The Relative Importance of Competition and Predation Varies with Productivity in a Model Community

Recent theory predicts that productivity can influence the relative importance of predation and competition in determining patterns in abundance, diversity, and community structure. In low-productivity systems, competition is predicted to be the major influence on community patterns, while at high productivity, the major influence is predicted to be predation. We directly tested this theory using a laboratory model community. Our model community consisted of the bacteriophage T2 (a virus that feeds on Escherichia coli) and two populations of E. coli, in glucose-limited chemostats. One E. coli population consisted of individuals that were sensitive to predation by T2 ("vulnerable" E. coli), and the other population consisted of individuals that were partially resistant to predation by T2 ("less vulnerable" E. coli). We manipulated productivity in this experiment by running replicate chemostats with different input concentrations of glucose. Our observations were consistent with theoretical predictions. We observed the decline of the more vulnerable prey population at higher productivity but not at lower productivity, and the decline of the less vulnerable prey population at lower productivity but not at higher productivity. However, the rate of decline in some replicates was slower than predicted, and extinctions were not observed during the experiments, contrary to theoretical predictions. We present some testable hypotheses that might explain the slow rate of decline observed.