The food niches of the invasive Dugesia tigrina (Girard) and indigenous Polycelis tennis Ijima and P. nigra (Müller) (Turbellaria; Tricladida) in a Welsh lake

Abstract

The invasion of Llyn Coron in North Wales by the American immigrant Dugesia tigrina resulted in the almost entire displacement of the native Polycelis tenuis and P. nigra populations. Because competition for food is the most important factor controlling and regulating populations of British lake-dwelling triclads, the diets of the invasive and native triclads in the lake were examined in an attempt to explain the successful invasion.

A serological technique, the precipitin test, identified the gut contents of field-collected triclads. Niche breadth, electivity and niche overlap indices were used in analysis of the data. A broad food niche was recorded for all the triclad species. Oligochaetes predominated in the diet of D. tigrina followed by Asellus and chironomids, in almost equal proportions, snails and caddisflies, with mayflies scarcely eaten. Both Polycelis species fed heavily on oligochaetes followed by Asellus. P. tenuis ate slightly higher proportions of chironomids and caddisflies than snails and mayflies, whilst P. nigra consumed similar proportions of these four prey taxa. Gammarus, scarce in Llyn Coron, and cladocerans were not eaten by the Polycelis species and by only a few D. tigrina.

To-date there has been no apparent shift in the diet of the Polycelis species as a consequence of the invasion by D. tigrina. In the absence of any evidence for the partitioning of food on the basis of prey size or condition, the considerable overlap in the diets of D. tigrina and the Polycelis species suggests the potential for severe inter-specific competition for food. The superior exploitation by D. tigrina of the available food resource requires explanation.