Dynamics of Natural Population as a Basis for Biological Control
Some species of insects consistently occur in large number, an important characteristics of insect population phenomenon, while others comparatively rare, e.g. Butterflies and Bollworms most abundant, Army population densities are maintained year after year, The absolute number of one or all of the species may vary from time to time. The particular observed population densities are governed by the effect of some intrinsic (biotic) or extrinsic (abiotic) factors.
1) The Ground Rules:
The study of the dynamics of natural population has a long history. Thomas Malthus (1798) was the first to inquire into the means by which population levels are maintained. He put theory of human population dynamics as “Population when unchecked increases in a geometrical ratio”. Human population is regulated by flood, famine etc.
Charles Darwin (1859) put forward the theory the struggle for existence, ‘stronger will survive’. He was the first biologist to deal with the relative importance of competition, predation and climatic factors in this regard. As early as 1700, Linnaeus considered the importance of certain factors in the gross mortality of pests. Kollar Fitch and Walsh during 1800 have notably emphasized particular mortality factors. Howard and Fiske (1911) reported the various factors in population regulation of gypsy moths. After 65 years these ideas have been accepted, rejected, discussed and elaborated.
2) Natural Control as the Ecological Basis for Biological Control:
Insect population density never remains static but it is always fluid and changing. The individuals which make up the population are dying of starvation, predation, exposure or accidents and new individuals come into the population immigration also take place throughout the year. Thus, it seems that population can be both stable and changing in numbers at the same time. The population density of an organism may be constantly changing, the value tends to oscillate about a mean which is comparatively stable, but may change under certain control.