The basic components and subsidiary factors comprising predation have been demonstrated in field and laboratory studies that analyzed the predation by small mammals of the European pine sawfly, Neodiprion sertifer (Geoff.). Predation was restricted to the cocoon stage of the insect and only three species of small mammals were important predators -- Sorex cinereus cinereus Kerr, Blarina brevicauda talpoides Gapper, and Peromyscus maniculatus bairdii Hoy and Kennicott.
The characteristics of the prey and of the field conditions were so simple that prey density and predator density were the only variables affecting predation. There were two basic responses to changes in prey density: the functional response, where the number of cocoons consumed per predator changed and the numerical response, where the density of predators changed. These responses differed for the three different predators. When the two basic responses were combined to reveal the relation between per cent predation and prey density, predation increased initially with increase in prey density (concurrent density dependence) and thereafter decreased (inverse density dependence). The forms of these peaked predation curves was different for each species of predator, for they reached different maxima and peaked at different prey densities.
The functional and numerical responses are the basic components of predation. Laboratory experiments, however, showed that subsidiary factors can exert an effect through these responses. Prey characteristics, by altering the strength of stimulus from prey, can change both the functional and numerical responses. Similarly, increase in the number or palatability of alternate foods can decrease the functional response and increase the numerical response.
A hypothetical predator - prey model showed that under certain conditions the peaked type of predation can regulate the numbers of a prey and can damp oscillations of prey numbers.
The scheme of predation revealed in this study can explain all types of predation and types of parasitism as well. It seems, however, that there are four major types of predation conceivable, each type having different functional and numerical responses.Science, Faculty ofZoology, Department ofGraduat
