Transformations that farmers bring to their traditional farming systems and their impacts on the conservation and evolution of maize varieties over a 12-year period are investigated using a longitudinal analysis. Despite the increased introduction and supply of improved maize variety seeds in the Yucatan Peninsula, Mexico, over the last 12 years farmers continue to maintain a substantial amount of traditional maize variety diversity.

In recent decades, markets have become widely used for environmental resources. Prime examples include water rights where trade enables water to be allocated to the most profitable crops, and allows farmers more flexibility to cope with climatic variability (Bjornlund 2003). Similarly, tradable rights for air pollution minimize the cost of meeting air quality targets (Stavins 1998). The same principles can potentially be widely applied to biodiversity conservation. In this issue, we are fortunate to have a short but diverse series of papers on tradable rights in conservation.

In many developing countries, people rely on natural resources for subsistence and cash income. The trade ban on species listed as critically endangered by the International Union for the Conservation of Nature (IUCN) Red List may be counter-productive, as increasing the rarity and thus price of these species acts as a stimulus to illegal markets rather than a deterrent. Since illegal markets cannot have legal property rights, there is no basis for any form of sustainable harvesting based on property rights.