Financial support provided by the Interdisciplinary Research and Education Fund (INREF) of Wageningen University and Research Centre, the Netherlands and the Dutch Ministry for International Cooperation (DGIS) is gratefully acknowledged. We thank E. Kwafo and K. Gyan for their assistance with the fieldwork. Ken Giller thanks the European Union for funding through AfricaNUANCES. Critical comments on an earlier version by three anonymous reviewers helped us to improve the manuscript.