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(Morand et al.
2002
), that there is reduced interspecific relative to intra-
specific aggregation (Morand et al.
1999
), and that null-model analysis found
little evidence for nonrandom co-occurrence patterns (Gotelli and Rohde
of ectoparasites of marine fishes have been described, and it is highly
unlikely that many exist: further evidence for the existence of many vacant
niches and the lack of competition for these niches (e.g., Rohde
1989
).
The findings of these studies have been corroborated by several other
authors, using marine and also freshwater fish. For example, the recent study
fish Siganus sutor did not change with increasing abundance of other species
present. Simkova et al.(
2000
,
2001a
,
b
,
c
) have made detailed studies of nine
species of the monogenean Dactylogyrus on the freshwater fish Rutilus rutilus.
They found that assemblages of the monogeneans at the local level (among
host populations) and at the level of seasons (among host populations within
localities) were nested, but that nestedness was uncommon at the level of
hosts (infracommunities of parasites). The authors suggest that nestedness
may arise from a variety of causes and is not necessarily due to competition.
Monogeneans have hard copulatory and attachment sclerites. Simkova et al.
(
2002
) measured them in detail, and the attachment sites on the gills for each
individual were determined. Morphometric distances of the attachment
organs and the copulatory sclerites, and Levin's niche size and Renkonen's
niche overlap indices were calculated. The results supported the view that
reinforcement of reproductive barriers is responsible for niche segregation,
because monogenean species sharing the same niche differ more strongly in
the structure of copulatory sclerites than species that are spatially segregated.
Furthermore, species with overlapping niches have similar attachment
organs, which suggests that morphologically similar species have similar
ecological requirements and that interspecific competition is of minor
importance, with little impact on the evolution of differences between
attachment organs.
In conclusion, evidence is convincing that ectoparasites of fish live in
largely unsaturated habitats with many vacant niches. Communities are
not significantly structured by interspecific competition.
Insects on bracken, and wasps: type I communities
with little evidence for interspecific competition
Among the most thoroughly studied insect communities are insects of the
bracken fern (Pteridium aquilinem). Lawton and collaborators have studied