PATTERN AND PROCESS IN COMPETITION
65
niches is in the region of the values x, and yl, and this space will be
occupied by the strongest individuals of species S,, with pressure on
other individuals of this species forcing them toward the less desirable
regions of N,. Although S , may be forced by competitive exclusion to
live in the niche space N, - N,, there will be pressure from within its
own population to expand into the preferred space of N, as well as
toward the region of lowest survival at the periphery of N, (Xg, y6).
Presumably, according to this hypothesis, the weakest individuals of
8, will be most directly in competition with the strongest individuals
of S,. Depending on the strength and effectiveness of interference from
S,, the region of immediate exclusion may exist as a “tension zone”
where the niche distributions are not clearly delineated. This may
explain distributions of Planaria naontenegrina and P . gonocephala when
they occur together in the same stream, as compared with the temperature tolerances they express when they live in separate streams. Rather
than a clear separation of niches at the extreme limit of tolerance of
P . motenegrina (16 to 17”C), population pressure from P . gonocephula
shifts the point of demarkation in their realized niches to approximately
13 to 14°C. This problem, while vaguely defined at present, may have
considerable bearing on the evolution of ecological differentiation and
of ecological and ethological isolating mechanisms.
VII. SPECIES DIVERSITY
Klopfer (1962) suggests the following ways in which it is hypothetically possible to increase the number of species in a fixed area:
(1) by increasing the amount of time during which speciation can occur,
(2) by increasing the “space” within which niches can be provided for
different species, and (3) by reducing the size of niche required by each
species. Niche size in this sense is the measurable range of conditions
which determine the presence or absence of a species. Klopfer and
MacArthur (1960) conclude that the increased faunal diversity of the
tropics is a result of the time available for speciation and smaller niche
sizes. One might also add that the other factor (2) of space is also
important, in that the structural diversity of tropical habitats provides
a greater amount of niche space in a given area. Klopfer (1962) carries
this line of reasoning further to state that a reduction in the volume of
the niche of a species also implies that the behavior of the animal has
become stereotyped, as reducing the niche size also reduces the range
of objects in the environment (or environmental variables) to which
the animal responds by feeding, nesting, or taking shelter. Hence, if
niche size is relatively large there is a wider range of behavior and a
given area will support fewer of such species. Support for this argument
is found in the relative abundance of passerine birds in temperate
65
niches is in the region of the values x, and yl, and this space will be
occupied by the strongest individuals of species S,, with pressure on
other individuals of this species forcing them toward the less desirable
regions of N,. Although S , may be forced by competitive exclusion to
live in the niche space N, - N,, there will be pressure from within its
own population to expand into the preferred space of N, as well as
toward the region of lowest survival at the periphery of N, (Xg, y6).
Presumably, according to this hypothesis, the weakest individuals of
8, will be most directly in competition with the strongest individuals
of S,. Depending on the strength and effectiveness of interference from
S,, the region of immediate exclusion may exist as a “tension zone”
where the niche distributions are not clearly delineated. This may
explain distributions of Planaria naontenegrina and P . gonocephala when
they occur together in the same stream, as compared with the temperature tolerances they express when they live in separate streams. Rather
than a clear separation of niches at the extreme limit of tolerance of
P . motenegrina (16 to 17”C), population pressure from P . gonocephula
shifts the point of demarkation in their realized niches to approximately
13 to 14°C. This problem, while vaguely defined at present, may have
considerable bearing on the evolution of ecological differentiation and
of ecological and ethological isolating mechanisms.
VII. SPECIES DIVERSITY
Klopfer (1962) suggests the following ways in which it is hypothetically possible to increase the number of species in a fixed area:
(1) by increasing the amount of time during which speciation can occur,
(2) by increasing the “space” within which niches can be provided for
different species, and (3) by reducing the size of niche required by each
species. Niche size in this sense is the measurable range of conditions
which determine the presence or absence of a species. Klopfer and
MacArthur (1960) conclude that the increased faunal diversity of the
tropics is a result of the time available for speciation and smaller niche
sizes. One might also add that the other factor (2) of space is also
important, in that the structural diversity of tropical habitats provides
a greater amount of niche space in a given area. Klopfer (1962) carries
this line of reasoning further to state that a reduction in the volume of
the niche of a species also implies that the behavior of the animal has
become stereotyped, as reducing the niche size also reduces the range
of objects in the environment (or environmental variables) to which
the animal responds by feeding, nesting, or taking shelter. Hence, if
niche size is relatively large there is a wider range of behavior and a
given area will support fewer of such species. Support for this argument
is found in the relative abundance of passerine birds in temperate