8102

• UCREFRP Catalog Number: 8102
• Author: Connell, J. H.
• Title: Some Mechanisms Producing Structure in Natural Communities
• USFW Year: n.d.
• USFW - Doc Type: a Model and Evidence from Field Experiments.
• Copyright Material: YES

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16 Some Mechanisms
Producing Structure in
Natural Communities:
a Model and Evidence
from Field Experiments

Introduction

Community Structure and
Niche Theory
If a local assemblage of organisms is to
be regarded as a community with some
degree of organization or structure, then
it is in the interactions between the orga-
nisms that we must look to provide this
structure. Two different interactions pro-
vide most of the organization: competition
and predation. I will use competition in
the sense of Birch's (1957) first meaning:
"Competition occurs when a number of
animals (of the same or of different spe-
cies) utilize common resources, the supply
of which is short; or if the resources are
not in short supply, competition occurs
when the animals seeking that resource
nevertheless harm one or~ another in the
process." I will use predation in the broad
sense of an animal's eating another orga-
nism for its main source of food, including
herbivores that eat plants as well as para-
sites or pathogens that eat their host.
Predation represents the interaction be-
tween trophic levels, whereas competition
mainly represents interactions within
trophic levels (although if space is the
resource, it is possible for plants to com-
pete with sessile animals in aquatic habi-
tats). These two interactions may them-
selves interact: the numbers of two

Joseph H Connell

competing species may be reduced by
their predators to such an eKtent that
competition is prevented. In addition, the
physical environment affects the intensity
of both sorts of interactions, and the orga-
nisms affect the physical environment.
In studying community structure, one
aspect of current interest is the theory of
the ecological niche. One speaks of how
niches are packed together, how much
niches can overlap, what determines the
breadth or shape of niches, etc. In these
discussions the emphasis has been on only
one of the two major interactions, compe-
tition, because of the way in which Hutch-
inson (1958) formally defined the niche.
In his definition, the boundary of the
"fundamental" niche is determined by the
limiting states of all possible ecological
variables, both physical and biological,
which permit a species to ell;st indefin-
itely. The "realized" niche is defined as
that portion of the fundamental niche
within which a species is constrained by
interactions with its competitors. "Inter-
action of any of the considered species is
regarded as competitive in sense 2 of
Birch (1957), negative competition being
permissible, though not considered here.
All species other than those under consid.
eration are regarded as part of the coordi-
nate system" (Hutchinson, 1958). Compe.
tition under sense 2 of Birch (1957)
includes, in addition to the item in his

460

16 Producing Structure in Natural
Communities

meaning quoted earlier, "an additional
item, namely the interference with one
species by another (with consequent
change in birth rate or death rate) even
when there is no demand for a common
resource of space or food. But they ex-
clude predation in which one animal eats
another for its main source of food."
Vandermeer (1972) has pointed out that
the idea of the niche presented by
Grinnell (1917, 1928) is close to the idea
of the fundamental niche, since he
thought of it as "the concept of the ulti-
mate distributional unit, within which
each species is held by its structural and
instinctive limitations." This concept of
the niche excludes interactions with other
species, both competitors and predators.
In contrast, Elton's (1927) definition ("the
'niche' of an animal means its place in the
abiotic environment, its relation to food
and enemies") is closer to the realized
niche, since it includes interactions with
other species.
The distinction between fundamental
and realized niches is a very useful one:
if communities are organized by inter-
actions, then the manner and degree of
that organization will be reflected in the
differences between the sizes and shapes
of realized and fundamental niches.
The modern theory of the niche, as
proposed by Levins (1968) and Mac-
Arthur (1968) and recently summarized
and extended by Vandermeer (1972), is
based upon the original definitions of
Hutchinson (1958).
Vandermeer categorizes Hutchinson's
distinction between fundamental and
realized niche as a distinction between a

<61 D~

461

pre interactive and a post interactive niche
(Vandermeer, 1972, p. 109), thus restrict.
ing the meaning of interaction to compe-
tition. This is also shown by his statement
that the species ("operational taxonomic
units") have only density-dependent feed-
back effects on each other and by his
equating of "operational habitat" with
"resource."
Thus, both in Hutchinson's original
definition and in Vandermeer's recent
summary and extension of the theory of
the niche, only competitive interactions
are considered. Other interactions, nota-
bly predation, are included in the factors
bounding the fundamental niche, in the
"conception of the niche as being pre-
interactive-the potential area within
which a species can live as opposed to the
area in which one actually finds it" (Van-
dermeer, 1972). Here, as elsewhere in dis-
cussions of niche theory, "preinteractive"
means all aspects except competition, and
"postinteractive" means "after competi-
tion occurs."
In this chapter I would like to challenge
the idea that competition is the sole or
even the principal mechanism determin-
ing the area in which one finds a species,
as opposed to the potential area within
which it can live. If this is not the case
then the formal structure of "niche the~
ory" needs to be extended so that inter-
actions other than competition are re-
garded as also constraining the realized
niche.
Of course, it has long been recognized
that predators can keep potential competi-
tors so rare that they do not compete.
Darwin's (1859, p. 67-68) experiment is