Biology Reference
In-Depth Information
and in benign environments parasites, predators, and herbivores become
more effective, also reducing population densities. As a consequence,
coevolution between competitors is most likely in moderately harsh
environments.
(2) Nonequilibrium reduces importance of competition
In a wide variety of community types (avian, insect, parasite, fish) non-
equilibrium conditions were shown to reduce the effects of interspecific
competition, or competition may not exist at all (e.g., Dayton
1971
;
Wiens
1974
; Sale
1977
;Connell
1979
;Grime
1979
;Price
1980
). This
phenomenon is widespread and well supported by observations. Examples
are discussed in various chapters (e.g., pp. 34; 37; 105-108).
(3) Population density and competition
Andrewartha and Birch (
1954
) point out that the terms ''competition''
and ''density-dependent factors'' are often used as synonyms. They
quote Nicholson (
1933
), and Varley (
1947
) who writes: ''The control-
ling factors which keep a population in balance must be affected in their
severity of action by the population density on which they act.'' Note
the ''must''. Andrewartha and Birch, in their detailed account and
dissecting many examples, conclude that the exaggerated emphasis on
competition is fallacious. Most natural populations never become suffi-
ciently numerous to use a substantial proportion of the resources needed
by them. Density-dependent factors and competition therefore do not
become operative. They list three possibilities of how numbers of animals
in natural populations can become limited. They are: (1) shortage of
resources; (2) inaccessibility of resources; and (3) shortage of time during
whichanincreaseinreproductiverates is positive. Based on their very
great experience with various animal populations, they conclude that
the first is probably least and the last most important in nature.
Importantly, the authors point out that populations which are usually
studied are atypical, i.e., they arecommonandnotrarelikethevast
majority. An excellent example for this point are the studies of larval
trematodes in snails. The impression is given that most snail populations
are heavily infected with many parasite species. However, many (and
probably most) marine snail species have no or few parasites, and the
same refers to terrestrial and freshwater snails. Authors don't bother
with uninteresting, poorly infected host populations, they rather con-
centrate on the more interesting, richly infected populations (see
Chapter
8
).
