38
RICHARD 9. MILLER
was seen to have an insignificant mean square, so that the values of a
and fl may be considered equal.
A considerable amount of ambiguity exists in interpretations of the
results of interspecies competition. It is necessary to distinguish between
two considerations: (1) maximum exploitation of the available resources
by one species, or (2) equitable utilization of the resources and possible
coexistence in a mixed species system. As most of the emphasis in
competition experiments has been on the extent to which a population
N , reduces the resources available to its competitor N , , there has been
a tendency to ignore the far more interesting fact that the resources
may be shared in such a way as to support two species. Terms such as
“degree of interference” or “intensity of competition” may be completely misleading in this respect (cf. Merrell, 1951; Moore, 1952;
Sokoloff, 1955). It is usually assumed that when the ecological requirements of two species are similar or nearly identical, the “intensity of
competition” will be high and coexistence impossible; when their
requirements are dissimilar they will compete less with each other and,
therefore, may coexist. As noted earlier, this was assumed by Gause
and Witt (1935) in their case for a stable equilibrium in mixed populations. The relationship between the sibling species Drosophila persimilis and D. pseudoobscura is remarkably like that between D.
melanogaster and D. simulans. Their ecological requirements are almost
identical and competitive superiority is only shown at extremely high
densities. Sokoloff (1955) concluded that when these two species comPete with each other there is intense “interference”, even though adult
survival is nearly the same for both species. When they compete with
D. miranda, a more distantly related species, interference with the
stronger competitor is not so great, even though the weaker competitor
may be eliminated entirely. These results pose an apparent contradiction
to the Gause principle, in that coexistence is usually more successful
between closely related species than between more distant relatives.
The expression ( K , - N , - aN,) merely implies that two species
which compete with each other cannot each survive at the same density
levels that are possible when only one species is present. Obviously N ,
cannot attain its maximum density K , when it shares the resources
with N,. This refers to the exploitation of the total resources by two
species, but does not exclude the possibility of coexistence, which
depends on the relative values of cc and 8. Analysis of the results of the
experiments with D. melanogaster and D. simulans showed that the
values of K , and K , were not significantly different and we may therefore assume that K , = K,. It was also shown that a = 1 and fl = 1
and a and fl were not significantly different, and we may conclude that
the competitive interactions between species were the same as those
RICHARD 9. MILLER
was seen to have an insignificant mean square, so that the values of a
and fl may be considered equal.
A considerable amount of ambiguity exists in interpretations of the
results of interspecies competition. It is necessary to distinguish between
two considerations: (1) maximum exploitation of the available resources
by one species, or (2) equitable utilization of the resources and possible
coexistence in a mixed species system. As most of the emphasis in
competition experiments has been on the extent to which a population
N , reduces the resources available to its competitor N , , there has been
a tendency to ignore the far more interesting fact that the resources
may be shared in such a way as to support two species. Terms such as
“degree of interference” or “intensity of competition” may be completely misleading in this respect (cf. Merrell, 1951; Moore, 1952;
Sokoloff, 1955). It is usually assumed that when the ecological requirements of two species are similar or nearly identical, the “intensity of
competition” will be high and coexistence impossible; when their
requirements are dissimilar they will compete less with each other and,
therefore, may coexist. As noted earlier, this was assumed by Gause
and Witt (1935) in their case for a stable equilibrium in mixed populations. The relationship between the sibling species Drosophila persimilis and D. pseudoobscura is remarkably like that between D.
melanogaster and D. simulans. Their ecological requirements are almost
identical and competitive superiority is only shown at extremely high
densities. Sokoloff (1955) concluded that when these two species comPete with each other there is intense “interference”, even though adult
survival is nearly the same for both species. When they compete with
D. miranda, a more distantly related species, interference with the
stronger competitor is not so great, even though the weaker competitor
may be eliminated entirely. These results pose an apparent contradiction
to the Gause principle, in that coexistence is usually more successful
between closely related species than between more distant relatives.
The expression ( K , - N , - aN,) merely implies that two species
which compete with each other cannot each survive at the same density
levels that are possible when only one species is present. Obviously N ,
cannot attain its maximum density K , when it shares the resources
with N,. This refers to the exploitation of the total resources by two
species, but does not exclude the possibility of coexistence, which
depends on the relative values of cc and 8. Analysis of the results of the
experiments with D. melanogaster and D. simulans showed that the
values of K , and K , were not significantly different and we may therefore assume that K , = K,. It was also shown that a = 1 and fl = 1
and a and fl were not significantly different, and we may conclude that
the competitive interactions between species were the same as those