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the infracommunity level. This conclusion was based on the findings that
mixed (multi-species) infections were fewer than expected under the null-
hypothesis (trematodes are randomly and independently distributed), that
in the few multiple infections rediae of one species were observed to
feed on rediae, sporocysts, and cercariae of another in a hierarchical order,
and that in marked snails with known infections, parasite species at the
bottom of the hierarchy were replaced over time by species higher in the
hierarchy. The first point, on its own, would not be convincing, since
infections may be acquired in a very heterogeneous fashion and a null-
hypothesis can therefore not be clearly defined, but joint evidence
from all three points is convincing. However, some of the observed
interactions were in fact predatory rather than competitive in the strict
sense: rediae feeding on other larvae.
In spite of the clear evidence for interspecific competition (plus pre-
dation) at the infracommunity level, evidence from observations at the
component community level, that is, not within a host individual but in a
host population, did not support the view that interspecific competition
has significant effects. This conclusion is based on the findings that species
richness and diversity of trematodes increased with snail size, i.e., com-
plete dominance by a few species did not develop and species accumu-
lated with time, and that neither numbers of uninfected hosts nor
variation in host size was correlated with parasite diversity; a greater
proportion of older than younger snails were infected. Sousa (
1990
)
stressed that the findings did not exclude the possibility that there were
some interspecific interactions, but any reductions due to competition
were ''more than compensated for by increases in both the number and
equitability of other parasite species in older host populations.'' The
findings on snail-trematode systems are of particular interest because of
the large size of the parasites relative to the snails. On these grounds alone,
interspecific effects could be expected (for a detailed discussion of trema-
tode communities in snails see pp. 131-134). Some parasites of birds are
supposed to have saturated component communities but resources at the
infracommunity level are underutilized (Bush
1990
).
Competition in mature and young communities
Holmes (
1973
) held the view that helminth communities are mature and
to a large degree structured by biotic interactions, i.e., competition,
apparently in the past, because most species exhibit ''selective site segre-
gation of niches'' (utilization of different sites that prevents interspecific
