Nonequilibrium in communities - Nonequilibrium Ecology - page 41

endoparasites of marine fishes. Parasites examined included Copepoda,
Isopoda, Branchiura, Monogenea, Turbellaria, Trematoda, Cestoda,
Hirudinea, Nematoda and Acanthocephala. Food (epidermis and blood for
ectoparasites; epidermis, blood and gut contents for endoparasites) for these
parasites is in unlimited supply and the only potentially limiting resource is
space (Rohde 1991 , 1994a ). All species inhabiting the same habitat can
thereforebeconsideredasbelongingtothesameguildandwereexamined
together. However, species of different large taxa (Platyhelminthes,
Arthropoda) were also examined separately. Some fish species from the
warm Pacific Ocean with large numbers of parasite species were selected,
because of their likely effects due to dense packing. Also examined were
parasites infecting a range of host species from various localities and latitudes
jointly, including endoparasites of 1808 fish belonging to 47 species,
justified since all fish species harbor similar parasites and a trend apparent
in a component community of parasites (i.e., parasites of all species infect-
ing one host population) should alsobeapparentorevenaugmented
in a compound community (i.e., all parasite communities within an eco-
system). Both size of parasites, defined as the maximum length width,
and volume, (biomass) defined as maximum length width depth, were
considered. In no case (with the exception of gill Arthropoda, which
showed a very weak effect) did the packing rules of Ritchie and Olff
apply. Examples are illustrated in Figure 2.8 . The effect in the gill
Arthropoda may be an artefact, due to the rarity of the very small males
of some copepods which may have been overlooked.
These negative results support the view that parasites of marine fish do not
live in saturated structured communities, but rather in assemblages not
significantly structured by interspecific competition. Recent studies support-
ing this view are by Mouillot et al.( 2003 ) who found a good fit of parasite
data with Tokeshi's Random Assortment model, and by Poulin et al.( 2003 ),
who demonstrated a linear relationship between parasite biovolume and
parasite diversity in parasite assemblages from 131 vertebrate host species,
suggesting an additive effect and nonsaturation (Figure 2.9 ) . The positive
results presented by Ritchie and Olff ( 1999 ) for Serengeti grazing mammals
and the North American savanna can be explained by the fact that they are
either vagile (mammals) or disperse well (savanna plants), and that both utilize
significant proportions of the resources for which different species compete,
plants in the case of the former and light and space in the latter; interspecific
competition is therefore expected (pp. 178-180).
Parasites, and this includes parasites of fish, represent a very large and
probably the largest component of the Earth's fauna and should therefore