Experiments on the Effect of Host Spatial Distribution on the Numerical Response of Parasitoids

Abstract

When parasitoids search at random, an assumption implicit in most parasitoid-host population models (see Royama (1971) for a review of such models), the distribution of their hosts does not influence their success (Rogers 1972). However, many parasitoids do not search randomly (Hassell 1971; Price 1972), since they are attracted to, and spend most of their time in areas of high host density (Corbet 1971), where they interfere with each other either directly by physical contact (Hassell 1971), or indirectly by leaving trails (Salt 1937). By remaining at a concentration of hosts an individual parasitoid will be most efficient, but the numerical response of a population of parasitoids behaving in this way requires a more complex interpretation. Furthermore most animal populations are aggregated in space, the degree to which they are clumped often varying with their density (Dybas & Davis 1962; Berthet & Gerard 1966). It is of imnportance, therefore, to know how parasitoid (or predator) populations respond to changes in their hosts' (or preys') spatial distribution, in conjunction with density changes, and to know under what circumstances the more or the less highly aggregated distributions will be the more advantageous. To test how parasitoid populations respond to different host distributions, an index of the intensity of their interactions under different circumstances is necessary. The mutual interference between searching parasitoids can be measured by the slope (M) of the regression of log10 area of discovery (A) and log10 parasitoid density (P) (Hassell & Varley 1969) where