1. Swiss Federal Institute of Technology, Department of Electrical Engineering, Circuits and Systems Group, 1015 Lausanne, Switzerland
2. Laboratoire d'Ecologie, CNRS URA 258, Ecole Normale Supérieure, 46 rue d'Ulm, 75230 Paris cedex 05, France
3. International Institute of Applied Systems Analysis, Adaptive Dynamics Network, 2361 Laxenburg, Austria