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 Euplotes
octocarinatus and Coleps hirtus in a nitrogen-limited chemostat system.
The ciliates competed for the microalgae Cryptomonas sp. (Cry) and
Navicula pelliculosa (Nav), and the bacteria present in the cultures over
a period of 33 days. We used monoclonal Euplotes and three different
Coleps clones (Col 1, Col 2, Col 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 Euplotes feeds on Cry and
on bacteria, the Coleps clones cannot survive on bacteria alone but feed
on both Cry and Nav with clone-specific rates. Experimental treatments
comprised two-species mixtures of Euplotes and one or all of the three
different Coleps 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 Nav quickly escaped top-down control and likely
reached a state of low food quality, ciliate competition was strongly
determined by the preference of different Coleps clones for Cry as opposed
to feeding on Nav. In addition, the ability of Euplotes to use bacteria as
an alternative food source strengthened its persistence once Cry was
depleted. Hence, trait variation at both trophic levels co-determined the
population dynamics and the outcome of species competition. --