Predator-prey dynamics in a rotifer-algal microcosm and the emergence of defensive traits in the prey

Predator-prey interactions are prevalent in all natural ecosystems and are regarded as a fundamental force in ecological as well as in evolutionary processes. A series of studies have investigated predator-prey dynamics using a combined approach of laboratory experiments and theoretical modelling of a rotifer-algal microcosm. The results from these studies have demonstrated that evolution of defensive traits in the algal prey can happen so rapidly that it alters the trajectory of ecological dynamics (termed eco-evolutionary dynamics). The overarching goal with this master project was to reproduce some of the previous findings from the rotifer-algal microcosm and, specifically, to generate two qualitatively different population dynamics (steady state vs. persisting population cycles) based on a mathematical model of the system. Additionally, a goal for this project was to investigate the mechanism for the defensive trait in the algal prey and its effects on the population dynamics. Two sets of preliminary experiments were conducted in order to compose a suitable growth medium and to estimate algal growth parameters specific to the experimental conditions. Next, eight continuous cultures of the freshwater rotifer Brachionus calyciflorus and the algae Chlamydomonas reinhardtii were established where all cultures were diluted by a rate of 0.3 /day, while four cultures were supplied medium with a nitrogen concentration of 50 µmol/l and the other four 250 µmol/l. The choice of dilution rate and nitrogen concentration was determined prior to the experiment by an analysis of the set of ordinary differential equations that describe the Chlamydomonas-Brachionus predator-prey system. The eight continuous cultures were maintained for 11 weeks and the density of rotifers and algae, distribution of algal size and concentration of dissolved nutrients in the cultures were estimated every other day in this period. The four rotifer-algal cultures that were supplied medium with a low nitrogen concentration were expected to develop into a steady state, but the rotifer population went extinct in three of the four cultures after approximately 40 days. Two of the high nutrient cultures exhibited large-amplitude and regular population cycles, while the cycles in other two cultures were small and irregular. A comparison of the model predictions and the experimental results revealed several aspects of the model that were ill-defined for the current rotifer-algal system. The phenotypic change in C. reinhardtii that is associated with predator defence (formation of cell clusters called palmelloids) were observed in all high nutrient cultures, but were absent from the low nutrient ones. Whether this phenotypic change in C. reinhardtii is incited by rapid evolution caused by predation pressure is disputable as C. reinhardtii is reported to induce formation of palmelloid cell clusters under several other conditions. Alternative views on the palmella formation in C. reinhardtii are discussed in relation to its evolutionary kinship to multicellular species, and it is proposed that this could serve as a case for genetic accommodation. It is also addressed whether the rotifer-algal system can demonstrate the significance of predation as an evolutionary force.