PATTERN AND PROCESS IN COMPETITION
33
has chosen to use the term “exclusion” as though it were synonymous
with competitive exclusion. It is obviously necessary to distinguish
between observed patterns of mutual exclusion in spatial distributions
and the cause of the distribution, which may or may not be competition.
The correct alternative is not usually evident in the distribution pattern
alone, but requires supporting evidence which will reveal the relationship between the fundamental niches of potential competitors.
Nevertheless, there is also a growing body of evidence that establishes
competition as an active force in species relationships in nature. When
ecological displacement occurs in the realized niches of sympatric
species that occupy broader niches outside the zone of sympatry, competition is clearly implied. If these observations are supported by
accurate descriptions of the intersection of their fundamental niches
with respect to critical factors which control their survival and habitat
occupancy, a stronger case for competition is established. This is especially true when there is a strong element of interference in the
interaction between the two species, as later discussions will show. It
would be more convincing to be able to show, with appropriate controls,
that the experimental addition or removal of a species affects the
realized niche distribution of another. This has seldom been attempted,
in spite of the potential value of such experiments.
V. CONDITIONS OF COEXISTENCE
It was stated earlier that the outcome of competition according to
equations (1) and (2) depends on the inequalities a > K , / K , and
fl > K,/K,. By reversing the aigns of the inequalities one by one it
can be shown that there are four possible outcomes of competition in
this model. Gause and Witt (1935) analysed the properties of this
equation system to consider the essential types of competition that
might exist between species. Two of the cases included in their analysis
are of particular interest:
Case 1: a > K J K , and j? > K 2 / K l
Case 2 : a < K , / K , and j? < K , / K ,
(4)
( 5 )
These cases are illustrated diagramatically in Fig. 6. In the diagrams,
neither species can increase above its saturation line K,/a, K , or K,,
RI/fl and below its saturation line each species will tend to increase. I n
the first case either N , or N , will be the sole survivor, depending on
the initial concentrations of each species. I n the second case competition
will lead to a stable, mixed-species population, regardless of the initial
concentration of each species. An important feature of the second case
is that there is homeostatic regulation of the composition of the stable
33
has chosen to use the term “exclusion” as though it were synonymous
with competitive exclusion. It is obviously necessary to distinguish
between observed patterns of mutual exclusion in spatial distributions
and the cause of the distribution, which may or may not be competition.
The correct alternative is not usually evident in the distribution pattern
alone, but requires supporting evidence which will reveal the relationship between the fundamental niches of potential competitors.
Nevertheless, there is also a growing body of evidence that establishes
competition as an active force in species relationships in nature. When
ecological displacement occurs in the realized niches of sympatric
species that occupy broader niches outside the zone of sympatry, competition is clearly implied. If these observations are supported by
accurate descriptions of the intersection of their fundamental niches
with respect to critical factors which control their survival and habitat
occupancy, a stronger case for competition is established. This is especially true when there is a strong element of interference in the
interaction between the two species, as later discussions will show. It
would be more convincing to be able to show, with appropriate controls,
that the experimental addition or removal of a species affects the
realized niche distribution of another. This has seldom been attempted,
in spite of the potential value of such experiments.
V. CONDITIONS OF COEXISTENCE
It was stated earlier that the outcome of competition according to
equations (1) and (2) depends on the inequalities a > K , / K , and
fl > K,/K,. By reversing the aigns of the inequalities one by one it
can be shown that there are four possible outcomes of competition in
this model. Gause and Witt (1935) analysed the properties of this
equation system to consider the essential types of competition that
might exist between species. Two of the cases included in their analysis
are of particular interest:
Case 1: a > K J K , and j? > K 2 / K l
Case 2 : a < K , / K , and j? < K , / K ,
(4)
( 5 )
These cases are illustrated diagramatically in Fig. 6. In the diagrams,
neither species can increase above its saturation line K,/a, K , or K,,
RI/fl and below its saturation line each species will tend to increase. I n
the first case either N , or N , will be the sole survivor, depending on
the initial concentrations of each species. I n the second case competition
will lead to a stable, mixed-species population, regardless of the initial
concentration of each species. An important feature of the second case
is that there is homeostatic regulation of the composition of the stable