Population Processes in the Vole and Their Relevance to General Theory

No animal population continues to increase indefinitely, and the problem is to find out what prevents this. Increase among voles is halted by declines that recur fairly regularly, and can be identified by certain associated characteristics as belonging to a single class of events. By examining enough of these instances, and contrasting them with control populations that are expanding, conventional types of answer to the problem can be eliminated. According to field evidence the individuals in a declining vole population are intrinsically less viable than their predecessors, and changes in the severity of their external mortality factors are insufficient to account for the increased probability of death. On the assumption that vole populations are a special instance of a general law, the hypothesis is set up that all species are capable of regulating their own population densities without destroying the renewable resources of their environment, or requiring enemies or bad weather to keep them from doing so. The existence of such a mechanism would not imply that it is always efficient, especially in situations to which a species is not adapted, or that species do not also occur in environments where the mechanism seldom, if ever, comes into effect. The hypothesis states that, under appropriate circumstances, indefinite increase in population density is prevented through a deterioration in the quality of the population. The hypothesis can be falsified, or shown to be irrelevant to a particular situation, by proving that there are no significant differences between expanding, stationary, and declining populations in the distribution of the properties of the individuals. Tests of this hypothesis are relevant to all theories about the factors limiting animal numbers, since the effects of most mortality factors depend upon properties of the organisms, and it cannot safely be assumed that so important an environmental variable as population density has no effect on the physiology of the individual or the genetics of the population. Contrary to the assumption often made, it is therefore a priori improbable that the action of the physical factors is independent of population density. It is therefore postulated that the effects of independent events, such as weather, become more severe as numbers rise and quality falls. This hypothesis, if true, overcomes two of the difficulties often met with in population studies: that there is no consistent evidence of (a) the mortality factors that are themselves influenced by population density in the manner required by one system of thought, or (b) the climatic catastrophes required by other systems.