Intraspecific trait variation alters the outcome of competition in freshwater ciliates.

Trait variation among heterospecific and conspecific organisms may substantially affect community and food web dynamics. While the relevance of competition and feeding traits have been widely studied for different consumer species, studies on intraspecific differences are more scarce, partly owing to difficulties in distinguishing different clones of the same species. Here, we investigate how intraspecific trait variation affects the competition between the freshwater ciliates and in a nitrogen-limited chemostat system. The ciliates competed for the microalgae sp. () and (), and the bacteria present in the cultures over a period of 33 days. We used monoclonal and three different clones ( 1, 2, and 3) in the experiment that could be distinguished by a newly developed rDNA-based molecular assay based on the internal transcribed spacer (ITS) regions. While feeds on and on bacteria, the clones cannot survive on bacteria alone but feed on both and with clone-specific rates. Experimental treatments comprised two-species mixtures of and one or all of the three different clones, respectively. We found intraspecific variation in the traits "selectivity" and "maximum ingestion rate" for the different algae to significantly affect the competitive outcome between the two ciliate species. As quickly escaped top-down control and likely reached a state of low food quality, ciliate competition was strongly determined by the preference of different clones for as opposed to feeding on . In addition, the ability of to use bacteria as an alternative food source strengthened its persistence once was depleted. Hence, trait variation at both trophic levels codetermined the population dynamics and the outcome of species competition.