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COMPETITIVE DISPLACEMENT, EXCLUSION
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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
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