PATTERN AND PROCESS IN COMPETITION
17
may exist to be occupied by subsequently invading species (cf. Darwin,
1859; Elton, 1958). It is in this connection that Weatherly (1963)
defines a niche as the nutritional role of an animal, existing to be
occupied in widely separated ecosystems. However, while the broad
functional concept of the niche allows us to describe the composition
of a community in such a way that particular roles can be occupied by
different species, it does not at the same time allow us to define degrees
of overlap in the ecological requirements of species competing at the
same trophic level.
Obviously the value of the niche concept as a theoretical tool is as
a means of describing environmental relationships in such a way that
the conditions of competition or coexistence can be defined and evaluated. As Crombie (l947), Hutchinson (1957) and others have pointed
out, the degree of similarity that leads to competition or, conversely,
the ecological differentiation that is necessary for coexistence, can only
be defined empirically. Thus we require a conceptual model for this
purpose which will accommodate both the functional and distributional
concepts of the niche (cf. Lack, 1944).
B. NICHE RELATIONSHIPS
Elton and Miller (1954) recognized certain of the difficulties inherent
in using the niche concept to define competitive interactions when they
pointed out, “There is no difficulty in seeing the reality of a very broad
distinction of ecological function, such as between herbivore and carnivore. The distinctions get a bit blurred if we divide up carnivores into
separate niches, since a carnivore may in fact ‘belong’ to more than one
consumer layer. At the other end of the scale, each species has a unique
ecological niche in the sense that its particular mosaic of abilities and
habits is unique. In between the broad consumer type and the species,.
one may create any number of ‘niches’, by choosing some well-marked
type of habit: ‘mouse-eater’, ‘conifer-needle feeder’, ‘bark.-beetle’ and
so on. What ecologists usually imply by such groupings is that within
them interspecific competition occurs or may occur. But when one
considers an actual example like the grain-beetle Rhizoperth dominica
competing with the grain-moth Sitotroga cerealella (in laboratory culture), it is seen that the similarities of habit (larvae feeding inside wheat
grains for example) that bring about competition can be matched by
other differences in properties of the species (they will have different
parasites; they have different life spans; they presumably have Werent
tolerance. . .) We think, therefore, that analysis of communities should
pay attention more to tracing the consequences af one species in a key
position being replaced by another, rather than trying to classify all
the functions of species into a few niches.”
17
may exist to be occupied by subsequently invading species (cf. Darwin,
1859; Elton, 1958). It is in this connection that Weatherly (1963)
defines a niche as the nutritional role of an animal, existing to be
occupied in widely separated ecosystems. However, while the broad
functional concept of the niche allows us to describe the composition
of a community in such a way that particular roles can be occupied by
different species, it does not at the same time allow us to define degrees
of overlap in the ecological requirements of species competing at the
same trophic level.
Obviously the value of the niche concept as a theoretical tool is as
a means of describing environmental relationships in such a way that
the conditions of competition or coexistence can be defined and evaluated. As Crombie (l947), Hutchinson (1957) and others have pointed
out, the degree of similarity that leads to competition or, conversely,
the ecological differentiation that is necessary for coexistence, can only
be defined empirically. Thus we require a conceptual model for this
purpose which will accommodate both the functional and distributional
concepts of the niche (cf. Lack, 1944).
B. NICHE RELATIONSHIPS
Elton and Miller (1954) recognized certain of the difficulties inherent
in using the niche concept to define competitive interactions when they
pointed out, “There is no difficulty in seeing the reality of a very broad
distinction of ecological function, such as between herbivore and carnivore. The distinctions get a bit blurred if we divide up carnivores into
separate niches, since a carnivore may in fact ‘belong’ to more than one
consumer layer. At the other end of the scale, each species has a unique
ecological niche in the sense that its particular mosaic of abilities and
habits is unique. In between the broad consumer type and the species,.
one may create any number of ‘niches’, by choosing some well-marked
type of habit: ‘mouse-eater’, ‘conifer-needle feeder’, ‘bark.-beetle’ and
so on. What ecologists usually imply by such groupings is that within
them interspecific competition occurs or may occur. But when one
considers an actual example like the grain-beetle Rhizoperth dominica
competing with the grain-moth Sitotroga cerealella (in laboratory culture), it is seen that the similarities of habit (larvae feeding inside wheat
grains for example) that bring about competition can be matched by
other differences in properties of the species (they will have different
parasites; they have different life spans; they presumably have Werent
tolerance. . .) We think, therefore, that analysis of communities should
pay attention more to tracing the consequences af one species in a key
position being replaced by another, rather than trying to classify all
the functions of species into a few niches.”