Competition in Tropical Stream Fishes: Support for the Competitive Exclusion Principle

Two collections of fish were made from the same Panama lowland stream, one during the dry season, and a second during the wet season. Food overlaps among nine sympatric fish species were calculated for each collection by examining fish stomach contents. Species habitats and feeding methods were determined from field and laboratory observations. Estimates of food abundance, measured independently from food overlap, were made by direct and indirect methods, and were used to determine relative levels of competition. The results show that food overlaps among the fishes are at a minimum during the dry season. Food abundance is also lowest during the dry season, which suggests increased competition for food at this time. This seasonal coincidence of the most distinct species separations with the time of increased competition are exactly those consequences predicted by the principle of competitive exclusion. It is concluded that this study provides strong support for the validity of this principle.