Interspecific competition: effects in communities and conclusion - Nonequilibrium Ecology - page 76

competition), and only few exhibit interactive site segregation, where
sites are reduced in size when competitors are present. Price ( 1980 ) holds
a contradicting view, that parasite communites are, in fact, not mature
and not significantly shaped by past competition. Competition, where it
occurs, is intermittent. However, age does not necessarily imply structure
by competition leading to selective site segregation. Platyhelminthes are a
very ancient group and they contribute a significant proportion of species
to parasite and other communities, but evidence for competition now or
in the past is poor (see Chapter 10 ).
Competition in saturated but less so in unsaturated communities
Rohde ( 1980a ) generalized his findings on ectoparasites of marine fish to
include groups of organisms other than parasites, and postulated that not
only parasites, many if not most of which have small population sizes, but
free-living species that are also rare, live under conditions where not
interspecific competition but selection to enhance mating opportunities
and reinforcement of reproductive barriers are important. However, for
large animals with great vagility, particularly birds and mammals, and free-
living insects with great vagility and large populations that can rapidly
spread into ''vacant niches,'' competition is probably important. He further
hypothesized that, for the vast majority of species, competition does not
reduce niches in the course of evolution, but there will be a gradual filling
of niche space, partly by sympatric speciation, and by separation of over-
lapping niches in order to avoid hybridization. Niches do not expand into
empty niche space, because the reproductive capacity of most species is too
low to guarantee mating and thus cross fertilization in suboptimal niches.
The hypothesis was tested by Gotelli and Rohde ( 2002 ) using null-model
analyses. They found that communities of large animals (birds, mammals)
are highly structured, whereas communities of herps (amphibians and
reptiles) and fish parasites are not. For a fuller discussion see Chapter 11 .
General aspects and conclusion
Competition does not decide the distribution and abundance
of organisms
White ( 1993 ) emphasizes the importance of seeing that selection is a
negative process, i.e., the unfit are selected against. Nitrogen is the most
limiting chemical for plants, and competition is a consequence of and not