PATTERN AND PROCESS IN COMPETITION
49
combination of factors which represents the limits of niche space within
which species S, is able to exert its competitive superiority and completely exclude S,, or vice versa. As long as interference prevents access
to the resources contained within a niche space, the realized niches of
the two species can be described in spatial terms relative to the
environmental variables that control the interaction.
The conditions of competitive exclusion through interference are
particularly well-illustrated in the case of an “included niche”, in which
the niche (N,) of species 8, is a proper subset of and is included within
the niche (N,) of species S,. As noted earlier, this situation immediately
imposes certain strict conditions on the interaction. I n order for S , to
survive in an included niche, it must be competitively superior to S,,
otherwise it would be completely eliminated from this biotope. This
relationship also implies that AS,, with its smaller niche space, is a more
specialized species with respect to the variables determining N, and N,.
A system of included niches therefore offers an opportunity for coexistence within the total biotope B by means of specialization of S,
and the ability of S, to survive within the difference subset N, - N,. It
is of considerable interest that several examples of this relationship
exist among different taxonomic groups. These examples will be reviewed in some detail in order to define their common properties.
A. MAMMALS
It has often been noted that pocket gophers (Geomyidae) do not
form mixed-species populations and are mutually exclusive, even in
local habitats. Their geographic distributions reflect this intolerance,
in that the ranges of two species may meet in contiguous allopatry but
do not become truly sympatric. An investigation of the ecological
relationships among four species of pocket gopher which are at or near
limits of their continental distributions in Colorado showed that their
habitat and geographic distributions are essentially governed by three
factors: soil depth, soil texture and competitive exclusion (Miller, 1964a).
All four species seem to prefer deep, sandy soils but, as illustrated in
Fig. 10, they differ in their abilities to burrow in shallower and coarser
soils. For example, Geomys bursarius is mostly confined to deep sand
or sandy loam soils, while indurate soils such as compacted clays or
coarse gravels are barriers to its habitat and geographic distributions.
At the other extreme, Thomomys tdpoides also prefers deep, sandy
soils but can burrow in extremely coarse soils and occurs throughout a
great variety of soil types and habitats. Although the four species differ
slightly in food habits and perhaps in other respects as well, these
differences do not appear to be critical and their fundamental niches
are essentially defined by their responses to the two variables of soil
49
combination of factors which represents the limits of niche space within
which species S, is able to exert its competitive superiority and completely exclude S,, or vice versa. As long as interference prevents access
to the resources contained within a niche space, the realized niches of
the two species can be described in spatial terms relative to the
environmental variables that control the interaction.
The conditions of competitive exclusion through interference are
particularly well-illustrated in the case of an “included niche”, in which
the niche (N,) of species 8, is a proper subset of and is included within
the niche (N,) of species S,. As noted earlier, this situation immediately
imposes certain strict conditions on the interaction. I n order for S , to
survive in an included niche, it must be competitively superior to S,,
otherwise it would be completely eliminated from this biotope. This
relationship also implies that AS,, with its smaller niche space, is a more
specialized species with respect to the variables determining N, and N,.
A system of included niches therefore offers an opportunity for coexistence within the total biotope B by means of specialization of S,
and the ability of S, to survive within the difference subset N, - N,. It
is of considerable interest that several examples of this relationship
exist among different taxonomic groups. These examples will be reviewed in some detail in order to define their common properties.
A. MAMMALS
It has often been noted that pocket gophers (Geomyidae) do not
form mixed-species populations and are mutually exclusive, even in
local habitats. Their geographic distributions reflect this intolerance,
in that the ranges of two species may meet in contiguous allopatry but
do not become truly sympatric. An investigation of the ecological
relationships among four species of pocket gopher which are at or near
limits of their continental distributions in Colorado showed that their
habitat and geographic distributions are essentially governed by three
factors: soil depth, soil texture and competitive exclusion (Miller, 1964a).
All four species seem to prefer deep, sandy soils but, as illustrated in
Fig. 10, they differ in their abilities to burrow in shallower and coarser
soils. For example, Geomys bursarius is mostly confined to deep sand
or sandy loam soils, while indurate soils such as compacted clays or
coarse gravels are barriers to its habitat and geographic distributions.
At the other extreme, Thomomys tdpoides also prefers deep, sandy
soils but can burrow in extremely coarse soils and occurs throughout a
great variety of soil types and habitats. Although the four species differ
slightly in food habits and perhaps in other respects as well, these
differences do not appear to be critical and their fundamental niches
are essentially defined by their responses to the two variables of soil