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<br />Joseph H. Connell
<br />
<br />queens in the next two years. In the colo-
<br />nies that were more crowded after new
<br />colonies had been moved adjacent to
<br />them, fewer queens were sometimes pro-
<br />duced. These effects were apparently more
<br />drastic on other colonies of the same spe-
<br />cies than on those of different species.
<br />In the absence of experimental manip-
<br />ulation, observations of aggressive dis-
<br />placement of one species by another are
<br />convincing evidence of competition. For
<br />example, Way (1953) observed one species
<br />of ant gradually driving another species
<br />back through a coconut plantation in
<br />Tanzania. Levins, Pressick, and Heatwole
<br />(1973) observed aggressive interactions
<br />between different species of ants when
<br />bait was placed on the ground on small
<br />Caribbean islands. Aggressive displace-
<br />ment of one species by another has also
<br />been observed in birds (Pitelka, 195];
<br />Orians and Collier, 1963; Orians and
<br />Willson, 1964) and mammals (Brown,
<br />1971; Heller, 197]; Sheppard, 1971).
<br />Indirect evidence of interspecific com-
<br />petition comes from observations of non-
<br />overlapping adjacent ranges (Diamond,
<br />1973), inverse correlations of abundance
<br />(Brown, Chapter 13), niche shifts on is-
<br />lands where competitors are absent
<br />(Crowell, 1962; Diamond, 1970 and
<br />Chapter 14), character displacement, etc.
<br />Such indirect evidence is open to explana-
<br />tions other than competition.
<br />In aquatic habitats. Benthic inverte-
<br />brates in fresh-water lakes sometimes
<br />occur at such high densities that they
<br />probably compete for space or food
<br />(Jonasson, 1971). Reynoldson and Bellamy
<br />(1971) have performed a field experiment
<br />
<br />466
<br />
<br />which indicated that one species of flat-
<br />worm was able to displace another in a
<br />small lake in Wales.
<br />Wilbur (1972) enclosed populations of
<br />various combinations of six species of am-
<br />phibians in pens along the edge of a pond.
<br />Competition occurred among the three
<br />local species of salamanders, affectin a
<br />survivorship, length of larval periOd,. and
<br />body weight. A fourth species not occur-
<br />ring locally was introduced; it survived well
<br />alone, but competed with the ]ocal species
<br />if grown with them. Another species of sal-
<br />amander was also a competitor unless it
<br />was able to grow quickly to a larger size,
<br />when it apparently became a predator on
<br />the other salamanders. Having frog tad-
<br />poles as prey may have given it this ad-
<br />vantage of faster growth. To carry out
<br />these complex experiments it was neces-
<br />sary to enclose the populations in pens.
<br />These excluded other species of predators,
<br />both invertebrate and vertebrate, which
<br />probably would have affected the out-
<br />come of the interactions under natural
<br />conditions. However, this study is one of
<br />the most complete and rigorous analyses
<br />of the complex interactions that produce
<br />structure in communities.
<br />Severa] controlled field experiments
<br />have demonstrated the existence of inter-
<br />specific competition in marine organisms.
<br />The usual procedure consists in changing
<br />the abundance of one species and observ-
<br />ing the survivorship or growth of another,
<br />with adjacent controls. All of the experi-
<br />ments have been done on rocky seashores.
<br />Connell (196Ib) found that one species of
<br />barnacle was eliminated from lower shore
<br />zones by another that grew faster and
<br />
<br />
<br />16 Producing Structure in Natural
<br />Communities
<br />
<br />
<br />either smothered, crushed, or undercut the
<br />first. Haven (1966, 1973) changed the
<br />population density of two species of graz-
<br />ing limpets and found that they affected
<br />each other's growth rates. A similar in-
<br />stance of competition for food between
<br />two predatory starfish was demonstrated
<br />in a controlled field experiment by Menge
<br />(1972). Competition for space producing
<br />a nonoverlapping mosaic between two
<br />species has been demonstrated with lim-
<br />pets by Stimson (1970, 1973) and with
<br />barnacles and anemones by Dayton
<br />(1971). Observations also suggest that
<br />competition may occur between barnacles
<br />and attached algae (Dayton, 1971).
<br />
<br />Where is Competition Prevented
<br />by Predation1
<br />On land. Much of the experimental
<br />evidence for predation as an important
<br />factor in community structure deals with
<br />predation on plants by mammals. Grazing
<br />by wild rabbits kept grasses from displac-
<br />ing dicotyledonous herbs in southern
<br />England. When rabbits were excluded by
<br />fences. or after they had been eliminated
<br />by an epidemic of myxomatosis, grasses
<br />invaded, forbs disappeared and woody
<br />vegetation began to invade (Tans]ey and
<br />Adamson, 1925; Hope-Simpson, 1940;
<br />Wall, 1957, 1960; Thomas, 1960). When
<br />mice and voles were excluded with fences
<br />for seven years, the same thing happened
<br />in a conifer plantation in Wales (Summer-
<br />hayes, 1941). Experimental exclusion of
<br />voles from grassland in California for two
<br />years showed that their grazing changed
<br />the relative abundance of species, reduc-
<br />ing the palatable species (Batzli and
<br />
<br />i
<br />
<br />467
<br />
<br />Pitelka, 1970). Thus small mammals, by
<br />selective predation, can determine the
<br />community structure of terrestrial vegeta-
<br />tion.
<br />Another example is the effect of insects
<br />on a species of hemiparasitic herb living
<br />in the understory of deciduous forests in
<br />Michigan. When these grazers were re-
<br />moved, the plant population increased
<br />greatly (Cantlon, 1970).
<br />The regeneration of trees may be almost
<br />completely prevented by grazing mam-
<br />mals. In three species of trees in the New
<br />Forest in England, the populations consist
<br />of three "generations," established in the
<br />intervals 1648 to 1763, 1858 to 1915, and
<br />since 1938 (Peterkin and Tubbs, 1965).
<br />The two latter generations coincide with
<br />periods when grazers such as deer, cattle,
<br />and ponies were removed from the forest.
<br />Regeneration is limited to occasional es-
<br />capes from predation. Once trees get tall
<br />enough, they are vulnerable to attack only
<br />by insects or pathogens. lnstances of com-
<br />plete defoliation by insects, sometimes
<br />with mass mortalities of adult trees, have
<br />occurred, e.g., balsam fir in Canada
<br />(Morris, 1963) and Eucalyptus de/ega/ensis
<br />in Australia (Readshaw and Mazanec,
<br />1969). As pointed out by Murdoch (1971),
<br />balsam fir is vulnerable to this heavy at-
<br />tack only when mature and in continuous
<br />even-aged stands, where, presumably, the
<br />dispersal of insects on to other vulnerable
<br />trees is facilitated.
<br />In the chaparral vegetation in Califor-
<br />nia the competitive interference through
<br />release of chemicals (allelopathy) between
<br />bushes and herbs is sometimes modified
<br />by grazing. The periodic fires which re-
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