INDIVIDUAL DIFFERENCES AS A FACTOR IN POPULATION DYNAMICS: THE DEVELOPMENT OF A PROBLEM

Abstract

The role of individual differences in the population dynamics of animals has been relatively neglected by ecologists. Individual requirements and responses to environmental pressures differ, however, and it is unwise to assume that the range of this variation within a population remains constant through successive generations. Moreover, the assumption that successive changes in the range of variation might affect the subsequent efficiency of some factors believed to regulate population density is worth further consideration. It cannot be tested adequately, however, by considering only the amounts of mortality that different extrinsic factors cause within a generation. Its adequate assessment also requires detailed observations of the qualities of different types of individuals. Therefore, in order to assess the importance of individual variation in the population dynamics of an insect, an outbreak of the western tent caterpillar, Malacosoma pluviale (Dyar), was studied in detail during 1956. Larvae were classified by differences in their total activity and behavior on emergence, and the proportions of the different types per egg mass were determined. Differences obtained were found to be associated with different feeding habits and rates of food consumption and development as well as with different chances for survival in particular environments or in the presence of disease. Colonies in new infestations consisted largely of active individuals, whereas less active individuals occurred with increasing frequency as infestations became older and heavier. Adult activity also varied, and more active moths appeared to be responsible for production of active colonies, especially in new infestations most distant from original sources. The population must be followed through its eventual decline to its next increase before complete information can be obtained, but present evidence is sufficient to formulate an acceptable working hypothesis, namely, that proportions of the different types within the population will show annual as well as areal changes. Moreover, the fact that the insect is colonial is not especially significant, since similar differences in total activity also may be detected among other species with solitary habits.