Infectious diseases and population growth The principle of population dynamics declares that it is environmental factors such as changes in physical conditions, food supplies and the occurrence of natural enemies that determine the number of an animal species in existence in a given area at a given point in time, and that these factors vary all the time, as the organic and inorganic environment is in a constant state of change. Every change that takes place in a species’ living conditions is reflected in its numbers. Examined over a period of time, this takes the form of a constant search for a state of equilibrium, which in an undisturbed situation would result in a fluctuation around a certain average figure.