Natural selection reduce detrimental effects or eliminate interaction altogether, as continued severe depression of a prey or host population by predator or parasite or parasite populations can only lead to extinction of one or both populations. A severe impact of predation or parasitism is most frequently observed, when interaction is of recent origin (when two populations have just become associated) or when large scale or sudden changes have occurred in ecosystem (as might be produced by humans). Over long term, parasite host or predator-prey interaction tends to evolve to coexistence (Odum and Barrett, 2005). Deer populations tend to irrupt when predator pressure is reduced.

Kaibab deer herd, increased from 4000 (on 700,000 acres on north side of Grand Canyon in Arizona) in 1907 to 100,000 in 1924, coincident with a predator removal campaign organized by the U.S. Government as originally described by Leopold (1943) on the basis of estimates by Rasmussen (1941). Caughley (1970) reexamined this case and found that dear increased, overgrazed and then declined with no evidence that it was due solely to removal of predators. Cattle and fire may also have played a part. Caughley believed that irruptions of ungulate populations are more likely to result from changes in habitat or food quality, which enables the population to “escape” from usual mortality control. Negative interactions become less negative with time, if ecosystem is sufficiently stable and spatially diverse to allow reciprocal adaptations.

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Parasite host or predator-prey populations introduced into experimental microcosms or mesocosms usually oscillate violently, with a certain probability of extinction. Violent oscillations occur when a host, such as, house fly (Musca domestica) and a parasitic wasp (Nasonia vitripennis) are first placed together in a limited culture system. When individuals selected from cultures that had managed to survive to violent oscillations for two years were then re-established in new cultures. Interactions reveal that through genetic selection an ecological homeorhesis evolve in which both populations could coexist in a much more stable equilibrium (Odum and Barrett, 2005).

Predators sometimes reduce prey population below carrying capacity of their resources and thus may reduce competition and promote coexistence (Parrish and Saila, 1970). Effects of predation on diversity are well studied in aquatic ecosystem. Introduction of predatory fish can significantly change the community of primary consumers and producers (Carpenter et al., 1987, 1988).

Top predators like humans can easily reach the balance of a competitive equilibrium, so that exploited species is replaced by another species. Human beings strive to become more efficient at fishing, hunting and harvesting plants are examples of such shifts. Lotka-Volterra Predator-prey model represents interaction between predator and prey and predicts that predator and prey population will tend to cycle as is observed in natural predator-prey dynamics. However, in a number of cases, there is clear evidence that predators have a considerable impact on prey number.

2. Competition:

Competition refers to interaction of two organisms striving for same resource. Interspecific competition adversely affects growth and survival of two or more species populations.

Competition occurs in two forms: (1) Interference competition: and (2) Exploitation competition. Competition to bring about an ecological separation of closely related species is known as competitive exclusion principle. Simultaneously, competition triggers many selective adaptations that enhance co-existence of diverse organisms in a community. Interspecific competition is considered in terms of direct physical interaction versus exploitation competition. Interference competition occurs, when two species come into direct contact with each other, such as, fighting or defending a territory. Exploitation competition occurs, when one species exploits a resource, such as, food, space and can provide a competitive advantage for one species against another. Competitive interaction may involve space, food, light, waste materials, susceptibility to carnivores, disease, and other types of mutual interaction. Interspecific competition results in equilibrium adjustments between two species or if severe, in one species population replacing another, or forcing other to occupy another space or to use another food (whatever was the basis of original competitive action).

Closely related organisms having similar habits or morphologies often do not occur in same places. When they occur in same places, they frequently use different resources or are active at different times. Explanation for ecological separation of closely related (or otherwise similar) species is known as Gause principle (Gause 1932). One of Gause’s original experiments with ciliates (Gause, 1935) Paramecium caudatum and Paramecium aurelia is an example of competitive exclusion. When grown in separate cultures, they exhibited typical sigmoid population growth and maintained a constant population level in culture medium that was maintained with a fixed density of food items. When both protozoans were placed in same culture, P.

aurelia alone survived after 16 days. Neither organism attacked the other or secreted harmful substances. P. aurelia populations had a more rapid growth rate and thus “out competed” P. caudatum for the limited amount of food under existing conditions. Both P.

caudatum and P. bursaria were able to survive and reach a stable equilibrium in the same culture medium. Although competing for same food, P. bursaria occupied a different part of culture, where it could feed on bacteria without competing with P. caudatum. Thus, habitat for two species was different to coexist, even though their food was identical.

Brian (1956) first distinguished between indirect or exploitation competition and direct or interference competition. Interference competition appears more frequently through phylogenetic tree of animal life, from simple filter-feeding protozoans and cladocera, which usually compete in gathering food, to vertebrates, with aggression and territoriality. Slobodkin (1964) concluded on the basis of competition experiments with Hydra that these two types of competition overlap. It is useful to distinguish between two processes on theoretical grounds. That competition is most severe and competitive exclusion mostly likely to occur in systems where immigration and emigration are absent or reduced, such as, in laboratory cultures or mesocosms or on island or other natural systems with substantial barriers to inputs and outputs are on record. Probability of coexistence is higher in more typical open systems of nature.



In such interaction, one species has a marked negative effect on other, but there is no detectable reciprocal effect (-0). Lawton and Hassell (1981) refer to this interaction as asymmetrical competition. Amensalism is just one evolutionary step from interactions such as allelopathy (- +). C.

H. Muller studied inhibitors produced by shrubs in the vegetation of California Chaparral which showed chemical nature and physiological action of inhibitory substances as well as their importance in regulating composition and dynamics of community (Muller, 1966, 1969). Volatile terpenes produced by two species of aromatic shrubs inhibit growth of herbaceous plants. Volatile toxins (notably cineole and camphor) are produced in leaves and accumulate in soil during dry season to such an extent that when rainy season comes, germination and subsequent growth of seedlings is inhibited in a wide belt around each shrub group. Other shrubs produce water soluble antibiotics of dominance. However, periodic fires, which are an integral part of Chaparral, effectively remove the source of toxins, denaturing toxicated soil and triggering germination of fire-adapted seeds.

Accordingly, fire is followed in next rainy season by a conspicuous blooming of annuals, which continue to appear each spring until shrubs grow back and toxins again become effective. Interaction of fire and antibiotics thus perpetuates cyclic changes in composition that are adaptive feature of this type of ecosystem (Odum and Barrett, 2005).



This is a simple type of positive interaction, first step toward the development of beneficial relations, especially common between sessile plants and animals as well as mobile organisms. Commensalism may be called as the neutral situation between two dissimilar organisms, host and parasite, in which one partner, the parasite is benefitted and the other, the host is neither benefitted nor harmed. Shellfish or sponge contains various “uninvited guest”, neither organisms that require shelter of host but do neither harm nor good in return. Oysters sometimes have a small, delicate crab in mantle cavity.

These crabs are usually commensals although sometimes they overdo their guest status by partaking of host’s tissues. Dales (1957) listed 13 species as guests in burrows of large sea worms (Erehis) and burrowing shrimp (Callianassa and Upogebia). Commensal fish, clams, polychaetes worms and crabs lives by snatching surplus or rejected food or waste materials from host. Many commensals are not host specific, but some are found associated with only one species of host.

5. Protocooperation:

Allee (1951) stressed the importance of cooperation and aggregation among species.

Allee’s principle of aggregation states that cooperation between species is found throughout nature. Crabs and coelenterates often associate with mutual benefit. Coelenterates grow on backs of crabs, providing camouflage and protection as coelenterates have stinging cells.

In turn, coelenterates are transported about and obtain particles of food, when crab captures and eats another animal. Crab does not absolutely depend on coelenlerate, or vice versa. A further step in cooperation results in mutualism or obligate symbiosis, when each population becomes completely dependent on other. Often quite diverse kinds of organisms are associated. Mutualisms are most likely to develop between organisms with widely different requirements (organisms with similar requirements are more likely to get involved in competition).

For example, mutualism develops between autotrophs and heterotrophs, which are not surprising, as these two components of ecosystem must ultimately achieve some kind of balanced symbiosis. Examples that would be labeled as mutualistic go beyond general community interdependence to the extent that one particular kind of heterotroph becomes completely dependent on a particular kind of autotroph for food, and latter becomes dependent on protection, mineral cycling, or other vital function provided by heterotrophs. Mutualism is also common between microorganisms that can digest cellulose and animals that do not have necessary enzyme systems for this purpose. Mutualism seems to replace parasitism as ecosystems evolve toward maturity, and it seems to be especially important when some aspect of environment is limiting.