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.Knowinging interests in exploiting phenotype A or pheno- that most, if not all, free-living species organismstype B gammarids.The second trematode species harbour parasites and knowing that a variety ofM.subdolum which also finishes its life cycle in host traits are altered by parasites, this new area ofaquatic birds prefers to infect phenotype B gam- research at the junction of ecology and parasitologymarids (Thomas et al.1997).The cercariae (infective appears promising.Clearly, further empirical andstage) of M.subdolum actively swim in the water theoretical studies on this subject are needed.column where they are more likely to encounterphenotype B gammarids (Thomas et al.1997).If shared interests exist between the manipulator 8.6 Parasite diversity and conservation(M.papillorobustus) and this hitch-hiker parasite biology(M.subdolum), there is conversely a clear conflict of8.6.1 The abundance of parasitesinterest between the manipulator M.papillorobustusand the nematode G.gammari.Indeed, the nema- The diversity and abundance of parasites istode uses amphipods as a habitat and source of remarkable: for example, at least 2000 species ofnutrition, not as a vehicle to be transmitted to nematodes have been named but there may wellbirds.As expected, there is evidence that the nema- exist 10 or 100 times this many species of nem-tode prefers phenotype A gammarids (Thomas et al.atodes, most of which still have yet to be classified2002).There is also the suggestion that the nem- (Poulin 1996b).Evidence from the phylogeny ofatode sabotages the manipulation exerted by nematodes illustrates that parasitism is somethingM.papillorobustus, turning back gammarids from a that has evolved many times even within this largephenotype B to a phenotype A (Thomas et al.2002).order of organisms (Anderson 1988; Adamson andA final example serves to illustrate that, in natu- Caira 1995).Even within the major families ofral conditions, the effects of parasites as ecological nematodes, we find that parasitism has evolvedengineers leads to a complex set of effects as several multiple times.Parasitism is a common way of lifeprocesses can occur simultaneously.Mouritsen and among other phyla of worms: all species in thePoulin (2002) reported an example of dramatic trematodes, monogeneans, cestodes, and acantho-change in an intertidal community resulting from cephala are parasitic.With the exception of thethe effects of shared parasitism and host mortality, monogeneans, all species in these taxa have com-ecosystem engineering and cascade effects.Due to plex life cycles in which two different hosts speciestrematode infections, the amphipod Corophium are parasitized sequentially.Parasitism is also avolutator disappeared from Danish mud flats.This very common life form in many of the protozoans,crustacean, because of its tube-building activity, the viruses, bacteria, and fungi.Peter Price was oneplayed a major role in stabilizing the substrate.The of the first people to make a conservative estimatelocal extinction of this allogenic engineers allowed of the proportion of all known species that are 136 PARASI TI SM AND ECOSYSTEMparasitic; by including sucking arthropods (an fish have on average one or two species of parasiticimportant analogy which we will return to later), helminth in them, birds may have somewhere from 2he concluded that around 50% of species are para- to 10 species of parasitic helminths, and mammalssitic (Price 1980).This work was more vigorously may have somewhere between 8 and 19 species offormalized by Cathy Toft, who formally counted parasitic helminth.If we look at the actual numbers,numbers of parasitic and non-parasitic species in all then fish will contain anywhere between 2 and 100known phyla (Toft 1986).This estimate also sug- worms per host, birds may contain anywheregests that as many as 50% of species are parasitic.between 20 and 1000 worms per host, and mammalsSome of the major advantages of a parasitic mode may have anywhere between 10 and 50,000 wormsof life are emerging from life history studies of par- per host.This implies that most populations of verte-asites (Calow 1979; Skorping et al.1991).brates, are effectively operating as a metapopulationComparative studies of nematodes illustrate that of patches that serve as resources for parasiticthe normal rules of allometric scaling and assimila- helminths, a theme we will explore further below.tion efficiency have to be reconsidered when we con-sider parasites (Skorping et al.1991; Arneberg et al.8.6.2 Host parasite models as metapopulation1998b; Morand and Sorci 1998).In contrast to mostmodelsfree-living animals, selection for increased body sizein parasitic nematodes leads to both increased Models for parasite host communities are related tolongevity and fecundity.This creates the potential those for metapopulations of organisms in patchyfor parasites to become Darwinian demons.environments.Indeed the simplest host parasiteWe can obtain some estimate of the ubiquity of model we could write would have a host popu-parasites by trying to estimate the number of para- lation of constant size divided into susceptible andsitic worms associated with any population of free- infected hosts.This model is identical to the classicliving vertebrates.If we briefly survey published Levin s population model (Levins and Culver 1971).data for 50 different species of North American The classic (SIR susceptible, infected, and recov-mammals we find that on average a mammal will ered) epidemiological models (Kermack andcontain 400 worms from four different taxa, tremat- McKendrick 1927) are a simple extension of this thatodes, cestodes, nematodes, and acanthocephalans.include an extra category of patch, those that wereAt the population level there are on average previously occupied, but are now resistant to fur-10 species of parasitic worms associated with any ther occupation.The other major modification ofone population of vertebrates (Dobson et al.1992b) [ Pobierz całość w formacie PDF ]