This study is based on the work of D. Knowler, B. Bradshaw and D. Gordon (all of Simon Fraser University, in Bernaby, British Columbia, Canada). The study benefited from the contributions of G. Evers (FAO), A. MacMillan (FAO), J. Plummer, J. Poulisse (FAO), J. Pretty (University of Essex, UK), F. Shaxson, B. Stewart (West Texas A&M University, Camion, Texas, USA) and Tanja van den Bergen (FAO).
The cover photograph was provided by T. Friedrich (FAO).