[ Pobierz całość w formacie PDF ]
.Throughout, however, there isivity.Some models discussed below suggest that assumed to be an implicit link to ecosystem function,spatial dynamics can characteristically produce sys- via impacts of pathogens on host abundance andtems with lower levels of virulence (viz., lower d ).stability.Moreover, even if this link is not of directIf so, then the total production of the host popu- interest, ecosystem context (e.g.habitat productivity,lation in the ecosystem context will be reduced.This patterns of spatial connectivity) can be of greatmay seem counterintuitive.The reason for this is importance in determining the population andthat with lower virulence, the host population will evolutionary dynamics of host parasite systems.equilibrate with fewer healthy hosts, and moreinfected hosts.In other words, with lower virulence5.3 Spatial variability in empiricalthe host carries a heavier load of parasites.We havepatterns of parasite distribution withinassumed that infected individuals do not reproduce,ecosystemsand that the parasite is the sole factor regulatinghost numbers; hence, this decrease in virulence can Before reviewing models of spatial dynamics inshift individuals from productive to nonproductive host parasite systems and their implications for thestates, and so depress host population productivity.understanding of epidemiology and evolution, weAlso, somewhat counterintuitively, an increase in present some empirical patterns that stress the rolethe recovery rate for individuals at the level of the of space at various levels within ecosystems andpopulation translates into an increase in total death highlight a series of factors that have been con-rate (for all individuals).Any ecosystem factors that sidered or need to be considered in theoreticalmight influence recovery rates (e.g.the presence of studies.bioaccumulated toxins) could thus indirectly alterpopulation productivity and flux rates to other5.3.1 Geographical distribution of parasitesecosystem compartments.species: availability of hosts and opportunityFinally (and to return to the spatial theme of thisfor transmissionchapter), assume that the above model applies ineach of a number of distinct habitats, which each Parasites need their host(s) to complete their lifereach their own respective demographic equilibria.cycle, either as an important source of nutrients (for THE SPATI AL DI MENSI ON 71example, for many ectoparasites like fleas, (depending on life cycle complexity) were comparedmosquitoes, or ticks parasitizing vertebrates) or as among these regions, statistically controlling fora habitat to live and reproduce (for example, for environmental exposure.The authors found 14some helminthes and microparasites such as bacte- species of digeneans, of which 13 have marine birdsria and viruses).The distribution of hosts in the as final hosts.The number of species per samplingenvironment will thus condition the distribution of station increased westwards, and was higher on thetheir parasites.This constraint is especially strong Norwegian coast than on the Russian coast.The fre-as most parasites are specialized to a limited number quency of occurrence of digeneans with more thanof hosts, and also because some parasite life cycles one intermediate host increased westwards, makingare complex and involve series of hosts, with some up a larger proportion of the digeneans amongplaying the role of vectors or of intermediate hosts infected snails.The prevalence of different species(Combes 2001).Before considering the factors showed the same pattern, and significantly moreaffecting the spatial variability in the distribution of snails of both species were infected with digeneansparasites within a given host population, a first step with complicated life cycles in the western regions.is thus to see how heterogeneity in the spatial The authors concluded that the causes of changingdistribution of parasite species within ecosystems species composition between regions are probablyrelates to the spatial distribution of their hosts.(1) the harsh climate in the eastern part of the studyAs most species are parasitized by several para- area reducing the probability of successful trans-sites, most of which are specialized to a given host mission of digeneans with complicated life cycles;species, the diversity of parasite fauna is spatially and (2) the distribution of different final hostsconstrained, within and among ecosystems, by the (Galatkionov and Bustnes 1999).diversity and ecology of their component species.The combined effect of spatial variability in hostThe spatial distribution of parasites is also affected availability and abiotic conditions on levels of para-by the opportunities for completing their cycle site infestation has also been addressed for otherwhich can be prevented by abiotic conditions out- ecosystems.Ecosystems at tropical latitudes areside their hosts.An example of this involves arctic well known for harbouring much higher number ofecosystems (see also Chapter 6) where an important animal and plant species than at higher latitudescomponent of the parasite fauna of seabirds are the (e.g.Rosenzweig 1995), and these areas are thusflukes (Digenea), and where a detailed study of expected to harbour more parasite species.The pic-such parasites compared their distribution between ture is not that simple though.For instance, despitetwo intermediate host species and among spatial some evidence of higher parasite richness in marinelocations (Galatkionov and Bustnes 1999).Different fish ectoparasites (Rohde and Heap 1998), fieldspecies of digeneans have life cycles which may studies conducted on communities of endopara-consist of one intermediate host and no free-living sites of freshwater fish as a function of latitude havelarval stages, two intermediate hosts and one free- reported lower richness in host species living inliving stage, or two intermediate hosts and two tropical areas than at higher latitudes (Choudhuryfree-living larval stages.The study examined the and Dick 2000)
[ Pobierz całość w formacie PDF ]