[An integrated theory of natural control of animal populations]

Abstract

Since the twenties of our century, at least 15 theories worth discussing have been developped which intend to explain the causes of natural control of animal populations (for details see SCHWERDTFEGER, 1968). An attempt is made to integrate the different-partly contrary-ideas and new results into a general theory. The basis to start from is the cybernetic principle of feed-back mechanism introduced into population dynamics by WILBERT (1962): an actual value (e.g. the inside temperature of a refrigerator) is permanently changed by perturbances (the always higher outside temperature); through a regulator (a thermostat), each change puts in action a regulating variable (a cooling device) which alters the actual towards the index value (required inside temperature).The often complicated processes that take part in the natural control of populations are summarized in Fig. 6. The actual value is the existing population density (Abundanz). The perturbances primarily causing its fluctuations (Fluktuation) are fertility and immigration which raise the abundance, mortality and emigration which lower it.The amplitude of the fluctuation must be limited, if the population is not to die out or to destroy its habitat by continuous increase. It is determined (Determination) as a sort of index value, the lower limit of which corresponds in the extreme to the minimal density guaranteeing the existence of the population, while the upper limit is formed by the environmental capacity. The latter is determined either by the total supply of requisites and the ability of the animals to use it or by the local minimum of adverse effects. The capacity of the environment and therewith the amplitude of fluctuations can be fixed or variable. It is fixed in a population of Great Tits with territorial behaviour: in an oak stand, the number of breeding pairs cannot be higher than the number of territories fitting in. It is variable in the case of bark beetles living in wind thrown spruce trees: they may find 2 suitable trees this year, 100 the next after heavy winter storms and 10 the year after next. In this case, the change of determination was a change of environment, specifically of the supply of a requisite; it can as well be a change in the constitution of the population characterized by its demands and efficiencies: in the final effect it makes no difference wether more breeding space is offered or less is demanded. The variability of the amplitude in the case of the bark beetles is caused by chance; it can also be governed or self-induced. It is governed e.g. by the seasonal rhythm of climate: the average level of density in tsetse flies is higher during the rain season than during the dry season. It is self-induced in the case of an entomophagous parasite changing the density of its host insect and by this varying the supply of a requisite.Fertility permanently tends to raise the density of populations. Processes of limitation (Limitation) work against this tendency. With increasing abundance, density dependent factors become more effective as general regulators. In simple feed-back control systems (einfacher Regelkreis), regulation is solely performed by perfectly density dependent factors. An example is the cyclically fluctuating Field Vole: increasing mutual interference causes a crash of the abundance which thereafter rises again. In complex feed-back control systems (komplexer Regelkreis) density is kept on a low level for a shorter or longer period by random influences and delayed density dependent factors; now and then a real regulating factor has to interfere as an emergency brake to prevent the transgression of the upper density limit. That applies to many insect populations. The effect of the limiting factors as regulating variables consists in lowering fertility and immigration and raising mortality and emigration.The processes causing fluctuations result in an equilibrium density (Gleichgewichtsdichte): increase and decrease are counterbalanced. The level of the equilibrium density is differently situated within the amplitude and approximately in accordance with the long-term average of the actual densities.