Some detailed examples at the community level - Nonequilibrium Ecology - page 123

Figure 8.3. Male and female copulatory sclerites of three species of Lamellodiscus
inhabiting the same parts of the gills of Acanthopagrus australis (see Figure 8.2 ).
From Roubal ( 1979 ), reprinted with permission of CSIRO Publishing and the
author.
share the same niche, whereas congeners with identical copulatory organs
are always completely or almost completely segregated into different niches,
strongly suggesting that reinforcement of reproductive barriers and not
interspecific competition is responsible for niche segregation (Rohde and
Hobbs 1986 )(Figures 5.2 , 8.2 , 8.3 ) . Monogenea use blood and mucus/
epithelial cells as food, which are fast replaced and not in limited supply as
long as the fish is alive; the only potentially limiting resource is space for
attachment. Nevertheless, different species of Monogenea, some of them
congeneric, on the gills of the same host species differ in the size and shape of
their feeding organs such as pharynx and oral suckers (Figure 3.1 ) . This
suggests that differences in feeding organs may be fortuitous (e.g., Rohde
1991 ) and not due to competition for different food resources, as frequently
suggested for other animal taxa by various authors. Nevertheless, the possi-
bility cannot be excluded that differences in feeding organs of monogeneans
have an adaptive value: they may permit extraction of food (blood) from
different parts of the gills.
Importantly, metazoan ectoparasites (and endoparasites) of fish do not
conform to the packing rules of Ritchie and Olff ( 1999 ) (Figure 2.8 ) ,
strong evidence that ectoparasite communities are not densely packed
and that competition for limiting resources has not been important in
evolution. Further evidence against an important role for interspecific
competition in ectoparasites of fish is that positive associations are much
more common than negative ones (e.g., Rohde et al. 1994 , 1995 ), that
nestedness is uncommon (Worthen and Rohde 1996 ; Rohde et al. 1998 ),
and - where it occurs - may be the result of epidemiological processes