Population growth in planktonic rotifers. Does temperature shift the competitive advantage for different species?
The numerical response of populations to different food concentrations in an important parameter to be determined for a mechanistic approach to interspecific competition. Theory predicts that the species with the lowest food level (TFL) should always be the superior competitor if only one food source is offered. However, TFLs are not species specific constants but may change along environmental gradients such as food size or temperature.
The hypothesis that temperature differentially affects the TFLs of three planktonic rotifers (Asplanchna priodonta, Brachionus calyciflorus and Synchaeta pectinata) was tested in laboratory experiments. Numerical responses were assessed for all three rotifers at 12, 16, 20, 24 and 28°C with Cryptomonas erosa as food alga. Growth rates of all three rotifers at high food concentrations (1 mg C l-1) increased as temperature increased until the limits of thermal tolerance were reached. This increase was very pronounced for Brachionus, but less for Synchaeta which already had relatively high growth rates at 12°C. Along the temperature gradient, the TFLs of Synchaeta increased from 0.074 to 0.66 mg C l-1, whereas those of Asplanchna and Brachionus stayed relatively constant at 0.3 and 0.2 mg C l-1, respectively. Hence, the zero net growth isocline (ZNGI) of Synchaeta crossed those of Brachionus and Asplanchna at 16 and 20.5°C, respectively. The results suggest that Synchaeta is better adapted to low temperatures than the other two rotifers and should be the superior competitor below 16°C.
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