THE FOOD NICHES OF THE INVASIVE DUGESIA-TIGRINA (GIRARD) AND INDIGENOUS POLYCELIS-TENUIS IJIMA AND POPULUS-NIGRA (MULLER) (TURBELLARIA, TRICLADIDA) IN A WELSH LAKE

The invasion of Llyn Coron in North Wales by the American immigrant Du gesia tigrina resulted in the almost entire displacement of the native Polycelis tenuis and P. nigra populations. Because competition for fo od is the most important factor controlling and regulating populations of British lake-dwelling triclads, the diets of the invasive and nati ve triclads in the lake were examined in an attempt to explain the suc cessful invasion. A serological technique, the precipitin test, identi fied the gut contents of field-collected triclads. Niche breadth, elec tivity and niche overlap indices were used in analysis of the data. A broad food niche was recorded for all the triclad species. Oligochaete s predominated in the diet of D. tigrina followed by Asellus and chiro nomids, in almost equal proportions, snails and caddisflies, with mayf lies scarcely eaten. Both Polycelis species fed heavily on oligochaete s followed by Asellus. P. tenuis ate slightly higher proportions of ch ironomids and caddisflies than snails and mayflies, whilst P. nigra co nsumed similar proportions of these four prey taxa. Gammarus, scarce i n 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 pot ential for severe inter-specific competition for food. The superior ex ploitation by D. tigrina of the available food resource requires expla nation.