Some estimates of competitive displacement and coexistence of exploiting species at various trophic levels have also been made. Even models to explain host-parasite and predator-prey relationships have recently been prepared (see Huffaker and Wilson, 1974). Appropriate techniques have been developed to assess the effects of various stressful conditions, including mortality factors, on natural populations. The balance of nature ensures prevention of the surplus reproductive potentials so characteristic of living organisms.
Although very large numbers of offspring may be produced by a species, not all of them can survive and in this respect, natural population sizes remain fairly uniform from year to year. This is due to the homoeostasis (self-regulation) factor. Some recent studies in America indicate that in a community greater diversity of enemies is correlated with greater stability. According to Huffaker and Wilson, a host-specific phytophagous insect effective in biological control will, by controlling a dominant plant species, pave the way for other plant species.
For a proper study of population dynamics, analysis of mortality factors is a vital requirement. The importance of a mortality factor is not determined merely by the mortality it causes in a given generation; a basic additional consideration is which of the factor or factors are more responsive to density change and can act compensatory, thereby leading to enhanced mortality in the absence of a previously acting factor. Suitable evaluation techniques for the assessment of such important and contemporaneously acting factors have been described and reviewed by Varley et al. (1974). In recent years much useful work on understanding the role of sex hormones (pheromones) in the mating behaviour of various parasites and predators has been reported and such pheromones have been increasingly used as monitoring tools for insect pests.
However, sometimes the attempts to use pheromones for control are not advisable in view of adverse effects on enemy populations. Jones et al. (1973) have shown that certain “kairomones” produced by host insects can attract certain natural enemies thereby encouraging ready host-searching. One central aspect of the life activity of all predators is undoubtedly the intense intra-specific competition for food.
A similar situation is applicable to phytophagous species in respect of their host plants. Some predators have a primary and an alternate prey. The indiscriminate use of pesticides in crop fields can sometime seriously disrupt and upset the ecological balance involving primary and alternate prey. Recent researches on biological control have led to the unravelling of intra- and inter-specific variations in the properties of the natural enemies of various pests. The diversity of natural enemy species seems to be a consequence of the high degree of ecological specialization involving, as it does, various differences in adaptation to a multitude of factors regulating host abundance.
Genetically variability in parasites is another factor of crucial significance since this makes possible to select distinct strains endowed with certain well-defined and/or biologically desirable traits such as pesticide tolerance, host-range, or specificity. Schippers (1971) has stressed the need for integrated research on stabilizing mechanisms in soil microorganisms. In an edaphic environment stability can be characterized as the stability of a dynamic equilibrium in which individual components or organisms may be constantly changing, but compensatory changes taking place in other units tend to maintain the overall functional balance. Some soils seem to possess a kind of buffering capacity against plant diseases and the recognition of such differences in the buffering potential of soils against plant diseases has led to the recent concept of the “antiphytopathogenic potential” (Reinmuth, 1963). Buffer functions in agricultural soils against epidemic development of pathogens include disease decline, existence of disease suppressive soils, and development of antiphytopathogenic potential induced or enhanced by organic amendments, crop rotation or weed tolerance. The last decade has witnessed substantial progress in the understanding of such phenomena in crop management as act against epidemics of certain pathogens (Mitchell, 1973).
Kennedy (1974) is of the opinion that intra-specific competition, host immune responses and parasite induced host mortality may function as negative feedback controls upon parasite populations, but that they may not do so in many natural habitats in view of the fact that parasite numbers are maintained at a low level by heavy mortality caused during transmission from one host to another. In the absence of feedback controls upon population numbers, parasite populations are unstable and hence susceptible to changes in their ecosystems resulting from human activities! Changes in the ecosystems can either increase or decrease the efficacy of transmission and thereby often lead to epizootics or even extinction.