PATTERN AND PROCESS IN COMPETITION
25
allopatry between the ranges of adjacent species (Miller, 1964a). There
are at least three possible explanations for this rather striking distribution pattern. Each species could be so highly adapted to some critical
factor such as soil type that the relationship between their fundamental
niches is that shown in Fig. 3 D; their fundamental niches may intersect
but some intervening barrier, such as a discontinuity between soil types
or a physical barrier such as a river, might prevent the intersection
from being expressed in their realized niches; or, in the absence of
effective geographical or habitat barriers, the intersection in their fundamental niches might be controlled by competition so that only one of
any species pair can occupy the niche space corresponding to the intersection subset. This example will be discussed in R later section when
the evidence for competition will be examined in more detail, but suffice
to say it can be shown that there is intersection in the fundamental
niches of all four species. I n fact, the relationship between their fundamental niches is essentially a system of included niches as illustrated
in Fig. 3 C , and competitive exclusion is the most reasonable explanation of the distribution patterns shown by these species.
A similar relationship is illustrated in Fig. 5, which shows the spatial
distributions of two species of triclnd when they occur separately and
together in highland brooks and streams (Beauchamp and UllyBtt,
1932). The distributions of fresh-water triclads are governed primarily
by the temperature and rate of flow of springs and streams. Both of
these species were found to exist alone in several springs where the
water temperature was 6.5 to 8.5"C, but Planaria montenegrina can
only tolerate temperatures up to 16 or 17°C while P. gonocephala inhabits waters as warm as 23°C. Their fundamental niches therefore
intersect (Fig. 3 C) in the range ofnpproximately 6.5 to 1 G or 17°C with
respect to this critical factor. \4heu the two species are sympatric in
the sense that they both occur in the same stream, P. gonocephala is
excluded from the spring head to a point where the temperature reaches
13 to 14"C, below which P. nronteticgrina is absent and P. gonocephala
is the only species present. It is interesting to note that competitive
exclusion does not occur a t the extreme of the fundamental niche of P.
montenegrina, but that there is a shift of 3 or 4 degrees to the exclusion
point of 13 to 14°C. Andrewartha and Birch (1954) contend that this is
not necessarily an example of Competitive exclusion, and that other
selection processes could have produced the same result. They offer no
alternative to what seems a clear case of exclusive realized niches
between two species whose fundamental niches intersect; the most
reasonable explanation of this phenomenon is that competitive exclusion operates predominaiitly in favor of 1'. rnontenegrina, the more
specialized species.
25
allopatry between the ranges of adjacent species (Miller, 1964a). There
are at least three possible explanations for this rather striking distribution pattern. Each species could be so highly adapted to some critical
factor such as soil type that the relationship between their fundamental
niches is that shown in Fig. 3 D; their fundamental niches may intersect
but some intervening barrier, such as a discontinuity between soil types
or a physical barrier such as a river, might prevent the intersection
from being expressed in their realized niches; or, in the absence of
effective geographical or habitat barriers, the intersection in their fundamental niches might be controlled by competition so that only one of
any species pair can occupy the niche space corresponding to the intersection subset. This example will be discussed in R later section when
the evidence for competition will be examined in more detail, but suffice
to say it can be shown that there is intersection in the fundamental
niches of all four species. I n fact, the relationship between their fundamental niches is essentially a system of included niches as illustrated
in Fig. 3 C , and competitive exclusion is the most reasonable explanation of the distribution patterns shown by these species.
A similar relationship is illustrated in Fig. 5, which shows the spatial
distributions of two species of triclnd when they occur separately and
together in highland brooks and streams (Beauchamp and UllyBtt,
1932). The distributions of fresh-water triclads are governed primarily
by the temperature and rate of flow of springs and streams. Both of
these species were found to exist alone in several springs where the
water temperature was 6.5 to 8.5"C, but Planaria montenegrina can
only tolerate temperatures up to 16 or 17°C while P. gonocephala inhabits waters as warm as 23°C. Their fundamental niches therefore
intersect (Fig. 3 C) in the range ofnpproximately 6.5 to 1 G or 17°C with
respect to this critical factor. \4heu the two species are sympatric in
the sense that they both occur in the same stream, P. gonocephala is
excluded from the spring head to a point where the temperature reaches
13 to 14"C, below which P. nronteticgrina is absent and P. gonocephala
is the only species present. It is interesting to note that competitive
exclusion does not occur a t the extreme of the fundamental niche of P.
montenegrina, but that there is a shift of 3 or 4 degrees to the exclusion
point of 13 to 14°C. Andrewartha and Birch (1954) contend that this is
not necessarily an example of Competitive exclusion, and that other
selection processes could have produced the same result. They offer no
alternative to what seems a clear case of exclusive realized niches
between two species whose fundamental niches intersect; the most
reasonable explanation of this phenomenon is that competitive exclusion operates predominaiitly in favor of 1'. rnontenegrina, the more
specialized species.