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.Morand species, for example, presence of empty niches.Inet al.(2002) have provided a cogent explanation mainstream ecology (see Elton 1958), it has beenfor the generation of nested patterns in parasite hypothesized among other predictions that greatercommunities, thus reinforcing the importance of diversity should increase resistance to invasionsboth spatial and demographic stochasticity in para- because the levels of limiting resources are gener-site species distribution and composition.Fig.2.6 ally lower in more diverse habitats (Tilman et al.illustrates the way Morand and collaborators 1996; Knops et al.1999), thus giving rise to thesynthesize the hypothetical distributions of para- so-called diversity invasibility hypothesis.Asite species among local communities and the many decrease in host species diversity allows remaining EMERGENCE OF SPATI AL PARASI TOLOGY AND EPI DEMI OLOGY 41Small area size Large area sizeSpecific habitat types Wide habitat typesHigh isolation Low isolationLow probabilityof invasionHigh probabilityof extinctionFigure 2.6 Nested (versus non-nested) speciessubset patterns are strongly associated with aninterplay of specific characteristics of parasiteParasite species(or microbe) species themselves) and local(by rank)habitat conditions to sustain or not a givenparasite (or microbe) species population andits life-cycle associates, for example, vectors,High probabilityreservoirs.Parasite (or microbe) communityof invasionecology would benefit greatly by considering theLow probability Localitiesimportance of epidemiological patterns inof extinction (by rank)generating the processes observed.the component diseases, a theorem which is calledspecies to increase in abundance as a resultthe species composition disease hypothesis (seeof decreased competition for limiting ressources,Elton 1958).This hypothesis predicts that anya phenomenon called density compensation bychanges in parasite or microbe community compos-ecologists (see Begon et al.1996).As a result, theincrease in remaining host abundances should facil- ition within a host community should impact onthe severity and virulence of one, or a group of,itate the invasibility of parasite or microbe speciesdiseases, which then proliferate to the detriment(Anderson and May 1978; Burdon and Chilversof other parasite or microbe species.Both these1982; Antonovics et al.1995).Indeed, host living inhypotheses, that is, the diversity disease hypothesishigh densities may harbour a high diversity ofand the species composition disease hypothesis,parasite species (Morand et al.2000; Poulin andhave received some empirical support from theMorand 2000; Stanko et al.2002), which in turnplant disease literature (Knops et al.1999; Mitchelloften achieve high abundances (Arneberg et al.and Power 2003), but they have never been tested1998a).Not all available results agree with this,to our knowledge in the case of both animal orhowever (Stanko et al.2002).Hence, both hosthuman pathogens.Research on community ecologydiversity and disease invasibility are related to hostof parasite or microbe species, at the differentspecies abundances within communities (Burdonhierarchical scales discussed in the present paper,and Chilvers 1982; Mitchell et al.2002).As alreadyshould clearly benefit from developing both experi-stated, this hypothesis has received some support inmental designs and comparative studies to exploregeneral ecology (Tilman 1997), but further workneeds to be done in both parasitology and epidemi- in greater detail the potential linkages betweenspecies diversity and composition, invasibility, andology to determine whether higher parasite (orother life-history traits such as virulence.microbe) species diversity at different hierarchicalIn addition, theoretical studies on parasite (orscales, that is, within-host, within-population,microbe) invasibility rest upon epidemiologicaland within-metapopulation community dynamicsprinciples, in which the local abundance of hosts isprotects parasite or pathogen communities fromthe key determinant of whether a parasite can estab-invaders (but see Torchin et al.2003).Another prediction available from the general lit- lish into a naïve host population (Anderson and May1978; Burdon and Chilvers 1982; May and Andersonerature is that greater species diversity of microbes1978).Within the study of human infectiousor parasites should decrease the severity of each of 42 PARASI TI SM AND ECOSYSTEMSdiseases, recent investigations have clearly illus- depending on the exact causes of diversity loss.trated the importance of local community size This loss may depend on habitat fragmentation,for the maintenance and diffusion of, in particular, which should impact on rare host species livingchildhood diseases (see Grenfell and Harwood 1997; at low abundance, or may be due to landscapeRohani et al.1999; Dieckmann and Heesterbeek 2000; changes, which may influence the more adaptedGrenfell et al.2001).Unfortunately, very few studies host species, and thus will favour either the spreadhave gone a step further in demonstrating that of rare species or invasion by exotic specieshigher local abundance in hosts should yield higher (Daszak et al.2001).Unfortunately, we cannot easilyparasite or microbe species diversity (but see Poulin refer to parasitological or epidemiological studiesand Morand 2000).on animals or humans that have specifically testedFor macroparasites, and especially for contact- these different hypotheses.Further empirical stud-transmitted microparasites, both high parasite ies are thus needed, which should contribute to aabundance and high parasite species richness better understanding of how native host speciesshould be attained more easily at high host abund- and their associated parasite or pathogen speciesance or density.This is expected from epidemi- may prevent or limit the attacks of invaders (seeological theory (see Grenfell and Dobson 1995) Torchin et al.2003).and supported by empirical studies on the deter- In summary, there are, of course, other interest-minants of parasite species richness (see Poulin and ing parasitological or epidemiological patterns thatMorand 2000, and the present chapter)
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