Studies on The Limitation of a Natural Population of Paramecium Aurelia

Abstract

A natural population of Paramecium aurelia was sampled over the 2 years 1963 and 1964. The habitat was a small seep in a mature hardwood forest, and the population consisted almost entirely of Variety 2. Of 3,397 specimens collected, 1 was Variety 4. During the 2 years, four major peaks in population density were observed. These occurred in June and November of 1963 and in August and November of 1964. The two November peaks followed the annual leaf fall; the other peaks were related to heavy rain. These factors would introduce additional nutrients into the habitat; and it was concluded that the population was limited primarily by food. This conclusion was reinforced by the failure of the population maxima to be related to seasonal climatic factors, and by loss rates during population declines which were unrelated to the density of paramecia or to water temperature. Two experiments were carried out in the habitat: the addition of water to the area surrounding the seep, and the addition of paramecia to part of the seep. The hypothesis of food limitation was confirmed in the first experiment; the second showed that at sufficiently high densities, loss rate is related to density, and thus can act as a regulatory mechanism. The results of this study indicate that competition for a common resource is likely betwen the varieties of P. aurelia, six of which are known to occur in the vicinity of the seep. Variety 2 is evidently superior throughout this habitat, but other more complex habitats contain two to six varieties, indicating that there should be sufficient ecological differences between the varieties to allow each to be superior to the others under some natural conditions.