Biology Reference
In-Depth Information
three parasite communities of fish from the Chilean coast. This means that
interactions between species are not important, and that these parasites
probably live in unsaturated communities (Mouillot et al.
2003
). The
packing rules based on fractal geometry are also based on the assumption
that species compete for resources in limited supply. They apply to herbi-
vorous mammals competing for food and to savanna plants competing for
light and space, but they do not apply to all parasites (see pp. 41-46, 76).
Lawton (
1984a
), in his study of community organization, refers to a
number of studies that suggest that certain ecological patterns may be
entirely or partly determined by interspecific competition, but he points
out that each of these patterns can also be explained by other hypotheses.
A search for competition should follow demonstration of interspecific
resource limitation, but such limitation has, for example, not been found
in insects inhabiting bracken, which are rare and, therefore, unlikely to
compete for resources. But Lawton admits that this does not exclude
evolution to minimize interspecific competition in the past. Wiens
(
1984
) points out, with regard to ''ghost of competition past,'' that such
hypotheses are not testable, and that demonstration of patterns of resource
partitioning does not say anything about the processes leading to such
partitioning (see also Connell
1980
and pp. 85-89). Both interactive and
non-interactive communities may contain species that partition resources.
He cites examples showing greatest resource overlap at times of scarcity,
others at times of abundance (p. 423). ''The introduction of even modest
levels of spatial heterogeneity into resource systems potentially permits an
almost unlimited number of species to coexist at equilibrium on a restricted set
of resources''; ''preconceptions about the systems must be recognized as such,
and not permitted to bias the research design or interpretation of results.''
Price (
1984
) emphasizes that the resource base of a community has to
be clarified, before community phenomena can be examined. He cate-
gorizes resource types as follows: rapidly increasing, pulsing or ephemeral,
steadily renewed, constant, or rapidly decreasing, and gives examples:
e.g., endoparasites use steadily renewed resources, constant resources are
physical ones, such as surfaces on which intertidal organisms can settle,
etc. Population responses to resource availability depend on generation
times, and permit prediction on whether resources will be in short supply
and whether competition will occur. Spatial distribution of resources
(whether uniform or patchy) will also permit predictions. However,
since most resources are patchy, and because patchy resources have a low
probability of colonization, Price assumes that interspecific competition is
not likely. Furthermore, disturbances will disrupt interactions between
