Abstract Biotic interactions such as competition and predation are known to play an important role in structuring communities. In this study, the intra- and interspecific competition between two congeneric sandy beach amphipods Bathyporeia pilosa and B. sarsi was examined by means of a two-way factorial mesocosm experiment. The amount of natural food resources as well as population densities of B. pilosa and B. sarsi were manipulated in order to investigate whether firstly, the species compete for food, and secondly, whether crowding effects, such as damaging encounter competition, are present within and between both Bathyporeia species. The absence of food negatively influenced survival and recruitment and enhanced aggressive behavior in the B. sarsi population, while there was no influence of the absence of food in the B. pilosa population. In the field, B. pilosa is occurring high in the intertidal where food supply is low, while B. sarsi is occurring in the mid-intertidal where food supply is higher. Hence, B. pilosa is more adapted to low food conditions than B. sarsi . Although an increased intraspecific density stimulated encounter competition within the B. sarsi population, no evidence of interspecific competition was found in this study. This first report on encounter competition for the deposit-feeding amphipod Bathyporeia sarsi suggests that zonation patterns of the two amphipods may be the result of the combined influence of species-specific physiological restrictions and biotic interactions within the B. sarsi population, indicating that on dissipative beaches, biotic interactions may be more common than generally considered.

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