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

Joseph H. Connell

gradient analyses, maps of distributions,
etc., cannot do.

The Pattern of Occurrence
of Competition

Where Is Competition Common
Under Natural Conditions?
On land Competition between the
roots of canopy trees and those in the
understory was demonstrated long ago by
controlled field experiments in natural
forests in temperate latitudes. Trenches
were dug around plots, cutting all roots
and so eliminating competition for water
or nutrients with adult trees (Fricke, 1904;
Toumey and Kienholz, 1931; Korstian
and Coile, 1938; Lutz, 1945; Shirley,
1945). In every case the smaller trees in-
side the trenched plots survived and grew
much better than in nearby control plots.
In the one pu blished instance of trenching
in a tropical forest, the plants in the ex-
perimental plot did not do better (Con-
nell, 1971). This may have been due to
the greater competition for light in this
very dense forest and/or to a greater in-
tensity of grazing.
Interference between bushes and herbs
in the chaparral vegetation of California
has been demonstrated in several studies.
In hiboratory experiments the bushes have
been shown to contain volatile or water-
borne chemicals that inhibit germination
and early growth of herbs (Muller, 1966).
New plants become established mainly
after fires, which destroy the "allelo-
pathic" effects of the original vegetation.
However, as will be discussed later, graz-

464

ing may also affect seedling establishment
in chaparral.
Direct evidence of interspecific compe-
tition between animals comes from field
experiments on vertebrates and social in-
sects. When DeLong (1966) reduced the
numbers of the meadow mouse Microtus,
the population of house mice (Mus) in-
creased as compared to that on a control
area. This increase was not due to greater
growth or survival of adults, but rather to
a greater production of young. This was
probably due to lack of disturbance by
Microtus of nests of Mus, since, in the
laboratory, female Mus either deserted
their nests or ate their young in the pres-
ence of Microtus. In another field experi-
ment Koplin and Hoffman (1968) found
that competition probably determined the
spatial distribution of two species of
Microtus. During a summer's trapping M.
mon/anus was never caught in the wetter
pans of the habitat where M. pennsyl-
vanicus lives. Over the subsequent au-
tumn and spring M. pennsylvanicus was
systematically removed, greatly reducing
its density. Then M. montanus was caught
in the wetter places for the first time.
Unfortunately the population densities of
both species on the control area were
much less than on the experimental area.
Therefore, the fact that M. montanus were
never caught in the wetter habitats on the
control plot may have been the result ei-
ther of the very small sample size or of
the lack of intraspecific pressure to explore
new areas, rather than from competitive
aggression by M. pennsylvanicus. Appar-
ently these two studies are the only field
experiments on interspecific competition

]6 Producing Structure in Natural
Communities

in rodents in which any sort of control
area was established. Grant's (1972) re-
view cites other field experiments that did
not use control areas.
To my knowledge, the only field experi-
ment testing for the existence of inter-
specific competition in birds is that of
Davis (1973). By trapping over a period
of three years in a field bordered by trees
and bushes, he found that most of the
golden-crowned sparrows were feeding
both on seeds and young plants along one
particular portion of the edge. Juncos,
which feed almost entirely on seeds, oc-
curred more or less equally around the
edge of the field. In the fourth year he
removed almost all the sparrows within
two months. For the next two months, in
the absence of the sparrows, a high pro-
portion of the junco population shifted
into the portion of the field where most
of the sparrows had been feeding. Then
half of the captured sparrows were re-
turned to the field and the other half re-
leased 0.64 km away; about half of these
reached the field within a week. Again a
high number were found along the same
portion of the edge where they had been
commonest before. Concurrently the pro-
portion of the total junco population in
that area fell back nearly to the size it had
been before the sparrows were removed.
Although no separate control area was
established, the fact that the junco popu-
lation rose when the sparrows were re-
moved and fell quickly when they were
returned is strong evidence that the spar-
rows were excluding the juncos from that
part of the habitat.
Other experimental evidence of compe-

465

tition between vertebrates comes from
populations confined in pens. Jaeger
(1971) found two species of salamanders
occupying adjacent habitats. The first spe-
cies, which normally lived only in shallow
soil, survived well in pens in areas with
deep soil, if the second species from the
deep soil was excluded. However, if the
species were penned together, the first
survived poorly. No aggressive behavior
was ever seen in the laboratory and the
mechanism for this apparent competition
is unknown. The number of animals used
was small and the effects of the pen are
unknown; manipulation oflarge numbers
without using pens would be a welcome
extension of these interesting experiments.
A series of controlled experiments with
rodents in large pens has been done by
Grant (see his review, 1972). He showed
that each species was restricted to one type
of habitat by competition with another
species. These were mice which mainly
used the surface of the ground rather than
burrowing or climbing, so that they were
likely to be competing for space.
Field experiments on competition be-
tween ant colonies have been done in at
least two instances. Brian (1952) set up
artificial nest sites by placing slabs of
stone or slate on the ground or building
turf banks. Initial colonization by different
species was "near random," but then cer-
tain species were observed to drive out
other species by direct aggression or to
move in when the other species had left
during a drought or cold spell. Pontin
(1969) moved whole colonies into new
positions. The colonies left behind, being
less crowded, often produced more alate