资料题：The example of seasonal changes in animal populations is to illustrate（　）。

A number of factors determine the growth of animal communities, usually measured in terms of population density patterns.Populations may mature and remain stable, may deviate regularly while remaining with a specified range, or may alternate irregularly between large growths or sudden declines.The annual increase in births, following the spring months, in conjunction with the heavy mortality of the winter months is a good example of regular deviation.The high density of a population might make a species vulnerable to limitations of such density dependent factors as space, food, or nesting sites.The increase of susceptibility(易感性)to disease and parasites is another factor.As animal communities grow, the likelihood of contracting disease by an individual member becomes great: the potential of disease transmission is frequently in proportion with the population density.A ubiquitous(普遍存在的)factor responsible for irregular changes in population density is the weather, which unlike most others is not density-dependent.If the temperature drops below freezing for 24 hours, 25 percent of a given group might die regardless of the original density.Such fluctuations are particularly common among insects.Interestingly enough, insects, which unlike large animals coping with weather changes by means of internal temperature controls are also the most likely creatures to develop massive population, increase when weather conditions are optimal.