Utriculariaʹs secret: the advantage of positive feedback in oligotrophic environments

An interesting example of positive feedback in ecosystems centers around the carnivory exhibited by various species of submerged aquatic plants belonging to the genus Utricularia. The autocatalytic loop of processes engendered by plant carnivory can be modeled by a very simple, but nonlinear system of 5 coupled differential equations. The stationary states of the model reveal how Utricularia harnesses the production of its own periphyton via intermediary zooplankton grazers. This subsidy to the plant apparently allows it to thrive in oligotrophic environments that would stress other macrophytes with similar direct uptake rates. As ambient nutrient levels rise, however, the advantage gained by positive feedback wanes, until a point is reached where the system collapses. For certain parameter values the model exhibits a Hopf bifurcation, which might help to explain dramatic differences in the composition of communities associated with different species of Utricularia. The results suggest that the role of positive feedback in ecosystems might be confined largely to oligotrophic environments.