Biology Reference
In-Depth Information
a cause of its limitation. In other words, ''competition does not decide the
distribution and abundance of organisms. That is decided by the inad-
equacy of the environment.'' There is growing evidence that interspecific
competition, although it occurs, and in contrast to intraspecific competi-
tion, is relatively uncommon, and of debatable significance for evolution.
He gives many examples of the non-existence of competition between
coexisting species, explained by the fact that their environments are
different, i.e., they use different niches. He discusses three examples
from the literature in more detail (for references see White
1993
), that
of the Australian smoky mouse Pseudomys fumeus and three other small
mammals seemingly sharing the same habitat, that of four species of
ground finches on the Galapagos Islands, and that of a gall-forming
aphid and a leaf-eating beetle. In each case, species seemed to share the
same habitat but closer examination revealed differences in microhabitat
and/or food use. Coexisting species were not excluded by competition
from particular parts of the habitat/food resources, but differed because of
the distribution and abundance of their respective foods. Emphasis on
competition deflects attention from what is ecologically significant for a
species. White concludes that there can be no doubt about the occurrence
of interspecific competition, because it ''is a real and potent cause of the
direction in which many organisms evolve'' but it is not ''necessary.'' The
inadequate environment performs the selecting out of unfit organisms,
even if competing species are not present. This implies nonsaturation of
habitats, empty niches, although this is not explicitly stated by White.
Concerning the existence of self-regulating mechanisms as, for example,
brought about by social behavior (e.g., Wynne-Edwards
1962
), or by
changes in the proportion of ''inferior'' and ''superior'' genotypes in a
population, White maintains that they do not exist.
The occurrence and significance of interspecific competition:
a conclusion
The discussions in this chapter and in Chapter
3
have shown that much of
the evidence for interspecific competition is faulty. A major problem in
demonstrating the effects of interspecific competition is the difficulty in
formulating valid null-hypotheses. Nevertheless, interspecific competi-
tion is likely to occur in certain taxa and under certain conditions (see
Chapter
11
). But even where it occurs, the outcome of competition may
be largely unpredictable (see pp. 65-67). The uncertain outcome of
interspecific competition due to chaos, environmental stochasticity or
