Biology Reference
In-Depth Information
larger than most invertebrates higher up in the continuum, but they
inhabit large volumes of water, which probably explains the lack of
community structure. They, indeed, are the only taxon of those included
in the figure, that live in a three-dimensional niche, all the others, and this
includes the birds, live on a two-dimensional surface at least for breeding.
However, many fish are territorial and demersal: a re-examination of data
for fishes from a variety of habitats seems advisable. On the other hand,
trematode larvae are small, in absolute terms, but they are very large
relative to the gonads/digestive glands of molluscs, where they live.
Consequently, communities of larval trematodes in molluscs are highly
structured (see pp. 131-134). Similar considerations apply to some commu-
nities of endoparasites of vertebrates. Digenean trematodes and cestodes
in the digestive tract of some fish and birds, for example, are quite large
relative to their habitat, and interspecific competition has been well
documented in several cases. The particularly thorough studies of
Kennedy (
1992
) are an excellent example: acanthocephalans in eels
Anguilla anguilla, were convincingly shown to interact negatively. This
has led Holmes and Price (
1986
) to distinguish interactive and isolationist
parasite communities, the first with much, the second with little or no
evidence for competition. However, the analysis on spatial scaling laws (see
Chapter
4
, p. 76, and p. 125) has shown that at least some of the endo-
parasite communities, namely those in marine fish, are not saturated. This
indicates that occasional interspecific competition may well occur even if
species are not densely packed, but that interactions have been without
evolutionary significance. Indeed, it is not unlikely that weak interactions
will be found in all or most communities, if investigators persist and
studies are of sufficient depth and duration. Therefore, it is doubtful
that the distinction between interactive and isolationist communities is
of much use, as already tentatively suggested by Holmes and Price (
1986
).
Many important studies have used modelling to make predictions, for
example on the effects of habitat fragmentation or reductions in species
diversity caused by humans. The example of the Glanville fritillary has
been discussed above (pp. 20-22). Many of these models use a demo-
graphic approach and we have to ask whether they are compatible with
the conclusion that many systems are nonsaturated and not in equilibrium.
According to Levin (
1998
), who was one of the early principal promoters
of nonequilibrial approaches to community ecology, patterns at higher
levels of ecosystems arise from localized interactions and selection pro-
cesses at lower levels. His statement that ''tight linkages between members
of interacting species provide reliable and rapid feedbacks for individual
