000).
9. Smith, H. M. & Kohler, A. J. A survey of herpetological introductions in the United States and Canada. Transactions of the Kansas Academy of Science80, 1-24 (1978).
10. USGS. US Geological Survey Nonindigenous Aquatic Species http://nas.er.usgs.gov., (2001).
11. Contreras-B, S. & Escalante-C., M. A. in Distribution, biology, and management of exotic fishes
(eds. Courtenay, W. R. & Stauffer, J. R.) 103-130 (Johns Hopkins University Press, Baltimore, 1984).
SUPPLEMENTARY TABLE I: PARASITE DATA COLLECTION FOR 26 HOST SPECIES
Table of host species used in our comparison of parasites in native and introduced* regions. *References with asterisks indicate introduced host populations.
Harada and Suguri, 1989; Rybakov and Lukomskaya, 1986; Shimura and
Ito, 1980; *Ching, 1991; *Torchin et al. (in review); *Whitlatch, 1974
Mattison et al., 1995; Probert, 1966; Toledo et al., 1998; *Lepitski and Scott,
1994; *Lepitski et al., 1994, *Ménard and Scott, 1987
Borggsteede et al., 2000; Gundlach, 1965; James and Llewellyn, 1967;
Owen and Pemberton, 1962; *Boyd, 1951; *Carter et al., 1973; *Cooper
and Crites, 1975; *Cooper and Crites, 1976; *Vincent, 1972
Allan et al., 1999; Blasco et al.,1996; Boag, 1985; 1987;1988; Boag and
Iason, 1986; Boag and Kolb, 1989; Butler, 1994; Molina et al., 1998; *Bull,
1964; *Casanova et al., 1996; *Dudzinski and Mykytowycz, 1963;
*Dunsmore and Dudzinski, 1968; *Hobbs et al., 1999; *Pisano et al., 2001
Hasegawa et al., 1994; Hasegawa and Syafruddin, 1995; Huq et al., 1985;
Mafiana et al., 1997; Casanova et al., 1996; *Miquel et al., 1996;
*Pisano et al., 2001
O'Callaghan and Moore, 1986; Presidente et al., 1982; *Stankiewicz
et al., 1998
Criado-Fornelio et al., 2000; Deblock et al., 1988; Hofer et al., 2000;
Papadopoulos et al., 1997; Richards et al., 1995; *Coman, 1973;
*Conti, 1984; *Davidson et al., 1992; El-Shehabi et al., 1999; *Ryan,
1976; *Sato et al., 1999
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