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.confirmed the reduced parasitization of intro-First, most studies on herbivorous insects (see duced organisms suggestive of an absence of satu-Dawah et al.1995; Frenzel and Brandl 2000) and ration in those parasite communities (see Figs.2(c)fish ectoparasites, that is, helminths and copepods and (d) in Torchin et al.2003).(see Morand et al.1999), have identified that empty Third, major advances in our understanding ofniches are common and that local communities are saturation versus non saturation of local parasiteunsaturated.As such, many natural enemy com- communities have been made in recent years.munities are subject to strong regional influences Cornell and Karlson (1997) and Srivastava (1999)then providing opportunities for new invasive drew attention to the necessity of the demonstra-species to become established.The opposite can be tion of other lines of evidences of niche and habitatobserved for many studies on internal parasites relationships combined with information on locallike intestinal helminths of fish (Kennedy and versus regional relationships.In particular, RohdeGuégan 1994), of amphibians and reptiles (Aho (1998) using randomization procedures high-1990), and of birds (Bush 1990; Calvete et al.2004) lighted the many scenarios in which a curvilinearwhere local forces may contribute to parasite com- local to regional relationship might be generatedmunity structure.Poulin (1996a, 1997) showed, on without requiring the necessity of species satura-the contrary, that for 31 intestinal helminth com- tion.Using a comprehensive survey of marine fishmunities in bird hosts and 37 in mammal hosts the ectoparasite communities, Morand et al.(1999)relationship between the maximum infracommu- examined the effects of interspecific aggregationnity richness and component community species on the level of intraspecific aggregation in infra-richness was linear, indicating the absence of communities, and they demonstrated that inter-species saturation and the availability of vacant specific interactions were reduced relative toniches in organisms accepted to generally have intraspecific interactions thus facilitating speciesspecies-rich helminth communities (see Bush and coexistence in rich communities (see Tokeshi 1999Holmes 1986a,b; Stock and Holmes 1988).The con- for further details on species coexistence).This pat-trasting results may in part be due to the fact that tern was highly coincidental with a positive linear 34 PARASI TI SM AND ECOSYSTEMSrelationship between infracommunity species and epidemiology.Third, one area in which therichness and total parasite species richness combination of these two methods should be fruit-obtained after controlling for the confounding ful is in the connection that might exist with theeffect exerted by phylogeny, indicative of no satu- density of parasites or microbes.Indeed, if compe-ration in ectoparasite communities of marine fish tition is important within parasite or microbe com-(see Rohde 1991, 1998).In a recent study, Calvete et munities, one would expect to see densityal.(2004) showed the existence of a curvilinear compensation in those communities with fewrelationship between local and regional species species.If complete compensation occurs, thererichnesses of intestinal helminth infracommunities should be no relation between parasite density orfor eight populations of the red-legged partridge in biomass and local species richness, while if thereSpain, even after checking for the confounding was no density compensation a linear trend wouldeffect of geographical distance among localities thus be expected (see Oberdorff et al.1998; Griffithson species richness calculations.Interestingly, 1999 for taxonomic groups others than parasites).this finding was confirmed by a demonstration of Furthermore, the linkages between interspecificnegative interspecific associations for the competition, (un)saturation and density compen-helminth species community, especially between sation in parasite or microorganism communitycestodes and other helminths parasitizing the bird assemblages will require more research from com-intestines.munity ecologists, parasitologists and epidemiolo-All these results illustrate a number of important gists.Notably, these issues should be highlyissues about the understanding of local regional relevant in the field of veterinary and medical sci-richness relationships in parasite or microbe com- ences since any alteration of local habitats (frommunity assemblages.First, demonstrating the the point of view of one parasite species, for exam-effects of saturation, or not, in infracommunity ple, one intestine) and other disturbances exertedassemblages requires that we simultaneously use by humans (e.g.the use of drugs like helminthi-additional investigations of interspecific interact- cides or antibiotics) should reduce parasite orions, or that published examples of the types of pathogen populations from time to time, makinginteractions exist, to test for the possible existence ways for more resistant species or aliens to increaseof interspecific competition.Neither of the two or to invade.The idea of saturation predicts that anpatterns for community assemblage organization, invasive species (like a crop or an emerging virus)that is, the local to regional richness relationship should not invade an infracommunity in individ-and the demonstration of interspecific competi- ual hosts, or should do so only with the conse-tion, is conclusive on its own.Interestingly, conclu- quence of excluding a resident member species,sions about the degree of concordance in the that is, density compensation by new individualsaturation of local communities, or not, between invaders.The study of interconnectedness betweentwo or more methods may yield generality, but these patterns and our efforts at understanding themost of studies to date have only considered one processes behind will require judicious choices ofoption to test for the shape of interspecific rela- both host and parasite or pathogen taxa, and at dif-tionships (but see Calvete et al.2004).Second, the ferent levels of spatio-temporal organization
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