Combined effects of food concentration and temperature on competition among four species of Brachionus (Rotifera)

Abstract

We evaluated the effect of algal food density (1.5 {\texttimes} 106, 3.0 {\texttimes} 106 and 4.5 {\texttimes} 106 cells ml−1 of Chlorella) and temperature (22{\textdegree} and 28{\textdegree} C) on competition among the rotifers Brachionus calyciflorus, Brachionus havanaensis, Brachionus patulus and Brachionus rubens, based on population growth experiments for 24 days. The growth experiments were conducted seperately for each individual rotifer species (i.e., controls), and in mixtures of all four species in equal initial proportions (i.e., under competition). The population growth of B. calyciflorus, B. havanaensis, B. patulus and B. rubens grown separately at two temperatures and at three algal food densities showed typical patterns of lag, exponential and retardation phases in the controls. This pattern differed considerably under competition. In general, we observed that in all of the test species, the highest growth rates were observed at higher food levels and in the absence of congenerics. At 22 {{\textdegree}C,} under the lowest food level, the differences in the population abundances of B. havanaensis, B. patulus and B. rubens grown alone and in the presence of competition were large. However, these differences reduced as food density was increased from 0.5 {\texttimes} 106 to 4.5 {\texttimes} 106 cells ml−1. At 28 {{\textdegree}C} and at the lowest food level, all of the other rotifer species eliminated B. havanaensis in mixed cultures. Each brachionid species had a higher rate when grown alone than when cultured with other species. The highest r (mean {\textpm} standard error: 0.54 {\textpm} 0.01 day−1) was recorded for B. havanaensis at 28 {{\textdegree}C} under 4.5 {\texttimes} 106 cells ml−1 of algal food density. At 28 {{\textdegree}C} at low algal food density, the presence of competitors resulted in negative population growth rates for three of the four rotifer species tested.