COMPETITIVE DISPLACEMENT, EXCLUSION
AND COEXISTENCE Among Arthropods
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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 and non-homologues.
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