The Effect of Prey Size on the Functional and Numerical Responses of a Protozoan Predator to Its Prey

The functional response of Didinium nasutum to variation in density of three Paramecium species was found to decrease with increasing prey size. The attack coefficient is similar on small and intermediate—sized Paramecium and declines sharply on the largest prey species. Handling time increases with increasing prey size. Actual division rates of the predator are highest, however, on the intermediate—sized prey and lowest on the smallest prey species. This suggest that there is an optimal Paramecium size at which Didinium division rates are highest. The density of prey and the size of prey were also found to have a significant effect on the volume of the Didinium cells. It is suggested that there may be contrasting advantages to Didinium of being of as large a size as possible and of dividing as quickly as possible.