A 4D model for a closed ecosystem with four compartments is derived. The model is reduced to a 3D dynamical system, described by a system of three nonlinear ordinary differential equations, depending on seven positive parameters. The local dynamics and bifurcations of this model are investigated. The system possesses at most three equilibrium points. It is found that at most one of the equilibria is locally asymptotically stable for each parameter strata. Several codimension one bifurcations are determined and analyzed. Various attractors and transitions scenarios are emphasized numerically.