Interspecific competition: definition and effects on species - Nonequilibrium Ecology - page 51

between species really necessary to permit coexistence? As pointed out by
Rosenzweig ( 1995 ), the original deterministic proposal by MacArthur
and Levins ( 1967 ), that two species, in order to survive, must differ to a
certain degree, is wrong. As already realized by May and MacArthur
( 1972 ), the limiting similarity in such a deterministic system is zero, i.e.,
even very similar competitors will not become extinct. Hubbell and
Foster ( 1986 , cited by Rosenzweig 1995 ) showed that extinction, even
in identical species, may take a very long time, and in populations of a few
thousand, extinction time may be as long as speciation time. Hence,
Gause's principle is wrong. This is not so in stochastic systems: extinction
may still take a long time, but a species too similar to the competing one
will finally be pushed over the rim by some ''accident'' (in Rosenzweig's
words, see also Crawley 1986 ). Also, the values of limiting similarity
change according to environmental conditions and properties of the
niche (Abrams 1983 ). Nevertheless, because systems are not strictly
deterministic, because they are stochastic to a certain degree, either
extinction or divergence due to competition is at least a possibility.
An early example of an explanation of differences between species as the
result of ''ecological character displacement'' (Grant 1975 ) is provided by
Grinnell ( 1904 ) , who postulated that more than one species can coexist in
the same habitat only by using different sorts of food and using different
ways of getting the food. Brown ( 1975 ), in his paper on the geographical
ecology of desert rodents, concludes that ''competition is a major force in
the structuring of rodent communities. Coexisting species subdivide food
resources by collecting seeds of different sizes and by foraging in different
microhabitats.'' Character displacement in body size is brought about by
selection of different seed sizes. When competitors are absent, species use a
much wider range of seed sizes than if they are present, and species that
have invaded isolated habitats occur in exceptionally high population
densities. One of the best examined examples of ecological character
displacement is that of Darwin's finches on the Galapagos Islands. Grant
and Schluter ( 1984 ) examined beak morphology of various species and
demonstrated that species co-occurring on the same island differ more
strongly in beak morphology than the same species occurring singly
on different islands. They concluded that interspecific competition was
responsible for the greater differences in sympatric species. Size differences
of the species was considered to correspond to size differences in food
particles. Studies of birds in New Guinea and elsewhere were also used
to support this hypothesis (Diamond 1973 , 1975 ;Cody 1974 ;discussion
in Roughgarden 1989 ). An example of supposed ecological character