Some detailed examples at the community level - Nonequilibrium Ecology - page 129

regional species richness, given as evidence for nonsaturation of habitats
(Figure 8.5 ) . Similar results were obtained by Hawkins and Compton
( 1992 ) for African fig wasp communities, both in temperate and tropical
habitats. Wasps examined comprised pollinating and non-pollinating
gallers, as well as their parasitoids. No evidence was found for saturation
in either gallers or parasitoids. For gallers, local species richness was more
or less the same at all latitudes (68Nto348S), whereas local parasitoid
community richness dropped slightly towards the tropics, suggestive of
less saturation in the tropics than at high latitudes. In all of these cases,
interspecific competition may still occur, but is too weak to result in
saturation, supporting the view that herbivore assemblages in general may
experience only weak interactions (Lawton and Strong 1981 ). The find-
ing on parasitoids is of particular importance, since parasitoids comprise
about 20-25% of all insect species (Godfray 1994 ).
In conclusion, interspecific competition for food is unlikely to be
important for these apparently unsaturated communities. Although
there is much predictability in the rank order of abundances in insect
communities of bracken, predictability decreases over time.
Larval trematodes in snails: evidence for interspecific
competition (and predation) in infracommunities,
and for nonequilibrium conditions
Snails are intermediate hosts for a large variety of trematode species.
Because of their small size and the possibility of dissecting large numbers
with relative ease, many studies have dealt with the community structure of
trematode communities of a range of snail species. Here the groundbreak-
ing studies of Kuris ( 1990 ); Kuris and Lafferty ( 1994 ); Lafferty et al.( 1994 );
Sousa ( 1992 , 1993 ), among others, must be mentioned. Curtis and
Hubbard ( 1993 ) examined infections of the 379 littoral snails of the species
Ilyanassa obsoleta on Cape Henlopen, Delaware, with 5 species of trema-
tode larvae, and found 22 trematode combinations in individual snails;
negative effects due to interspecific combinations could not be statistically
demonstrated. Kuris and Lafferty ( 1994 ) assessed data from 62 studies,
including the one by Curtis and Hubbard, in which 62 942 snails out
of a total of 296 180 host snails examined were infected. An average of
24%of the snails had multiple infections. Statistical tests showed that of the
14 333 expected double infections, only 4346 were observed. The authors
estimated that 13% (10% average) of the trematode infections were lost as
the result of interspecific competition (although some of the interactions