Life History Responses to Resource Variation in a Sessile Predator, The Ciliate Protozoan Tokophrya Lemnarum Stein

To identify the responses of a Predator to resource variation, age—specific fecundity and survivorship were measured at several food levels for the sessile, predatory ciliate protozoan Tokophrya lemnarum Stein. The major life history characteristics identified were: (1) a direct relationship exists between net reproductive rate and food level, (2) age—specific fecundity closely follows food intake rate (3) each embryo is endowed with a nutrient supply upon birth and (4) an inverse relationship exists between net reproductive rate—food level and life span. Totally starved individuals had the longest mean life spans. Increased survival time at low food levels probably results from low metabolic rate between meals and a slowed accumulation of toxic waste products. Fecundity is directly related to prey number and prey size. It is postulated that this set of adaptations acts to decrease variability of population size in a predator with cannot control its hunting rate and in which prey density must vary greatly during the lifetime of an individual.