The Numerical Response of Avian and Mammalian Predators during a Gradation of the Spruce Budworm

Abstract

It seems to be rather generally agreed that the most effective species of predators affecting an animal population are likely to be those that show a response to changes in its density. Such a response can take either or both of two forms. The population density of the predators may change as a result of changes in prey density, and this Solomon (1949) has aptly termed the response. The present study shows that some species of predators increase when prey density increases while others decrease; thus it is necessary to distinguish between a direct numerical response and an inverse numerical response. The numerical response to prey density is more likely to be a than an immediate response. Varley (1953) has already pointed out that the term delayed density dependence is more appropriate than densityv dependence, because the correlation coefficient between predator and prey populations may be approximately zero when there is a direct but response. The second type of response, or response, is concerned with food habits. When one species of prey becomes abundant, the predator may respond to the increased availability of this species by concentrating its searching activities upon it. Curves showing both the numerical and the functional response can be combined to give the total response. In studies on the spruce budworm, Choristoneura fumiferana (Clem.) on the Green River Watershed in northern New Brunswick we are attempting to express both of these responses quantitatively over a long period of years. This is but one aspect of a broader investigation on the population dynamics of the insect, the approach to which has already been outlined (Morris and Miller 1954; Morris 1957). Considerations of time and space make it necessary to limit the present paper to avian and mammalian predators, and to deal only with their numerical response. The field data were gathered during the 10-year period 1947-1956. Results will be presented in more detail at a later date and will include the functional and total responses. Those who may be interested in the subspecific identities of the birds and mammals listed here are referred to Squires (1952) for the birds of New Brunswick and Morris (1948) for the mnammals.