Threshold policies are defined and analysed for different types of one species and predator-prey type models. It is shown that such policies can be designed by suitable choice of so called virtual equilibrium points. The simplest threshold policies are discontinuous, which leads to some drawbacks. It is also shown how to design continuous threshold policies that retain most of the advantages of their discontinuous versions but do not have the major drawback of chatter in the control. Threshold policies are also seen to be robust to uncertainty of the model parameters, and the initial conditions, as well as to delays between stock assessment and policy enactment. The models studied are the Noy-Meir herbivore-vegetation model, subject to linear consumption curves, the two species Rosenzweig-MacArthur model, and a three-dimensional chemostat model.