Introduction The world’s stock of biodiversity is declining at an alarming rate: species are thought to be going extinct at rates unprecedented in the history of mankind. To the extent that species have an intrinsic value, biodiversity decline constitutes a welfare loss, but also because of the resulting decrease in the world’s stock of genetic material. Opinions differ with respect to the actual rate of extinction as well as with respect to how costly it is for society when species disappear. But the fact remains that many governments nowadays implement policies specifically aimed at conserving individual species as well as species diversity in general, which suggests that there is widespread consensus about the urgency of protecting biodiversity. The main direct threats to biodiversity conservation are excessive harvesting of specific species and habitat destruction (see Reid and Miller 1989). A common feature of these two types of threats is that there is little incentive for individual agents to take into consideration the impact of their (economic) activities on species conservation. In many instances, property rights with respect to the use of natural resources are either simply lacking or not well enforced. Hence, agents who invest in larger future stocks of a particular species or resource - by reducing current extraction - cannot claim an exclusive right to the future benefits of their investment.