Food Overlap of Lake-Dwelling Triclads in the Field

In an earlier paper (Reynoldson & Davies 1970), the ability of the four common British species of triclads to co-exist in lotic habitats was explained in terms of 'food refuges' and found to be related to the productivity of the water-body. A food refuge was regarded as that part of the diet on which a particular triclad species was competitively superior. This Sexplanation has been tested by studying the performance of laboratory populations provided with varying combinations of the specific food refuges; prediction of the outcome of interspecific competition based on the food refuge concept was confirmed in all six of the experimental regimes which were completed (Reynoldson & Bellamy 1973). In the 1970 paper which dealt with differences rather than overlap, each food item was expressed as a percentage of the diet so that variations in absolute amounts of prey eaten were not taken into account. In analysing contrasts no use was made of the various indices which have been developed (e.g. Horn 1966; Ricklefs 1966; MacArthur, Recher & Cody 1966). Here, niche overlap is examined using the index of MacArthur & Levins (1967) as described by Ricklefs (1973) for comparison with the earlier results; the competition coefficient of species j with species i is given as