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were in fact predatory rather than competitive: rediae feeding on larvae of
other trematode species). Importantly, spatial and temporal heterogeneity
and differential prevalences among host size classes, in most but not all
cases, did not reduce but increased the likelihood of multiple infections
(Figure 8.6 ) . This result is not unexpected because vertebrates that are final
hosts to different trematode species are often attracted to the same sites.
The few exceptions are a consequence of hosts being attracted to different
sites. In spite of these seemingly clear results, the authors caution ''Despite
the large number of studies encompassed in our analysis, we feel that the
definitive study of trematode communities has yet to be done. Future
analyses of the effects of recruitment and post-recruitment contributions
to community structure should include evaluation of the impact of hetero-
geneity in snail densities at different sites'' by weighting samples according
to density. Such an evaluation was performed by Lafferty et al.( 1994 )in
their analysis of trematode communities in Cerithidia californica,aCalifornian
salt marsh snail. They demonstrated that interspecific competition is
indeed the most important structuring force in trematode communities
of this snail, and again, spatial heterogeneity was found to increase the
likelihood of co-occurrence of trematode species. It is also important to
note that interactions occur in a hierarchical order, i.e., certain trematode
species are better survivors in multi-species infections than others
(e.g., Sousa 1992 , 1993 ; Kuris and Lafferty 1994 ), and that interactions
may be positive, i.e., infection with one trematode species may facilitate
infection with another. An extreme case was described by Walker ( 1979 ):
the schistosome Austrobilharzia terrigalensis could only infect snails that
were already infected with another trematode species.
Importantly, in spite of the distinct interspecific effects at the level of rich
infracommunities, that is, within host individuals, discussed above, effects
at the level of host populations appear to be minimal. Sousa ( 1990 ) based
this conclusion on the findings that, in the host-parasite system studied by
him, species richness and diversity of trematodes increased with snail size,
i.e., complete dominance by a few species did not develop, and parasite
species accumulated with time. Furthermore, neither numbers of unin-
fected hosts nor variation in host size was correlated with parasite diversity.
Any reductions as the result of competition were ''more than compensated
for by increases in both the number and equitability of other parasite species
in older host populations.'' Overall, no evidence for equilibrium conditions
was found at the levels of infra- and component communities.
Rich trematode faunas in snails are common; for example, Kube et al.
( 2002 ) found 10 species in the mudsnail Hydrobia ventrosa in coastal lagoons
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