FILE: <bc-12.htm>                                                                                                                                                                                        GENERAL INDEX          [Navigate to   MAIN MENU ]




     AND COEXISTENCE Among Arthropods




---- Please CLICK on desired underlined categories [to search for Subject Matter, depress Ctrl/F ]:




Competitive Displacement of Non-homologues

Mechanisms of Competitive Displacement

Competitive Displacement and Biological Control



The Coexistence Principle



[Please refer also to Selected Reviews  &  Detailed Research ]



          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.


REFERENCES:   [Additional references may be found at  MELVYL Library ]

Aitken, T. H. G. & H. Trapido. 1961. Replacement phenomenon observed amongst Sardinian anopheline mosquitoes following eradication measures. In: "The Ecological Effects of Biological and Chemical Control of Undesirable Plants and Animals." p. 106-14. Symp. 8th Tech. Meeting Intern. Union for Conserv. Nature & Nat. Resources, Warsaw. E. J. Brill Publ., Leiden, Netherlands.

Andrewartha, H. G. 1963. Introduction to the Study of Animal Populations. Univ. of Chicago Press, Chicago. 281 p.

Andrewartha, H. G. & L. C. Birch. 1960. Some recent contributions to the study of the distribution and abundance of insects. Ann. Rev. Ent. 5: 219-42.

Andrewartha, H. G. & T. O. Browning. 1958. Williamson's theory of interspecific competition. Nature 181(4620): 1415.

Andrewartha, H. G. & L. O. Birch. 1954. The Distribution and Abundance of Animals. Univ. of Chicago Press, Chicago. 782 p.

Arbuthnot, K. D. 1955. European corn borer parasite complex near East Hartford, Connecticut. J. Econ. Ent. 48: 91-93.

Ayala, F. J. 1969. Experimental invalidation of the principle of competitive exclusion. Nature 224: 1076-79.

Ayala, F. J. 1972. Competition between species. Amer. Scient. 60: 348-57.

Bartlett, M. S. 1957. On theoretical models for competitive and predatory biological systems. Biometrika 44: 27-42.

Bartlett, B. R. & J. C. Ball. 1964. The developmental biologies of two encyrtid parasites of Coccus hesperidum and their intrinsic competition. Ann. ent. Soc. Amer. 57: 496-503.

Beauchamp, R. S. A. & P. Ullyett. 1932. Competitive relationships between certain species of freshwater triclads. J. Econ. 20: 200-208.

Beddington, J. R. et al. 1978. Characteristics of successful natural enemies in models of biological control of insect pests. Nature 273: 513-19. [A discussion of attributes of effective natural enemies based on theoretical models].

Beirne, B. P. 1960. Biological control research in Canada. In: "Biological Control of Insects of Medical Importance." Amer. Inst. Biol. Sci. Publ. Tech. Rept. 94-97.

Bellows, T. S., Jr. & T. W. Fisher, (eds) 1999. Handbook of Biological Control: Principles and Applications. Academic Press, San Diego, CA.  1046 p.

Bess, H. A., R. van den Bosch & F. R. Haramoto. 1961. Fruit fly parasites and their activities in Hawaii. Proc. Hawaiian Ent. Soc. 17: 367-78.

Birch, L. c. 1957. The meanings of competition. Amer. Naturalist 91: 5-18.

Birch, L. C. 1953. Experimental background to the study of the distribution and abundance of insects. III. The relation between innate capacity for increase and survival of different species of beetles living together on the same food. Evolution 7: 136-44.

Birch, L. C. 1961. Natural selection between two species of tephritid fruit flies of the genus Dacus. Evolution 15: 360-74.

Bowers, D. E. 1964. Natural history of two beach hoppers of the genus Orchestoidea with reference to their complemental distribution. Ecology 45: 677-96.

Brian, M. V. 1956. Segregation of species of the ant genus Myrmica. J. Anim. Ecol. 25: 319-37.

Brower, L. P. 1962. Evidence for interspecific competition in natural populations of the monarch and queen butterflies, Danus plexippus and D. qilippus berenice in south central Florida. Ecology 43: 549-52.

Brown, W. L., Jr. & E. O. Wilson. 1956. Character displacement. Syst. Zool. 5: 49-64.

Caldwell, L. D. & J. B. Gentry. 1965. Interactions of Peromyscus and Mus in a one-acre field enclosure. Ecology 46: 189-92.

Campbell, A. et al. 1974. Temperature requirements of some aphids and their parasites. J. Applied Ecology 11: 431-38. [Last paragraph addresses why some parasitoids may be incapable of controlling their hosts].

Chaing, C. L. 1954. Competition and other interactions between species. In: "Statistics and Mathematics in Biology," p. 197-215. Iowa St. Coll. Press, Ames.

Christenson, L. D. & R. H. Foote. 1960. Biology of fruit flies. Ann. Rev. Ent. 5: 171-92.

Clark, A. H. 1931. The extirpation of one butterfly by another. Sci. Monthly 33(2): 173-74.

Cole, L. C. 1960. Competitive exclusion. Science 132(3423): 348-49.

Connell, J. H. 1961. The influence of interspecific competition and other factors on the distribution of the barnacle Chthamalus stellatus. Ecology 42: 710-23.

Cooper, D. M. & T. Dobzhansky. 1956. Studies on the ecology of Drosophila in the Yosemite region of California. I. The occurrence of species of Drosophila in different life zones and at different seasons. Ecology 37: 526-33.

Crombie, A. C. 1945. On competition between different species of graminivorous insects. Proc. Roy. Soc. (London) B, 132: 362-95.

Crombie, A. C. 1946. Further experiments on insect competition. Proc. Roy. Soc. (London) B, 133: 76-109.

Crombie, A. C. 1947. Interspecific competition. J. Anim. Ecol. 16: 44-73.

Cunha, A. B. da, T. Dobzhansky & A. Sokoloff. 1951. On food preferences of sympatric species of Drosophila. Evolution 5: 97-101.

Dawson, P. S. & I. M. Lerner. 1962. Genetic variation and indeterminism in interspecific competition. Amer. Naturalist 96(891): 379-80.

Darwin, C. 1909. On the Origin of Species (1859). Reprinted by Cassell & Co., Ltd., London. 430 p.

DeBach, P. 1954. Relative efficacy of the red scale parasites Aphytis chrysomphali Mercet and Aphytis "A" on citrus trees in southern California. Boll. Lab. Zool. Agr., Portici, 33: 135-51.

DeBach, P. & P. Sisojevic. 1960. Some effects of temperature and competition on the distribution and relative abundance of Aphytis lingnanensis and A. chrysomphali. Ecology 41: 153-60.

DeBach, P. 1962. Ecological adaptation of parasites and competition between parasite species in relation to establishment and success. Proc. 11th Intern. Congr. Ent., Wien, 1960, 11: 686-90.

DeBach, P. 1964. Some ecological aspects of insect eradication. Bull. Ent. Soc. Amer. 10(4): 221-24.

DeBach, P. 1965. Some biological and ecological phenomena associated with colonizing entomophagous insects. In: "Genetics of Colonizing Species." Academic Press, N.Y.

DeBach, P. 1966. The competitive displacement and coexistence principles. Ann. Rev. Ent. 11: 183-212.

DeBach, P. & R. A. Sundby. 1963. Competitive displacement between ecological homologues. Hilgardia 34: 105-66.

Diver, C. 1940. The problem of closely related species living in the same area. In: "The New Systematics." p. 303-28. Huxley, J. (ed.), Oxford Univ. Press, London.

Dobzhansky, T. 1961. Man and natural selection. Amer. Scientist 49(3): 285-99.

Doutt, R. L. & P. DeBach. 1964. Some biological control concepts and questions. In: "Biological Control of Insect Pests and Weeds." Chap. 5, 124-28. P. DeBach (ed.). Chapman & Hall, London, Reinhold, N.Y. 844 p.

Dumas, P. C. 1956. The ecological relations of sympatry in Plethodon dunni and Plethodon vehiculum. Ecology 37: 484-95.

Dumas, P. C. 1964. Species-pair allopatry in the genera Rana and Phrynosoma. Ecology 45: 178-81.

Dybas, H. S. & M. Lloyd. 1962. Isolation by habitat in two synchronized species of periodical cicadas. Ecology 43: 432-44.

Elton, C. 1927. Animal Ecology. Sedgwick & Jackson, Ltd., London. 207 p.

Elton, C. 1946. Competition and the structure of animal communities. J. Anim. Ecol. 15: 54-68.

Elton, C. S. 1958. The Ecology of Invasions by Animals and Plants. Methuen & Co., Ltd. London 181 p.

Elton, C. S. & R. S. Miller. 1954. The ecological survey of animal communities, with a practical system of classifying habitats by structural characters. J. Ecol. 42: 460-96.

Flanders, S. E. 1958. Moranila californica as a usurped parasite of Saissetia oleae. J. Econ. Ent. 50: 247-48.

Flanders, S. E. 1964. Some biological control aspects of taxonomy exemplified by the genus Aphytis (Hymenoptera: Aphelinidae). Canad. Ent. 96: 888-93.

Flanders, S. E. 1965. Competition and cooperation among parasitic Hymenoptera related to biological control. Canad. Ent. 97: 409-22.

Flanders, S. E. 1966. The circumstances of species replacement among parasitic Hymenoptera. Canad. Ent. 98: 1099-24.

Foott, W. H. 1963. Competition between two species of mites. II. Factors influencing intensity. Canad. Ent. 95: 45-57.

Force, D. C. 1972. r and K-strategists in endemic host-parasitoid communities. Bull. Ent. Soc. Amer. 18: 135-37.

Force, D. C. & P. S. Messenger. 1968. The use of laboratory studies of three hymenopterous parasites to evaluate field potential. J. Econ. Ent. 61: 1374-78.

Frank, P. W. 1957. Coactions in laboratory populations of two species of Daphnia. Ecology 38: 510-19.

Franz, J. M. 1973a. Quantitative evaluation of natural enemy effectiveness. Introductory review of the need for eavluation studies in relation to integrated control. J. Appl. Ecol. 10: 321-23.

Franz, J. M. 1973b. The role of biological control in pest management. Bull. Lab. Entomol. Agraria 30: 235-43.

Furman, D. P., R. D. Young & E. P. Catts. 1959. Hermetia illucens (Linn.) as a factor in the natural control of Musca domestica Linn. J. Econ. Ent. 52: 917-21.

Forbes, S. A. 1880. On some interactions of organisms. Bull. Ill. Nat. Hist. Surv. 1: 3-17.

Gause, G. F. 1934. The Struggle for Existence. William & Wilkins Co., Baltimore. 163 p.

Gause, G. F. & A. A. Witt. 1935. Behavior of mixed populations and the problem of natural selection. Amer. Naturalist 69: 596-609.

Gause, G. F. 1936. The principles of biocoenology. Quart. Rev. Biol. 11: 320-36.

Gilbert, O., T. B. Reynoldson & J. Hobart. 1952. Gause's hypothesis: an examination. J. Anim. Ecol. 21: 310-12.

Gillies, M. T. & A. Smith. 1960. The effect of a residual house spraying campaign in East Africa on species balance in the Anopheles funestus group. The replacement of A. funestus Giles by A. rivulorum Leeson. Bull. Ent. Res. 51: 243-52.

Goeden, R. D. 1983. Critique and revision of Harris' scoring system for selection of insect agents in biological control of weeds. Prof. Ecol. 5: 287-301.

Goeden, R. D. 1976. Biotic interference with insects imported for weed control. Ann. Rev. Ent. 21: 325-42.

Grinnell, J. 1904. The origin and distribution of the chestnut backed chickadee. Auk 21: 364-82.

Grinnell, J. 1928. The presence and absence of animals. Univ. of Calif. Chronicle 30: 429-50 (Reprinted in: Joseph Grinnell's Philosophy of Nature; selected writings of a western naturalist. Univ. of Calif. Press, Berkeley 1943. 237 p.)

Hairston, N. G. 1951. Interspecies competition and its probable influence upon the vertical distribution of Appalachian salamanders of the genus Plethodon. Ecology 32: 266-74.

Hairston, N. G. 1959. Species abundance and community organization. Ecology 40: 404-16.

Hairston, N. G. & S. L. Kellerman. 1965. Competition between varieties 2 and 3 of Paramecium aurella: the influence of temperature in a food-limited system. Ecology 46: 134-39.

Hardin, G. 1960. The competitive exclusion principle. Science 131(3409): 1292-97.

Harper, J. L., J. N. Clatworthy, I. H. McNaughton & G. R. Sagar. 1961. The evolution and ecology of closely related species living in the same area. Evolution 15: 209-27.

Hartman, W. D. 1957. Ecological niche differentiation in the boring sponges. Evolution 11: 294-97.

Haskins, C. P. & E. F. Haskins. 1965. Pheidole megacephala and Iridomyrmex humilis in Bermuda--equilibrium or slow development? Ecology 46: 736-40.

Hassell, M. P. 1969a. A population model for the interaction between Cyzenis albicans (Fall.) (Tachinidae) and Operophtera brumata (L.) (Geometridae) at Wytham, Berkshire. J. Anim. Ecol. 38: 567-76.

Hassell, M. P. 1969b. A study of the mortality factors acting upon Cyzenis albicans (Fall.), a tachinid parasite of the winter moth, Operophtera brumata (L.). J. Anim. Ecol. 38: 329-39.

Hassell, M. P. 1978. The Dynamics of Arthropod Predator-Prey Systems. Princeton Univ. Press, Princeton, New Jersey.

Hassell, M. P. 1980. Foraging strategies, population models and biological control: A case study. J. Anim. Ecol. 49: 603-28.

Heatwole, H. & D. M. Davis. 1965. Ecology of three sympatric species of parasitic insects of the genus Megarhyssa. Ecology 46: 140-50.

Hokyo, N. & K. Kiritani. 1963. Two species of egg parasites as contemporaneous mortality factors in the egg population of the southern green stink bug, Nezara viridula. Japan J. Appl. Zool. 3: 214-27.

Holloway, J. K. 1958. The biological control of the Klamath weed in California. Proc. 10th Intern. Congr. Ent., Montreal, 1956 4: 557-60.

Huffaker, C. P., P. S. Messenger & P. DeBach. 1971. The natural enemy component in natural control and the theory of biological control. In: "Biological Control," C. B. Huffaker (ed.), pp. 16-67. Plenum Press. 511 p.

Hughes, R. D. et al. 1974. The selection of natural enemies for the biological control of the Australian bushfly. J. Applied Ecol. 11: 483-88. [Addresses the concept of pre-introductory evaluation for biological control of a native pest].

Hurlbert, S. H. 1975. Secondary effects of pesticides on aquatic ecosystems. San Diego St. Univ. Center For Marine Studies, Contrib. No. 6. Springer-Verlag, New York. p. 81-148.

Hurlbert, S. H. & M. S. Mulla. 1981. Hydrobiologia 83: 125-51.

Hurlbert, S. H., J. Zedler & D. Fairbanks. 1972. Ecosystem alteration by mosquito fish (Gambusia affinis) predation. Science 175: 639-41.

Hutchinson, G. E. 1953. The concept of pattern in ecology. Proc. Acad. Nat. Sci., Phila. 105: 1-12.

Hutchinson, G. E. 1957. Concluding remarks. Cold Spring Harbor Symp. Quant. Biol. 22: 415-27.

Hutchinson, G. E. 1964. The lacustrine microcosm reconsidered. Amer. Scien. 52: 334-41.

Hutchinson, G. E. & E. S. Deevey, Jr. 1949. Ecological studies on populations. In: "Survey of Biological Progress." Academic Press, N.Y. p. 325-59. 396 p.

Kiritani, K., N. Hokyo & J. Yukawa. 1963. Coexistence of the two related stink bugs Nezara viridula and N. antennata under natural conditions. Res. Pop. Ecol. 5: 11-22.

Klomp, H. 1961. The concepts "similar ecology" and "competition" in animal ecology. Arch. Nederl. Zool. 14: 90-102.

Kostitsin, V. A. 1937. Biologie Mathematique. Librairie Armand Colin, Paris 223: 193 p.

Kuenzler, E. J. 1958. Niche relations of three species of lycosid spiders. Ecology 39: 494-500.

Lack, D. 1944. Ecological aspects of species formation in passerine birds. Ibis 86: 260-86.

Laird, M. 1959. Biological solutions to problems arising from the use of modern insecticides in the field of public health. Acta Trop. 16: 331-55.

34.   Legner, E. F.  1966.  Competition among larvae of Hippelates collusor (Diptera: Chloropidae) as a natural control factor.  J. Econ Entomol. 59(6):  1315-1321.


64.   Legner, E. F.  1970.  Attraction of Hippelates eye gnats and other minute Diptera to baits and man with considerations on competitive  displacement by exotic non-problem species.  Proc. Calif. Mosq. Contr. Assoc., Inc. 37:  119-126.


216.   Legner, E. F.  1983.  Imported cichlid behaviour in California.  Proc. Intern. Symp. on Tilapia in aquaculture, Nazareth, Israel, 8-13 May, 1983.  Tel Aviv Univ. Publ. 59-63.


226.   Legner, E. F.  1986a.  Importation of exotic natural enemies.  In:  pp. 19-30, "Biological Control of Plant Pests and of Vectors of Human and Animal  Diseases."  Fortschritte der Zool. Bd. 32:  341 pp.


227.   Legner, E. F.  1986b.  The requirement for reassessment of interactions among dung beetles, symbovine flies and natural enemies.  Entomol. Soc. Amer. Misc. Publ. 61:  120-131.


212.   Legner, E. F. & F. W. Pelsue, Jr.  1983.  Contemporary appraisal of the population dynamics of introduced cichlid fish in south California.  Proc. Calif. Mosq. & Vector Contr. Assoc., Inc. 51:  38-39.


217.   Legner, E. F. & R. D. Sjogren.  1984.  Biological mosquito control furthered by advances in technology and research.  J. Amer. Mosq. Contr. Assoc. 44(4):  449-456.

Leslie, P. H. & J. C. Gower. 1958. The properties of a stochastic model for two competing species. Biometrika 45: 316-30.

Lotka, A. J. 1925. Elements of Physical Biology. Williams & Wilkins Co., Baltimore. 460 p.

Luck, R. F. 1985. Competitive exclusion of Aphytis lingnanensis by A. melinus: potential role of host size. Ecology 66: 904-13.

MacArthur, R. H. 1958. Population ecology of some warblers of northeastern coniferous forests. Ecology 39: 599-619.

Mayr, E. 1947. Ecological factors in speciation. Evolution 1: 163-88.

Mayr, E. 1948. The bearing of the new systematics on genetical problems. The nature of species. Advan. Genet. 2: 205-37.

Mayr, E. 1966. Animal Species and Evolution. The Belnap Press of Harvard Univ. Press, Cambridge, Mass. 797 p.

McIntosh, R. P. 1963. Ecosystems, evolution and relational patterns of living organisms. Amer. Scien. 51: 246-67.

Merrell, J. J. 1951. Interspecific competition between Drosophila funebris and Drosophila melanogaster. Amer. Naturalist 85: 159-69.

Miller, J. C. 1983. Ecological relationships among parasites and the practice of biological control. Environ. Ent. 12: 620-24.

Miller, R. S. 1964a. Larval competition in Drosophila melanogaster and D. simulans. Ecology 45: 132-48.

Miller, R. S. 1964b. Ecology and distribution of pocket gophers in Colorado. Ecology 45: 256-72.

Mills, N. J. 1983. Possibilities for the biological control of Choristoneura fumiferana (Clemens) using natural enemies from Europe. Biocontrol News and Information 4: 103-25. [A preintroductory assessment of how to bring about classical biological control of spruce budworm in its native home].

Milne, A. 1961. Mechanisms in biological competition: definition of competition among animals. Symp. Soc. Exptal. Biol. 15: 40-61.

Moore, J. A. 1952. Competition between Drosophila melanogaster and Drosophila similans. I. Population cage experiments. Evolution 6: 407-20.

Neyman, J., T. Park & E. L. Scott. 1956. Struggle for existence: the Tribolium model: biological and statistical aspects. Proc. Symp. Math. Statistics and Probability, 3rd ed. Berkeley 4: 41-79.

Nicholson, A. J. 1933. The balance of animal populations. J. Anim. Ecol. 2: 132--.

Nicholson, A. J. 1957. The self-adjustment of populations to change. Cold Spring Harbor Symp. Quant. Biol. 22: 153-72.

Park, T. 1948. Experimental studies of interspecies competition. I. Competition between populations of the flour beetles, Tribolium confusum Duval and Tribolium castaneum Herbst. Ecol. Monog. 18: 265-307.

Park, T. 1954. Experimental studies of interspecies competition. II. Temperature, humidity and competition in two species of Tribolium. Physiol. Zool. 27: 177-238.

Park, T. 1955. Experimental competition in beetles, with some general implications. In: "The Numbers of Man and Animals." p. 69-82. Oliver & Boyd, Ltd., London.

Park, T. 1955. Ecological experimentation with animal populations. Sci. Monthly 81: 271-75.

Park, T. 1957. Experimental studies of interspecies competition. III. Relation of initial species proportion to competitive outcome in populations of Tribolium. Physiol. Zool. 30: 22-40.

Park, T. 1962. Beetles, competition and populations. Science 138: 1369-75.

Park, T., E. V. Gregg & C. Z. Lutherman. 1941. Studies in population physiology. X. Interspecific competition in populations of granary beetles. Physiol. Zool. 14: 395-430.

Park, T., P. H. Leslie & D. B. Mertz. 1964. Genetic strains and competition in populations of Tribolium. Physiol. Zool. 37: 97-162.

Patten, B. D. 1961. Competitive exclusion. The exclusion principle is recast in the context of a generalized scheme for interspecific interaction. Science 134(3490): 1599-1601.

Patten, B. C. 1964. Effects of radiation stress on interspecific competition. Oak Ridge Natl. Lab., Radiation Ecol. Sect., Publ. No. 107: 104-8.

Pemberton, C. E. & H. F. Willard. 1918. Interrelations of fruit fly parasites in Hawaii. J. Agric. Res. 12: 285-95.

Polnik, A. 1960. Effects of some intraspecies processes on competition between two species of flour beetles, Latheticus oryzae and Tribolium confusum. Physiol. Zool. 33: 42-57.

Pontin, A. J. 1960. Field experiments on colony foundation by Lasius niger (L.) and L. flavus (F.). Insectes Sociaux 7(3): 227-30.

Pontin, A. J. 1961. Population stabilization and competition between the ants Lasius flavus (F.) and L. niger (L.). J. Anim. Ecol. 30: 47-54.

Pontin, A. J. 1963. Further considerations of competition and the ecology of the ants Lasius flavus (F.) and L. niger (L.). J. Anim. Ecol. 32: 565-74.

Price, P. W. 1972. Methods of sampling and analysis for predictive resultsin the introduction of entomophagous insects. Entomophaga 17: 211-22. [Preintroductory evaluation based on niche parameters].

Remington, C. L. 1968. Suture zones of hybrid interaction between recently joined biotas. Evol. Biol. 2: 321-428.

Richards, O. W. 1963. Some factors controlling insect populations living on scotch broom. Proc. 16th Intern. Congr. Zool., Wash., D.C. 3: 353-56.

Rivnay, E. 1964. Influence of man on insect ecology in arid zones. Ann. Rev. Ent. 9: 41-62.

Ross, H. H. 1957. Principles of natural coexistence indicated by leafhopper populations. Evolution 11: 113-29.

Savage, J. M. 1958. The concept of ecological niches with reference to the theory of natural coexistence. Evolution 12: 111-12.

Schuster, M. F. & H. A. Dean. 1976. Competitive displacement of Anagyrus antoninae ([Hym.: Encyrtidae] by its ecological homologue Neodusmetia sangwani [Hym.: Encyrtidae]. Entomophaga 21: 127-30.

Schwerdfeger, F. 1942. Uber die Ursachen des Massenwechsels der Insekten. Z. angew. Ent. 28: 254-303.

Simpson, G. G. 1964. Organisms and molecules in evolution. Science 146(3651): 1535-38.

Slobodkin, L. B. 1962. Growth and regulation of Animal Populations. Holt, Rheinhart & Winston, N.Y. 184 p.

Smith, H. S. 1929. Multiple parasitism: its relation to the biological control of insect pests. Bull. Ent. Res. 20: 141-49.

Solomon, M. E. 1957. Dynamics of insect populations. Ann. Rev. Ent. 2: 121-42.

Trujillo, E. E. & G. E. Templeton. 1981. The use of plant pathogens in biological control of weeds. In: D. Pimentel (ed.) Agric. Handbk. Ser: Integrated Pest Management. Boca Raton, Florida. CRC Press, Inc.

Turnbull, A. L. 1967. Population dynamics of exotic insects. Bull. Ent. Soc. Amer. 13: 333-37.

Udvardy, M. D. F. 1951. The significance of interspecific competition in bird life. Oikos 3: 98-123.

Udvardy, M. D. F. 1959. Notes on the ecological concepts of habitat biotype and niche. Ecology 40: 725-28.

Utida, S. 1953. Interspecific competition between two species of bean weevil. Ecology 34: 301-07.

Utida, S. 1957. Population fluctuation, an experimental and theoretical approach. Cold Spring Harbor Symp. Quant. Biol. 22: 139-50.

van den Bosch, R. & F. H. Haramoto. 1953. Competition among parasites of the oriental fruit fly. Proc. Hawaiian Ent. Soc. 15: 201-06.

van den Bosch, R., E. I. Schlinger, E. J. Dietrick, J. C. Hall & B. Puttler. 1964. Studies on succession, distribution and phenology of imported parasites of Therioaphis trifolii (Monell) in southern California. Ecology 45: 602-21.

Van Valen, L. 1960. Further competitive exclusion. Science 132(3440): 1674-75.

Varley, G. C. 1949. Special review: Population changes in German forest pests. J. Anim. Ecol. 18: 117-22.

Venkatraman, T. V. 1964. Experimental studies in superparasitism and multiparasitism in Horogenes cerophaga (Grav.) and Hymenobosmina rapi (Cam.), the larval parasites of Plutella maculipennis (Curt.). Indian J. Ent. 16: 1-32.

Volterrra, V. 1931. Variations and fluctuations of the number of individuals in animal species living together [Trans. in "Animal Ecology." Chapman, p. 409-48 (1931).].

Weatherley, A. H. 1963. Notions of niche and competition among animals with special reference to freshwater fish. Nature 197(4862): 14-17.

Whittaker, R. H. 1965. Dominance and diversity in land plant communities. Numerical relations of species express the importance of competition in community function and evolution. Science 147(3655): 250-60.

Williams, C. B. 1947. The generic relations of species in small ecological communities. J. Anim. Ecol. 16: 11-18.

Williamson, M. H. 1957. An elementary theory of interspecific competition. Nature 180: 422-25.

Yoshida, T. 1960. Adult longevity under the condition of interspecific competition. Mem. Fac. Liberal Arts Educ. Miyasski Univ. 9: 463-72.

Zimmerman, J. R. 1960. Seasonal population changes and habitat preferences in the genus Laccophilus. Ecology 41: 141-52.