What's the difference between economists and ecologists? According to one wag, economists think they are God's gift to the world, whereas ecologists think the world is God's gift to them.

This might seem to offer a poor start to a close working relationship — and, indeed, beyond this flippant answer there are many cultural differences between the two disciplines. The traditional gulf between the natural and social sciences is further widened in this case by a tendency towards opposite political leanings: the economics that is in the ascendant today is closely aligned with capitalism, yet rampant capitalism is often accused of being the enemy of the environment.

But the academic discipline of ecology is not the same as ‘environmentalism’, and economists are at pains to explain that the real purpose of their discipline is the efficient allocation of scarce resources — a description that is, sadly, becoming increasingly relevant to the Earth's environmental assets. There are even technical similarities between the two fields: for example, an ecologist's description of competition and cooperation between species has much in common with the treatment of the behaviour of companies in microeconomics.

Given the crucial importance of both fields, it is not so surprising that, as described elsewhere in this issue (Briefing, page 426; Commentary, page 433), ecologists and economists are working closely together on ways to value and preserve the natural environment. Thus at one level, much of the success of the Kyoto Protocol on climate change hinges on efforts to set up an efficient and effective system of greenhouse-gas emissions trading. At another, cooperation between ecologists and economists will be needed in a $2.3 million United Nations project announced last week to assess the demand for water in African cities, and its impact on freshwater resources and aquatic ecosystems.

In the days when humans were a small perturbation on the face of the planet, economics had little more to say about natural ecosystems than it does today about sunshine. But today, a viable combination of economics and ecology is essential; for while natural science may be able to describe what is needed for a sustainable world, only social science can articulate a path by which such a goal can be reached.