COMPETITIVE INTERACTIONS BETWEEN HERBIVOROUS ROTIFERS - IMPORTANCE OFFOOD CONCENTRATION AND INITIAL POPULATION-DENSITY

We studied the outcome of competition between a large (Brachionus caly ciflorus) and a small (Anuraeopsis fissa) rotifer species at five alga l (Scenedesmus acutus) concentrations (0.5 x 10(6) to 40.5 x 10(6) cel ls ml(-1)) and with varying initial densities in mixed populations (10 0 to 0% of B. calcyciflorus or A. fissa), the combined initial biomass being 0.2 mu g ml(-1) in all test jars. Experiments were conducted at 28 +/- 1 degrees C. Regardless of food concentration, B. calcycifloru s showed a greater increase in biomass than A. fissa, peak densities ( mean +/- standard error) at the lowest food concentration in the contr ols being 1.34 +/- 0.31 mu g dry weight ml(-1) and 0.82 +/- 0.08 dry w eight ml(-1), respectively At the lower food concentrations, A. fissa displaced B. calyciflorus and vice versa at the higher food concentrat ions. At the intermediate food concentrations of 4.5 x 10(6) cells ml( -1), B. calyciflorus outcompeted A. fissa only if its initial populati on density was three times higher. The rates of population growth in c ontrols varied from 0.792 +/- 0.06 d(-1) to 1.492 +/- 0.13 d(-1) for B . calyciflorus and 0.445 +/- 0.04 to 0.885 +/- 0.01 for A. fissa depen ding on food level. When both species were introduced together, low fo od levels favoured higher abundance of A. fissa than B. calyciflorus, suggesting, in nature, it is likely that small Anuraeopsis colonize ol igotrophic water bodies more successfully than larger Brachionus. The results also suggest that the outcome of competition depends not only on the size of the competing species and food availability but also on their colonizing density.