1. Complex Systems Research Group, Department of Physics, FEN-UPC, Campus Nord, Mòdul B4, 08034 Barcelona, Spain
2. Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, NM 87501, USA
3. Department of Ecology, Facultat de Biologia, Universitat de Barcelona, Diagonal 645, 08045 Barcelona, Spain
4. National Center for Ecological Analysis and Synthesis, University of California at Santa Barbara, 735 State Street, Suite 300 Santa Barbara CA 93101-5504, USA
5. Fritz-Haber Institut der Max Planck Gesellschaft, Abteilung PC, Faradayweg 4-6, 14195 Berlin, Germany