Reliability in Measuring Diet Overlap

Intraspecific diet overlaps were calculated for young-of-the-year largemouth bass (Micropterus salmoides), juvenile bluegills (Lepomis macrochirus), and inland silversides (Menidia beryllina) by randomly dividing collections of each species into two sets, measuring the diet (average of the volume percentage) and calculating the diet overlap by Schoener's method. The randomization process and overlap calculation were repeated 30 times and an upper confidence limit (95%) for intraspecific overlap was determined. Upper confidence limits of intraspecific overlap were generally below the 100% overlap expected if diets were adequately characterized. Intraspecific overlap varied with collection date, fish species, and size of sample. Increasing sample size did not increase intraspecific overlap when the larger sample included a wider size range of fish. The upper confidence limit of intraspecific overlap can be used as a measure of reliability for interspecific diet overlap calculations. We suggest that intraspecific overlaps approaching 100% indicate high reliability for the corresponding interspecific overlap value. The intraspecific overlap is most useful in judging the reliability of interspecific overlap when sample size is limited.Key words: diet overlap, intraspecific overlap, interspecific overlap