UTRICULARIAS SECRET - THE ADVANTAGE OF POSITIVE FEEDBACK IN OLIGOTROPHIC ENVIRONMENTS

An interesting example of positive feedback in ecosystems centers arou nd the carnivory exhibited by various species of submerged aquatic pla nts belonging to the genus Utricularia. The autocatalytic loop of proc esses engendered by plant carnivory can be modeled by a very simple, b ut nonlinear system of 5 coupled differential equations. The stationar y states of the model reveal how Utricularia harnesses the production of its own periphyton via intermediary zooplankton grazers. This subsi dy to the plant apparently allows it to thrive in oligotrophic environ ments that would stress other macrophytes with similar direct uptake r ates. As ambient nutrient levels rise, however, the advantage gained b y positive feedback wanes, until a point is reached where the system c ollapses. For certain parameter values the model exhibits a Hopf bifur cation, which might help to explain dramatic differences in the compos ition 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.