[An approach to a graphical representation of regulation and determination in population dynamics].

The present paper continues a line which has been marked by publications of WILBERT (1962) and SCHWERDTFEGER (1968) and aims at a cybernetical interpretation of the dynamics of animal (especially insect) populations. Here, the author tries to apply this conception to concrete situations in the population dynamics of two Arthropoda.On two spruce plantations, one of them being experimentally differentiated in plots, the plants of the other being classified according to their needles' length, differences in population density ("infestation mosaic") of the phytophagous Arthropoda were overcompensated by density-dependent factors. This could be represented by a regression line in a diagram with - both as logarithms - the initial population density in the abscissa and the multiplication index in the ordinate. Then, regulation appears as the inclination of the regression line, determination as its intersections with the abscissa (=momentary equilibrium density).Peculiar to this method is that the analysis - unless it is repeated - deals with variations in space instead of fluctuations in time. Example 1 (see Fig. 1). Eucosma tedella (CL.) (Microlep.). The regression lines 1962/63 and 1963/64 differ in their inclination. Regulation was 1963/64 stronger than 1962/63. In neither generation was the equilibrium density (see intersection with abscissa) attained; the population density of the insect continued to rise. Example 2 (see Fig. 2). Oligonychus ununguis (JAC.) (Acari). The four parallel regression lines range corresponding to the length of the spuce needles. Parallelism means that regulation had the same value; the different position of the lines which can be read off either on the abscissa or on the ordinate represents differences in determination.The question as to the causes of regulation and determination is, for the present, disregardable. However, it can be generalized that the disposition of a food plant (for which the needles' length may be representative) is, in the language of cybernetics, a "controlling variable", except it be, itself, density dependent.