compare the monetisation of ecosystem services to the enclosures of common land. What price the biosphere? Before we rush to financialise ecosystem services, or to counter this tendency with cries of horror about corporate greed, why not acknowledge that we don't yet know how ecosystem services -- or biodiversity -- actually work? Ecosystems have functional values in dimensions that science can measure. Surely the first thing to do is to demonstrate the dependence of all terrestrial life on ecosystems in a practical, measurable way that everyone can understand. It should then be obvious that humanity has a special responsibility in respect of the biosphere which cannot possibly be discharged by selling chunks of it to anyone. Michael Dunwell Staunton, Gloucestershire  George Monbiot is making a big mistake in arguing for a market-free world. Demand for access to fragile areas for tourism and extraction of resources, combined with scarcity, creates a market whether he likes it or not. The issue is whether it is shaped in a rational way and how it is regulated. The creation of incentives to invest in the conservation of biodiversity creates the possibility of a win for communities (investment in eco-tourism and its community infrastructure), for nature (price denotes real value creating a disincentive to destroy), and for investors (income from bonds over time). Those following the mythical path, denying the reality of market mechanisms, create obstacles to sustainable conservation. Jeremy Ross Ashtead, Surrey  When land is taken into private ownership, in effect duty-free