Food supply as a factor regulating population size in freshwater triclads

Summary

1.	The population biology of the three planariid species, Polycelis nigra (Müll.), P. tenuis (Ijima) and Dugesia higubris (O. Schmidt), based on nonquantitative sampling, is reviewed.
2.	They all suggested two basic features, recurring periods of food shortage associated with breeding and subsequent mortality. This interpretation was supported by field experiments in which the food supply was increased artificially.
3.	The population biology of P. tenuis in a small pond is re-examined using a quantitative technique which recorded changes in absolute numbers per unit area.
4.	The data obtained on seasonal changes in size-structure, fecundity, recruitment and mortality entirely support the earlier interpretation.
5.	Changes in numbers of prey are not related to predatory pressure by the planariid species but in Dendrocoelum lacteum (Müll.) there is evidence of this.
6.	The annual cycle in the population biology of the potentially longer-lived planariid species is compared briefly with that of the shorter-lived Dendrocoelum lacteum. Although the same kinds of events occur, their timing differs; there are related contrasts in size-structure, fecundity and recruitment.
7.	Population regulation in these triclads fits the hypothesis developed by Nicholson and it is dependent on self-induced, intraspecific competition for food.