Biological Control – Natural Control and Balance of Nature
Natural control is defined as the ‘maintenance of the population dynamics equilibrium within particular upper and lower limits over a period of time by a complex combination of all the environment factors affecting upon the population’. The rebounding of the population in returning to the characteristic mean density after periods of positive or negative excess is most important. Consequence of this result to a particular species continued to increase or decrease in number. This is called “Balance of Nature”.
The ‘Balance of nature’ is the result of natural regulative processes in the environment of every living thing and it assures that a species will neither decline in numbers to extinction not increase to infinite density. Regardless of whether a species is abundant or scarce, the average characteristic density of its population in a given habitat is constant. The importance of this ecological principle should be self evident for without it the living natural world would cease to exist.
It is customary in biological control work to describe the species of animals and plants which live at the expense of other animals and plants as natural enemies of the latter. Any given species in a community with few exceptions is attacked and fed on by one or more such natural enemies, and indication of the tremendous potential for biological control.
All organisms are capable of increasing in numbers through process of reproduction. Most insects are notable for their high potential rate of numerical increase because their relatively very high fecundities and short life cycles. But the fact is that most of organism including rapidly breeding insect do not increase over successive generations or for prolong period. On the contrary they increase only periodically and to limited extent a consequence of “Natural controls” present in their environments (Solomon 1949).
The natural control generally limits the number of insects. Such checks to numerical growth include limited resources (Food, Space, and Shelter) periodically occurring inclement (severe) weather or other hazards (Heat, Cold, Wind, Rain, and Draught), competition from themselves or from other kind of animals and natural enemies (Predator, parasites and pathogens)
The major component of natural control of population numbers:
Natural Control: 1. Density independent factors, 2. Density dependent factors
1. Density independent factors:
a. Physical: E.g Temperature, Humidity, Air movement, Exposure.
b. Biological: E.g Host Suitability, Food Quality.
2. Density dependent factors:
a. Non reciprocal: E.g Some Foods, Space territoriality.
b. Reciprocal: Parasites, predators, pathogen, herbivorous, some food sources.
In nature probable every insect population is attacked to some degree by one or more natural enemies, referred to entomophagous insects. Clausen (1940) stated that every phytophagous insect species is attacked by such one or more parasitic or predatory insects. Other predatory animals e.g. some birds, certain mammals, toads, frogs, Lizards etc. can also act as natural control agents.
The results of natural control are the regulation of numbers, preventing the population from becoming too high or relaxing certain suppressive influence when the population becomes low. The occurrence of this situation the long term maintenance of the population at a characteristic level of abundance relative to other organisms in the community is the demonstration of the “Balance of Nature”. The mechanisms and interactions between the population and its environment which bring about the relative of numbers constitute the natural control of populations “Natural control” includes the collective forces of the environment which serve to hold a given population in check against its own capability for numerical growth. The natural control is governed through influences of mortality factors and reproductive factors includes climatic factors such as excessive heat, or cold or disappearance or deterioration of food resources, the action of competing species and natural enemies. The environmental factors which act as mortality agents at intensities unaffected by the size of the population e.g. weather this limiting factor is called a ‘Density Independent mortality factor’. Whereas those limiting factor whose intensities of action vary with the abundance of the species in question e.g. predator which consumes proportionally more prey when they later are more abundant then when they are scares is called ‘Density Independent mortality factor’. Natural enemies have the capability of acting as density dependent mortality factor though they may not act this way depending their own environmental limitations. All natural enemies used in contributing to successful biological control programmes act in this way.
Density depended factors may be classified according to whether they vary in number (or magnitude) as the host numbers change (reciprocal action) or whether their numbers (or magnitude) remain fixed (non reciprocal action). The parasitoids and predators are reciprocal in action, since they commonly increase in numbers when their host or prey become numerous as decrease as the hosts or prey become suppressed. Thus the enemies control their hosts and the host controls the enemies. Space is an example of a non reciprocal factor since it does not mold (wax) or wane (decrease in area) as the user population rises or fall “space” can control the numbers of the users, but the users do not alter the amount of space present e.g. some herbivorous insects can be limited by the amount of “food” (host plant) available to them but often do not influence the numbers of plants present for insect populations to remain in relative numerical balance within their normal communities for substantial periods of time, it appears to us necessary that there be one or more density dependent mortality agents involved in the natural control of such populations. Such mortality agent being responsive to increase in the density of the population serve as regulators to check this increase as the population density declines the regulative action of these agents’ moderates allowing the population to rise again.
When an insect population is maintained at a characteristic level of abundance by the effects of natural control agents (including all mortality agents, both density dependent and density independent in action) usually there is a substantial contribution from natural enemies to the total mortality occurring in any generation. There are about 10,000 to 30,000 insects’ species out of 1 million or more species of to be recognized as economically important. When insect species invade new geographic regions either commercial activities of man or accidentally they may increase to extra ordinarily high numbers mainly because they have escaped the controlling influences of their customary natural enemies. Overall generation mortality is greatly diminished while reproductive capacity of the space is remain high. The population of such high invades increases in numbers at an exponential rate, causing population outbreak. When such an invading insect is injurious, pest outbreaks occurs and the control efforts must be needed. These efforts directs to search for and colonization of any adopted natural enemies that remain behind in the native home of the invading species. Virtually all successful classical biological control programs to date have resulted from the re-association of invading pests of foreign origin with their adopted natural enemies (De Bach, 1964). However, not all insects’ species are of foreign origin, some may include native also e.g. Cotton bollworm.
The biological control of native pest species is more complicated than foreign species because it is presumed that the native species have already associated with their natural enemies. There are some other factors that favors unduly increase in number of native pest species. The agricultural practices of the growers may often interfere with the efficiency of natural enemies on exerting their share of natural control.
Biological Control of Such Native Pest can then Assume Several Routes:
1) The introduction of natural enemies of the foreign origin which are associated with related pest species.
2) The modification of agricultural and other practices with the intention of enhancing native natural enemy action.
3) The employment of other pest control techniques, chemical control in particular, to bring about the integrated control of such pests.