Trait variability and defense costs in coupled bi-trophic plankton - biofilm systems: effects on predator-prey dynamics and coexistence

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The present project investigates the effect of trait variability on the dynamics of predator-prey systems. During the first funding period of DynaTrait, we established an experimental plankton – biofilm system and a corresponding mathematical model framework, in which trait variability in the prey guild is realized by phenotypic plasticity (by the bacterium Pseudomonas putida with both plankton and biofilm phenotypes). Trait variability in the predator guild can be manipulated by adding either a selective plankton feeder being not able to feed on biofilms (i.e., the ciliate Paramecium tetraurelia) or a selective biofilm feeder being not able to feed on plankton (i.e., the amoebae Acanthamoebae castellanii). A novel chemostat system has been developed successfully, which allows selective manipulation of both the plankton and the biofilm phase separately and which thus offers the unique opportunity to investigate the costs of defense against grazing by either plankton or biofilm feeders. Besides system development and parameter determination, we focused within the first funding period on the effects of variation on biomass allocation and dynamics within the coupled plankton-biofilm systems. In the second funding period, we aim to test four advanced hypotheses, which were supported by our mathematical modelling:1) Trait variation in the predator guild strongly increases the carbon flow of the total system because it avoids long-term accumulation of biomass in a defended prey phenotype.2) Phenotypic plasticity in prey can drive indirect facilitation and coexistence between predators. The second hypothesis is supported by our model results, which predicts extinction of the specialized biofilm feeders in single-predator communities as the prey is pushed towards non-edible plankton.3) Third, we will focus on the system dynamics in testing the hypothesis that equalizing the costs of defense for each of the two bacteria phenotypes will lead to cycling dynamics of the system. To test this, we will manipulate both the plankton growth rates (selective reduction by antibiotic addition) and biofilm growth rate (selective enhancement) in order to equalized plankton and biofilm growth.4) Finally, we will add complexity to the system by adding a second prey species with increasing biofilm growth rates and decreasing plankton growth and thus allow trait variation in the prey guild generated by both phenotypic plasticity and genotypic variation. We will test the hypothesized that trait variation in predators increases the range of coexistence of two competing phenotypically plastic prey species.The hypotheses will be tested in close iteration with chemostat experiment and mathematical modelling. It will be closely linked to other bi-trophic systems within the DynaTrait community and to the common core group.