A population study ofScapholeberis rammneri Dumont and Pensaert (Cladocera: Daphniidae)

Scapholeberis rammneri does not hatch at temperatures below 19°C, and its temperature optimum, as derived from field and culture data, is situated close to 21°C. In a small pond, free of vertebrates and with few insects present, its population dynamics were governed entirely by temperature-dependent parameters, such as reproductive success and fertility. From cultures conducted at four different temperatures, it was found that culture reproductive success (the fraction of females in a culture that reproduce at least once before death occurs) leads to an estimate of net birth rates, and its complement (the fraction of females that fail to reproduce before death occurs) produces an estimate of mortality due to natural causes. Because in the pond predation was insignificant, postembryonic culture mortality provides a reasonable estimate of field mortality due to natural causes as well. When postembryonal culture mortality is substracted from culture birth rates, a net rate of population increase somewhat lower than that in the pond is observed. Because of strong temperature effects, this transfer of culture data to field conditions strictly applies only to early summer conditions: an expandingScapholeberis population, and fairly constant field temperatures close to the temperature optimum of the species.