untitled

In a test of these hypotheses Pimm and Pimm (1982) recorded the feeding choices

of three nectar-feeding bird species (Himatione,Loxops,Vestiaria) on the island of

Hawaii. There were two main tree species, Metrosiderosand Sophora, which came

into flower at different times of the year. The evidence for the distinct preference

case is seen in Fig. 9.15. When the number of flowers is high, all three species feed

on both trees. When flowers per tree are low (and assuming that this indicated

limiting resource) only Loxops feed on Sophora, and only Himatione feed on

Metrosideros. Thus, both species reduce their niche width and specialize. There was

also evidence of shared preference. Vestariafeed on both tree species but only at high

flower numbers, and physically exclude the other species by visual and vocal dis-

plays. In contrast, both Himationeand Loxopsspend much of their feeding time on

trees with few flowers. Thus, these two species are confined to poorer feeding areas

during times when resources are low, as predicted by the theory.

Rosenzweig’s theory predicts that niches contract when resources are limiting and

there is interspecific competition. We have seen that the Hawaiian honeycreepers may

conform to the predictions, but what about other species? Information from wildlife

both agrees and disagrees with the predictions. The overlap in diet of sympatric moun-

tain goats (Oreamnos americanus) and bighorn sheep (Ovis canadensis) is high in sum-

mer but reduced in winter (Dailey et al. 1984), as predicted by the theory. In ducks

we have already seen that during winter there is a decrease in overlap (Fig. 9.9).

Burning grasslands increases the nutrient content of regenerating plants and may pro-

duce locally abundant food. Under these conditions mountain goats and mule deer

(Odocoileus hemionus) actually increase dietary overlap (Spowart and Thompson

Hobbs 1985). In contrast, elk and deer in natural forests increased dietary overlap

in winter when resources were assumed to be least available, contrary to expectation

(Leslie et al. 1984).

152 Chapter 9

6.0

4.0

2.0

20

10

0

1000 200 100

100 20 10 7

Number of flowers per tree

Handling times (s / flower)

Handling times (s / flower)

(a) Sophora

(b) Metrosideros Loxops

Himatione

Vestaria

Fig. 9.15
Demonstration of
distinct and shared
preferences in habitat
selection by three
species of honeycreepers
in Hawaii. The two
main flowering trees
are (a) Sophoraand
(b) Metrosideros. At low
flower numbers Loxops
(
) fed on Sophora, and
Himatione() fed on
Metrosideros, showing
“distinct preference.”
Vestaria() fed on
both trees, excluding
the other species, but
only at high flower
numbers, indicating
“shared preference.”
Note the reverse scale
on the x-axis. (After
Pimm and Pimm 1982.)