PATTERN AND PROCESS IN COMPETITION
51
relationship (Miller, 1964a). We conclude from this that the superior
competitor is in some way more specialized and that its distribution is
governed solely by its relation to the variables of soil depth and texture.
These four species show morphological differences in body size and
fossorial development that correspond to their competitive rankings.
Gemys bursarius is the most obviously fossorial, with a relatively
massive skull that is flattened doro-ventrally, reduced eyes and ears,
and large forefeet and claws. Because of the pronounced fossorial
character of its claws and feet it walks with difficulty, with the feet
splayed outward like those of a mole. Thomomys talpoides, at the other
extreme, is a more generalized, mouse-like rodent. I n body size, C.
castanops is somewhat larger than G. bursarius, T . bottae is next in size,
and T . talpoides is the smallest. As there is a correlation between body
size and soil type in pocket gophers, with the largest individuals and
subspecies occurring in deep, sandy soils, it is difficult to judge the
extent to which observed differences in body size are hereditary or
phenotypic. On the one hand, large body size might confer an advantage
in aggressive encounters, which undoubtedly occur between these highly
territorial animals, but smaller size may be advantageous when an
animal is forced to live in more indurate soils. Kennedy (1954, 1959)
suggests that the relatively smaller size of (2. bursarius may have adaptive value, allowing it to survive in less favorable habitats when it
competes with G. personatus.
The ranges of the yellow-pine chipmunk (Eutamias amoenus) and the
least chipmunk (E. minimus) overlap narrowly in the eastern foothills
of the Rocky Mountains in Alberta, where E. amoenus occupies the
open forests and E . minimus is mostly confined to alpine slopes.
Wherever other species of Eutamias occur, E. minimus generally seems
to be displaced either to sagebrush desert or to alpine habitats, although
in regions of North America where there are no other western chipmunks, E. minimus also occurs in forest habitats, suggesting that its
fundamental niche is in fact relatively broad.
In a study of the oompetitive relationships between Eutamias amoenus
and E . minimus in their region of sympatry in western Alberta, Sheppard (unpublished) observed that the most consistent difference between
the habitats of the two species is in the amount of vegetative cover
present. Using the classification of vegetation layers proposed by Elton
and Miller (1954), Sheppard recorded the percent occurrence of each
type in vegetation samples from habitats occupied by each species. As
shown in Table VII, the typical habitat of E. amoenus contained more
vegetation of all types, especially shrubs and trees. Both species use
shrubs and trees as “ewape routes” when alarmed, but E. minimus will
apparently tolerate more open habitats in this respect, moving across
51
relationship (Miller, 1964a). We conclude from this that the superior
competitor is in some way more specialized and that its distribution is
governed solely by its relation to the variables of soil depth and texture.
These four species show morphological differences in body size and
fossorial development that correspond to their competitive rankings.
Gemys bursarius is the most obviously fossorial, with a relatively
massive skull that is flattened doro-ventrally, reduced eyes and ears,
and large forefeet and claws. Because of the pronounced fossorial
character of its claws and feet it walks with difficulty, with the feet
splayed outward like those of a mole. Thomomys talpoides, at the other
extreme, is a more generalized, mouse-like rodent. I n body size, C.
castanops is somewhat larger than G. bursarius, T . bottae is next in size,
and T . talpoides is the smallest. As there is a correlation between body
size and soil type in pocket gophers, with the largest individuals and
subspecies occurring in deep, sandy soils, it is difficult to judge the
extent to which observed differences in body size are hereditary or
phenotypic. On the one hand, large body size might confer an advantage
in aggressive encounters, which undoubtedly occur between these highly
territorial animals, but smaller size may be advantageous when an
animal is forced to live in more indurate soils. Kennedy (1954, 1959)
suggests that the relatively smaller size of (2. bursarius may have adaptive value, allowing it to survive in less favorable habitats when it
competes with G. personatus.
The ranges of the yellow-pine chipmunk (Eutamias amoenus) and the
least chipmunk (E. minimus) overlap narrowly in the eastern foothills
of the Rocky Mountains in Alberta, where E. amoenus occupies the
open forests and E . minimus is mostly confined to alpine slopes.
Wherever other species of Eutamias occur, E. minimus generally seems
to be displaced either to sagebrush desert or to alpine habitats, although
in regions of North America where there are no other western chipmunks, E. minimus also occurs in forest habitats, suggesting that its
fundamental niche is in fact relatively broad.
In a study of the oompetitive relationships between Eutamias amoenus
and E . minimus in their region of sympatry in western Alberta, Sheppard (unpublished) observed that the most consistent difference between
the habitats of the two species is in the amount of vegetative cover
present. Using the classification of vegetation layers proposed by Elton
and Miller (1954), Sheppard recorded the percent occurrence of each
type in vegetation samples from habitats occupied by each species. As
shown in Table VII, the typical habitat of E. amoenus contained more
vegetation of all types, especially shrubs and trees. Both species use
shrubs and trees as “ewape routes” when alarmed, but E. minimus will
apparently tolerate more open habitats in this respect, moving across