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COMPETITIVE DISPLACEMENT, EXCLUSION
AND COEXISTENCE Among Arthropods
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Overview All organisms
have certain habitable zones delimited by physical parameters outside of
which they cannot persist by themselves. This can be a result of parasitism
and predation, or of gross physical stresses. Within the habitable zone long
established species usually exhibit a typical average density with generally
narrow fluctuations. Species may be designated as rare, common or abundant. Ecologists
have paid most attention to fluctuations of abundance, while too little
thought has been given to reasons for the rarity or absence of a species
altogether. Such scarcity is especially intriguing when physical conditions
seem optimum. Some species reach these areas from time to time, but they do
not persist. Extinction will often occur in a particular area when residence
had been temporarily established. The absence
of a species from a habitat may be due to unsuitable physical factors or the
lack of physical or biological requisites, geographic isolation (islands,
mountains), or interspecific actions. Interspecific actions in the form of
multiple parasitism was probably best illustrated by H. S. Smith (1929).
DeBach (1966) discussed the competitive
displacement "principle." Various synonyms for this idea are Gause's Law (1934), Grinnell's Axiom (1943),
the Volterra-Gause Principle (Hutchinson 1957, 1960),
and the Competitive Exclusion Principle
(Hardin 1960). DeBach's
definition of the competitive displacement principle, "different species
having identical ecological niches (= ecological homologues) cannot coexist
for long in the same habitat," admits that all species differ
biologically no matter how closely related they are, or however similar they
may be in habits. Competitive exclusion is also included in the definition
because the complete exclusion of an invader rarely occurs. More than likely,
some individuals gain a foothold and competitive displacement follows. Verification
of competitive displacement in the field was rare prior to the 1960's.
Connell (1961) learned that the intertidal distribution of barnacles was
limited by interspecific competition. DeBach & Sundby (1963) reported
that Aphytis lingnanensis, within 10 years
following its importation in 1948, had displaced its ecological homologue, Aphytis chrysomphali (mercet) from nearly the entire geographic
distribution of the latter (ca. 4,000 sq-miles). Sarotherodon (Tilapia)
hornorum has displaced S. mossambica and Tilapia
zillii from drainage
channels in the south coastal area of California, probably because S. hornorum is the most euryhaline (tolerant of salt water).
Daily ocean tides bathe the primary breeding habitat (Legner 1986a, Legner &
Sjogren 1984). Another
possible case of displacement involves the apparent replacement of Hippelates robertsoni by H.
impressus, a recent invader
from Mexico, in the Riverside, California area. Mechanisms
of Competitive Displacement The basis of
competitive displacement is simple. The winner is the species which produces
the most female progeny which survive to reproduce per unit of time. Other
mechanisms may complicate the process of competitive displacement by
affecting the progeny production of one species relative to the other. These
include host-finding, host recognition, active interference between species,
cannibalism, disease, predation, genetic drift and changes in the physical
conditions
Ernst Mayr (1948) writing
on natural selection stated that "Individuals of two species with
identical ecological requirements would be subject to the same competition
for space and food as if they were members of a single species. However,
since the two species are genetically different, one of them will undoubtedly
be slightly superior to the other in a given habitat. Natural selection will
discriminate against the less efficient individuals [presumably less fecund
with respect to R] and thus eventually eliminate the less efficient species." Nicholson
(1957) on the subject of natural selection, wrote "Within a species
population all individuals have essentially the same properties and
requirements and no competition amongst them is complete. Consequently, if by
mutation or some other change in their genes, individuals appear which have
an advantage over other individuals that causes them to leave more surviving
offspring than individuals of the original form, this new form will
inevitably displace the original form from all places in which they have the
advantage, no matter how small this advantage may be." It is
generally believed that requisites must be in short supply for competition
and displacement to occur. DeBach opposed this viewpoint and refers to
Dobzhansky's (1961) statement that natural selection may take place when
resources are not limiting. Fitness is merely a measure
of reproductive proficiency. DeBAch stated that inasmuch as most insect
populations in nature are under natural control by factors which hold their
densities below a ceiling where food shortage becomes critical and begins to
limit their populations, short supply of food or space is usually not a
factor. Additionally, DeBach and Sundby (1963) showed that competitive
displacement between species of Aphytis
occurred both in the field and laboratory when food (hosts) was abundant in
relation to immediate needs. In
competitive displacement, the winner may not always be the same species.
There can be different outcomes in different habitats (eg., Gause 1934,
Hutchinson & Deevey 1949). Also involved are differences in temperature,
humidity, disease, pH, food quality and perhaps irradiation. The initial
numbers usually are not important in influencing which species wins, except
under special conditions (Crombie 1945, Park 1957). If competitive abilities
of the two contestants are evenly balanced, chance determines the outcome.
However, greater probability may lie with the one having the greatest initial
population density. Genetic
heterogeneity may influence the outcome: the more the genetic variation is
reduced by inbreeding, the more determinate the outcome of competition
becomes. Most past
cases of competitive displacement are history and difficult to verify. There
remain numerous cases where closely related species are allopatric except for
a narrow band of overlap where they come together. These overlapping bands
are believed to represent cases of competitive displacement. However, they
also could involve adaptation to different physical conditions (see Remington
1986). Some
more examples
of field competitive displacement are as follows: 1. Wheat stem
sawflies in the northeastern United States. Cephus pygmaeus
(L.) occurs east of the Delaware-Erie line, while C. tabidus
Fab. occurs west of this line. They overlap narrowly in the center. Elton
calls this "Mutually exclusive distribution." 2. DeBach
& Sundby (1963) and Luck (1985) present the very decisive case of Aphytis parasitoids on red scale
in southern California. 3. Connell
(1961) gives experimentally decisive evidence with barnacles off the coast of
Scotland. 4. DeBach
(1966) showed how Aphytis melinus DeBach rapidly
displaced A. lingnanensis in the interior
citrus areas of southern California, but more slowly in coastal areas. Aphytis lingnanensis became virtually extinct in the interior
areas by 1964. 5. The exotic
Mediterranean fruit fly, Ceratitis
capitata (Wiedemann), was
replaced around Sydney, Australia by the Queensland fruit fly, Dacus tryoni (Froggatt) which invaded from the north
(Andrewartha & Birch 1954). 6. In Hawaii,
the Mediterranean fruit fly was displaced by the Oriental fruit fly, Dacus dorsalis Hendel, in littoral areas. The Mediterranean
fruit fly is now restricted entirely to cool climates at higher elevations. 7. The
introduced parasitoids of Dacus
dorsalis also showed
displacements in Hawaii. Opius
longicaudatus (Ashmead) and Opius vandenboschi Fulla.
A corollary of the Competitive Displacement Principle is the
Coexistence Principle. Coexistence maintains that different species which
coexist indefinitely in the same habitat must have different ecological
niches; i.e., they cannot be ecological homologues. Coexistence
between ecological homologues is theoretical. it might occur if both species
exist at such low densities that competition does not occur (Crombie 1947,
Dumas 1956). It probably never will actually occur, however. What probably
happens is that displacement at low densities is greatly lengthened. It might also
be possible for two species to coexist homologously if each has different
regulatory factors (Harper et al. 1961, Klomp 1961, MacArthur 1958, Nicholson
1957). There is no argument about the coexistence of such species since by
having different regulatory factors, they are not true ecological homologues. The continued
reversal of habitat variation has been suggested as a mechanism whereby two
homologues can coexist (Hutchinson 1949). Klomp (1961) thought this can occur
only if habitat variation is dependent on the numerical ratio of the species
involved. This is very improbable. way were extremely scarce after Opius oophilus
fullaway was introduced. 8. The
California red scale, Aonidiella
aurantii, has completely
replaced the yellow scale, Aonidiella
citrina (Coquillett) in the
presence of abundant food in southern California (DeBach & Sundby 1963). Aonidiella citrina is thought to have been handicapped by more
effective natural enemies in its competition with A. aurantii. 9. The
imported black scale parasitoid, Scutellista
cyanea Motschulsky, largely
replaced its indigenous ecological homologue Moranila californica
(Howard) (Flanders 1958). 10. The
European cabbage butterfly, Pieris
rapae (L.) displaced the
native Pieris oleracea Harris entirely from a
large area. The checkered white butterfly, Pieris protodice
Boisduval & LeConte, also greatly decreased in density. 11. In Israel
the mealybug parasitoid, Clausenia
purpurea Ishii, displaced
the established parasitoids Leptomastix
flavus Mercet and Anagyrus kivuensis Compere (Rivnay 1964). 12.
Displacement of Rhodesgrass scale parasitoid, Anagyrus antoninae
by Neodusmetia sangwani in Texas (Schuster
& Dean 1976). The
Coexistence Principle Utida (1957)
believed that the superior ability of one homologue to utilize a common requisite
is offset by the superior ability of the other to discover and exploit
unutilized sources of the common requisite. Klomp (1961) challenged this
because obviously the second species occurs in parts of the habitat in which
the first is absent; hence, they are not true homologues. It has been
proposed that two ecologically homologous species of parasitic wasps, if not
host regulative can coexist on a common host whose population fluctuates, if
one has an advantage at high host densities and the other at low host
densities. Utida (1957) thought this probably applies to the parasitoids
attacking different host stages, in which case they are not homologues and
could coexist. Other
examples where it was thought that homologues coexisted are reported by
Heatwole and Davis (1965), who observed that three species of Megarhyssa coexisted on the
same host. In this instance they were not homologous because each possessed
ovipositors of different lengths. Ross (1957) discussed six closely related
species of the lawsoni
complex of the leafhopper genus Erythroneura.
All six breed on sycamore, appear to have identical habits, mature
synchronously in each locality, hibernate together and feed in the same
manner, often side-by-side on the same leaf. Coexistence was possible
probably because certain species have advantages in different habitats. Diver
(1940) declared three species of closely related syrphid flies homologous.
However, he did not study the habits and host specificity of the larvae. Schwerdtfeger
(1942) documented the coexistence of four genera of caterpillars in Germany
from 1880 to 1940: Panolis, Hyloicus, Dendrolimus, and Bupalis
on Pinus sylvestris L. Again, this
coexistence can be explained on the basis that each caterpillar was different
"ecologically." Utida (1957) has some exceptions which might
require closer examination. Otherwise, generally speaking, laboratory
experiments usually show one species with different requirements, habits,
etc., when examined carefully. Competitive
Displacement of Non-homologues
Non-homologues have similar but not identical ecological niches.
Competitive displacement of one by the other requires that the broad niche of
one must completely overlap the narrow niche of the other. Examples are as
follows: 1. If Dutch
elm disease should kill all American elm trees, it would eliminate all
insects specific to the American elm. 2. Highly
effective insects, such as the Klamath weed beetles, which reduce the Klamath
weed to very low population densities, may be responsible for the elimination
of other insects specific to the weed because the area of discovery of the
other insects is too low to permit existence. 3. A highly
effective parasitoid of one stage of an insect is compared to an ineffective
one on a later stage: the first would reduce host populations and eliminate
the second in the same habitat. 4. Generally,
an herbivorous mammal might exterminate a moth through excessive reductions
of their common food supply (Nicholson 1957). Contemporary ecologists believe
that this would only happen locally but not generally, because a moth can
survive on much less food than the herbivore. 5.
Terrestrial organisms that alter large habitats, such as scarab beetles, are
especially risky biological control candidates because their activity may
overlap portions of the niche of other species, so that potential disruptive
side effects among organisms in different guilds exist. The outcome for
future symbovine fly control may be undesirable in that some potentially
regulative natural enemies, such as certain predatory arthropods, may now be
difficult to establish in the disrupted habitat. In the southwestern United
States, the predatory staphylinid genus Philonthus
is severely restrained from colonizing the drier dung habitat created by Onthophagus gazella F. activity. Thus, the
scarab, a non-homologue, may largely displace members of the genus Philonthus (Legner 1986b ). One might reasonably surmise that all
competitive displacement actually occurs between non-homologues, especially
when in the final analysis it is extremely difficult to find true homologues.
Even two individuals of the same species are never exactly the same in the
genetic sense. An informative review of competitive displacement and
exclusion is given by Ayala (1969), where it is demonstrated that two species
of Drosophila competing for
limited resources of food and space can coexist. Although the principle of
competitive exclusion was rejected, along with Gause's principle (Ayala
1969), there were sufficient differences in the competing species to account
for their coexistence. Competitive
Displacement and Biological Control Biological control
offers a good arena for the study of competitive displacement because natural
enemies which share the same food and which may approximate the ecological
homologue status are purposely and commonly brought together into the same
habitat. Biological control work since Smith (1929) has shown that
competition between parasitoids in multiple introductions has never caused a
less effective host regulation level. A second importation can only add to
the effectiveness of the first if chosen carefully (Legner 1986a ). Competitive
displacement may prove of practical value in insect eradication. The use of
an ecological homologue which itself is not a pest, may be used for
displacement of a pest. For example, Hermetia
illucens (L.), the soldier
fly, can eliminate Musca domestica breeding by larval
competition. The action comes about by Hermetia
changing the substrate to a semi-liquid, which is not suitable for Musca. Hermetia is effective in this capacity only in certain
relatively humid areas and not broadly throughout any given area, so that
competition results in a reduction and not elimination of Musca. It is been
suggested that mosquitoes and other pests of medical importance might be
replaced through larval competition of a pest of humans by an ecological
homologue which only attacks animals. In Sardinia, Anopheles labranchiae
Falleroni, a vector of malaria, was largely replaced by A. hispaniola
(Theobald), a non-vector. Relative survival of the non-vector was favored
under the eradication measures used. Eradication did not continue long
enough, however, to allow for complete displacement to occur. In East
Africa, spraying houses with dieldrin to control Anopheles funestus
Giles, a serious malaria vector, led to the mosquito's replacement by A. rivulorum Leeson. Anopheles
rivulorum is zoophilous,
preferring cattle, so its increase did not obstruct the goal of malaria
control. Fruit flies
might also be promising targets for competitive displacement, as exemplified
by the accidental cases of displacement in Australia and Hawaii that were
previously discussed. Hippelates
eye gnats might also be controlled with this method, although the alternative
should be carefully screened for possible undesirable attributes (Legner 1970). Exercise
12.1--How may competitive displacement be used to our advantage in
pest management? Exercise
12.2--What is an ecological homologue? Exercise
12.3--Describe in some detail at least 6 examples of competitive
displacement in nature. Exercise
12.4--Distinguish competitive displacement, exclusion and
coexistence. Exercise
12.5--Distinguish between competitive displacement by homologues
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