PATTERN AND PROCESS IN COMPETITION
57
occupied the emergent vegetation in deep water near the center of the
marsh, and had displaced the redwings to more peripheral nest sites in
spite of persistent harassment by the redwings. The territories established by the yellow-headed blackbirds in their first few days on the
marsh remained intact throughout the remainder of the breeding season.
c. AMPHIBIA
W. F. Blair (personal communication) has followed events for several
years in a temporary pond containing populations of the bullfrog (Rana
catesbiana) and leopard frog (Rana pipiens). R. catesbiana requires a
habitat of standing water but R. pipiens can also live in more terrestrial
environments. When drought reduced the pond to almost nil, R. catesbiana disappeared and only a few, scattered R. pipiens remained. When
the pond refilled after a wet spring, the 12. pipiens population “exploded” to a very high density, far greater than the pond could normally sustain. Gradually, the numbers of R. catesbiana also increased
and the numbers of R. pipiens declined. When R. pipiens was the only
species present it filled the entire habitat, but when the R. catesbiana
reappeared this species occupied the water environment and displaced
R. pipiens to drier sites near the edge of the pond.
The salamanders Plethodon dunni and P. vehiculum are abundant in
the rocky outcrop-talus slope ecosystem of the coast range of Oregon,
where they occupy approximately the same habitats. P. dunni tolerates
slightly lower temperatures and wetter substrates than P. vehiculunz
while P. vehiculum is tolerant of lower relative humidities and higher
temperatures than P. dunni, although their preferences for cool, humid
conditions are not significantly different (Dumas, 1956). Although temperature does not significantly alter the humidity preference of the two
species within the normal range of humidity encountered in their
natural environments, there are specific differences in their substrate
distributions which are apparently due to the presence of P. dunmi ill
the preferred sites and displacement of P. vehiculum to lion-preferred
but tolerated habitats. This interaction becomes especially critical in
spring and fall, when the weather is variable and there is great risk of
exposure to lethal environmental conditions, and when P. vehiculum is
found far more often than P. dunni in suboptimal “transient habitats”.
Dumas (1956) examined 36 filled stomachs of P. dunni and 37 of P.
vehiculum to determine the extent of ecological differentiation in their
food habits, because of the stress that is often placed on this factor in
competitive interactions. He found that collembolans are the greatest
food source for both species, but that P. dunni consumes a greater
variety of foods than P. vehiculwm. There was a total of 39 types of
food with a mean of 8.36 items per stomach in the P. dunni sample,
57
occupied the emergent vegetation in deep water near the center of the
marsh, and had displaced the redwings to more peripheral nest sites in
spite of persistent harassment by the redwings. The territories established by the yellow-headed blackbirds in their first few days on the
marsh remained intact throughout the remainder of the breeding season.
c. AMPHIBIA
W. F. Blair (personal communication) has followed events for several
years in a temporary pond containing populations of the bullfrog (Rana
catesbiana) and leopard frog (Rana pipiens). R. catesbiana requires a
habitat of standing water but R. pipiens can also live in more terrestrial
environments. When drought reduced the pond to almost nil, R. catesbiana disappeared and only a few, scattered R. pipiens remained. When
the pond refilled after a wet spring, the 12. pipiens population “exploded” to a very high density, far greater than the pond could normally sustain. Gradually, the numbers of R. catesbiana also increased
and the numbers of R. pipiens declined. When R. pipiens was the only
species present it filled the entire habitat, but when the R. catesbiana
reappeared this species occupied the water environment and displaced
R. pipiens to drier sites near the edge of the pond.
The salamanders Plethodon dunni and P. vehiculum are abundant in
the rocky outcrop-talus slope ecosystem of the coast range of Oregon,
where they occupy approximately the same habitats. P. dunni tolerates
slightly lower temperatures and wetter substrates than P. vehiculunz
while P. vehiculum is tolerant of lower relative humidities and higher
temperatures than P. dunni, although their preferences for cool, humid
conditions are not significantly different (Dumas, 1956). Although temperature does not significantly alter the humidity preference of the two
species within the normal range of humidity encountered in their
natural environments, there are specific differences in their substrate
distributions which are apparently due to the presence of P. dunmi ill
the preferred sites and displacement of P. vehiculum to lion-preferred
but tolerated habitats. This interaction becomes especially critical in
spring and fall, when the weather is variable and there is great risk of
exposure to lethal environmental conditions, and when P. vehiculum is
found far more often than P. dunni in suboptimal “transient habitats”.
Dumas (1956) examined 36 filled stomachs of P. dunni and 37 of P.
vehiculum to determine the extent of ecological differentiation in their
food habits, because of the stress that is often placed on this factor in
competitive interactions. He found that collembolans are the greatest
food source for both species, but that P. dunni consumes a greater
variety of foods than P. vehiculwm. There was a total of 39 types of
food with a mean of 8.36 items per stomach in the P. dunni sample,