Effect of prey heterogeneity on the response of a model food chain to resource enrichment

We demonstrated that the presence of invulnerable prey can result in a shif t in the balance between top-down and bottom-up control of a model food cha in. Our model food chain consisted of the bacterium Escherichia coli and th e bacteriophage T4 (a virus that feeds on E. coli) in chemostats supplied w ith different concentrations of glucose. The E. coli population consisted o f individuals that were susceptible to predation by T4 ("edible" E. coli) a nd individuals that were resistant to predation by T4 ("inedible" E. coli). The equilibrium density of a heterogeneous prey population (consisting of edible and inedible E. coli) increased strongly in response to an enrichmen t of its resources. This response consisted of an increase in the inedible fraction of the prey population but no change in the edible fraction. In co ntrast, a homogeneous prey population (edible E. coli only) increased only marginally. The equilibrium density of the predator population (bacteriopha ge T4) did not significantly increase in response to enrichment when its pr ey were heterogeneous, but it increased strongly when its prey were homogen eous.