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in the southern Baltic Sea, and many other examples are listed in Kuris and
Lafferty ( 1994 ). Nevertheless, these findings should not create the impres-
sion that snails are always filled to such a degree with parasites that multi-
species infections and, as a result, interspecific competition are common.
For example, the two most common snail species, Cerithium (Clypeomorus)
moniliferum and Planaxis sulcatus on beachrock at Heron Island, Great
Barrier Reef, both had a great diversity of trematodes (Cannon 1979 ;
Rohde 1981b ), but the less common predatory snail Peristernia australiensis
whose habitat overlapped with those of these two snails, had no digenean
trematodes and only one species of aspidogastrean trematode. Many other
rare species of molluscs were never infected at that locality, but results of
surveys were not published because snail numbers were too small and
negative results are considered to be less interesting.
Farrell ( 1998 ) examined Lymnaea tormentosa, the intermediate host of
Fasciola hepatica in northern New South Wales, Australia. Twelve proper-
ties, each with a history of Fasciola infections in sheep/cattle, were
targeted. Population densities of snails were low, but altogether almost
400 snails were examined. None was found to be infected, indicating that
even low prevalences of parasites in an intermediate host can keep an
infection going, and that great parasite diversities in molluscs are by no
means general. Koch ( 2003 ) made a survey of all aquatic snails in the same
area, the New England tablelands of northern New South Wales. She
extensively sampled all nine snail species at a great number of sites and
found only two that were infected. One species, Gabbia vertiginosa, had
three, and another, Glyptophysa sp., had one species of larval trematode.
Prevalence of infection was low. The tablelands have many ponds and
creeks, which are suitable habitats for molluscs, and there is a rich fauna of
birds, reptiles, amphibians, and mammals that are potential definitive
hosts. However, disturbances due to droughts, and also man-induced
disturbances such as pollution due to herbicides and grazing cattle and
sheep, frequently reduce population numbers of intermediate and defi-
nitive hosts, and are the likely cause of nonequilibrium conditions result-
ing in low diversity and prevalence of trematode infections.
We conclude that in rich trematode communities interspecific com-
petition is important at the level of infra- but not of component com-
munities. Evidence does not support the view that equilibrium
conditions are of any significance. In poor communities (which are
common but have been relatively little studied), large and unpredictable
environmental conditions (such as rainfall) prevent communities from
becoming so rich that interspecific competition can become important.
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