Predators and planariid competitors of the triclad Phagocata vitta (Dugés)

Abstract

When Phagocata vitta, Crenobia alpina and Polycelis felina were exposed separately to each of seventeen potential invertebrate predators in the laboratory, only two stonefly species, Dinocras cephalotes and Perlodes microcephala, fed on the three triclad species, whilst the trichopteran Rhyacophila dorsalis ate the last two triclads. On exposing pairs of triclad species to D. cephalotes, significantly more P. felina than Ph. vitta were consumed, whereas similar numbers were eaten in each of the other two triclad combinations. Cannibalism and interspecific predation by triclad species were not observed. It is concluded that predation is unlikely to have a major influence in determining the observed distribution and abundance of triclad species in a Welsh study stream which harbours low numbers of effective predators.

The de Wit model of competition was used to examine the competitive relationships between Ph. vitta, and C. alpina and P. felina, using chironomids or tubificid worms as food. In mixed cultures of Ph. vitta and P. felina fed on tubificids a stable equilibrium existed within the range of relative densities used in the experiments, whereas Ph. vitta was competitively superior to C. alpina in cultures fed on each of the food types, and to P. felina fed on chironomids. However, in theory, an equilibrium could occur when 10 or 6–7 times as many Ph. vitta as P. felina and C. alpina respectively are in the culture, when intraspecific rather than interspecific competition would become more important. Where the three triclad species coexist in the Welsh study stream, they are in similar numbers. This could imply that food is not limiting, with no consequent interspecific competition, or that the laboratory experiments were too simplistic to allow any interpretation of the field situation.