We have seen in Chapter 8 that a pest species is only a problem when its population density exceeds the damage threshold level. For different pests and different situations the population that is considered harmful will vary greatly, but the principle remains the same. A consideration of the factors that regulate insect numbers is thus of fundamental importance in relation to pest control. 

   The discussion on these matters that follows is in fact pertinent (恰当的；有关的) to all living organisms, not just to those we designate as (称作)pests, because all species of animals (and plants) are subject to（受支配的；易受……的） the same basic biological laws. Pests are only a special case because their activities conflict with human interests.

Put in its simplest terms, the numbers of an organism at any particular point in time are a consequence of the interaction between birth rate and death rate. The reproductive potential (biotic potential) of a species is always towards an increase in numbers which is counterbalanced (平衡，抗衡) by various factors in the environment which tend to restrict or depress numbers. The  outcome（结果） is reflected in the population density at any particular time. Environmental factors do not remain constant however, but tend to fluctuate in either a regular (e.g. seasonal) or erratic（不稳定的，无规律的） (e.g. daily weather) fashion. Insect populations do not therefore remain constant but also fluctuate with time. However, such fluctuations tend to be about outcome（结果） is reflected in the population density at any particular time. 

Environmental factors do not remain constant however, but tend to fluctuate in either a regular (e.g. seasonal) or erratic（不稳定的，无规律的） (e.g. daily weather) fashion. Insect populations do not therefore remain constant but also fluctuate with time. However, such fluctuations tend to be about a mean which does remain fairly constant over long periods of time, provided drastic alteration of the environment does not take place. In other words, populations of insects (and of other organisms) in the long term must operate on a “replacement only” basis otherwise they would tend towards either extinction or massive increase. The study of changes in insect numbers and of the factors that are responsible for them is that of insect population dynamics. It is a fast developing field which to some extent must involve mathematical treatment. Only the briefest discussion of basic concepts is attempted here.