[Association coefficients of three planktonic rotifer species in the field and their explanation by interspecific relationships (competition, predator-prey relationships)].

Coefficients of interspecific association of three rotifer species (Brachionus calyciflorus, Brachionus rubens, Asplanchna brightuelli) of 314 plankton samples from about 100 ponds in South Germany were determined: two coefficients basing on present-absent data (Table 2: C 7, C 8), one coefficient regarding population density (Table 2: C 9), and one coefficient of partial interspecific association taking in mind the presence or absence of the third species (Table 3: C p). Some of the computed coefficients are significantly positive or negative indicating that joint appearances are sometimes more and sometimes less frequent than expected by chance. Most of these results cam be interpreted by the knowledge of interspecific relationships, vz. competition between Brachionus calyciflorus and B. rubens, and predator-prey relationships between Asplanchna brightwelli and B. rubens which had been detected in former studies. In each case the association between two of the species is modified by the presence of the third. The causes (alternative prey, destruction of the competitors etc.) are discussed.