The influence of environment and competition on the distribution of Bdellocephala punctata (Turbellaria, Tricladida) in Britain

Summary

Bdellocephala punctata (Pallas), the largest British freshwater triclad, has been recorded from thirty-four, widely distributed sites in the British Isles. This distribution does not appear to be associated with any chemical or physical characteristics of the habitats. Although B. punctata has a simple pattern of population dynamics, which normally enables it to avoid competition for food with other triclads, it is suggested that this strategy is sensitive to environmental fluctuations. A decrease in relative food supply, particularly if this is due to increased breeding overlap with a confamilial, would lead to drastic reductions or extinctions of local populations and produce the observed disjunct distribution. The possibility that it may be a comparatively recent immigrant to Britain cannot be excluded.