untitled

populations were not responding to the experiment. In few cases have all these con-

ditions been met. Because of these difficulties an entirely different approach to the

study of interspecific competition has measured the pattern of overlap in the use of

resources. We now consider this approach.

In Chapter 3 we saw that different species on different continents appeared to adopt

the same role in the community and often these species have evolved similar

morphological and behavioral adaptations. This place in the community is called the

niche, defined by Elton (1927) as the functional role and position of the organism

in its community. (We provide the modern definition later.)

For practical reasons the niche has come to be associated with use of resources.

Thus, we can plot the range and frequency of seed sizes eaten by different bird species,

as a hypothetical example, in Fig. 9.7a. Species that exploit the outer parts of the

COMPETITION AND FACILITATION BETWEEN SPECIES 143

100

50

0

100

50

0

100

50

0

100

50

0

Small Large

Resource spectrum (seed size)

Amount of resource

(a)

1

2

3 4

5

6

(b)

Redpoll

Linnet

Greenfinch

Hawfinch

ABCDE

Seed size

% of diet

Fig. 9.7(a)
Hypothetical frequency
distribution of seeds of
different sizes indicating
the range and overlap
of potential niches
for granivores. (After
Pianka et al. 1979.)
(b) Range of seed sizes
eaten by British finches
feeding on herbaceous
plants. Seeds are in five
size categories A to E.
The finches are redpoll
(Carduelis flammea),
linnet (C.cannabina),
greenfinch (Chloris
chloris), and hawfinch
(Coccothraustes
coccothraustes). (After
Newton 1972.)

9.4 The concept of the niche