Why do rotifer populations present a typical sigmoid growth curve?

To determine the underlying processes to population growth in the rotifer B rachionus plicatilis, we conducted an experiment using 1.5 ml cultures for 70 days. All individuals were transferred daily to culture media containing algae, and the number of individuals, clutch sizes and number of deaths we re counted. The population dynamics showed a typical sigmoid curve. The pop ulation density increased exponentially from 10 to 682 individuals during t he first 7 days (exponential growth phase), and gradually up to about 1500 individuals during the next 30 days (post-exponential growth phase). The po pulation density then remained at a constant level with small fluctuations during the rest of the experimental period (stationary phase). Mortalities appeared from the post-exponential growth phase and were almost constant at about 2% throughout the experimental period. The clutch size decreased fro m 5 to 1 during the first 5 days, and afterwards females laid only one egg each. The proportion of non-reproductive females increased from 30% (expone ntial growth phase) to 80% (post-exponential growth phase) to 90% (stationa ry phase). These results suggest that the exponential growth phase resulted from the imbalance between a high birth rate and a low death rate, while t he stationary phase was maintained by the compensation between low birth an d death rates.