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Synopsis How ecosystems work and what part they play in biodiversity remain a mystery. But we do know that they perform a host of invaluable services for the human species. Biodiversity's fundamental value is neither aesthetic nor economic but environmental, even though most people are largely unaware of this. The value of biodiversity is often measured in terms of the number of species living in a given area. But the interactions between the many species in an ecosystem, and between them and the environment's physical and chemical components are also very important. This highly intricate web of relationships makes an ecosystem more valuable than the sum of the species it contains. Ecosystems perform services that are essential for the survival of the human species. They fix carbon in the atmosphere and produce oxygen, protect soil from erosion and keep it fertile, filter water and replenish aquifers, provide pollination and anti-parasite agents and so on. The first two of these services are closely related to each other. They result from photosynthesis, whereby green plants, staring with algae, absorb carbon dioxide (CO2) and emit oxygen. For millions of years, the balance between the various gases in the atmosphere remained stable. But with the coming of the industrial revolution, humans began burning increased amounts of fossil fuels. Today, three billion tonnes of carbon build up in the atmosphere each year, and natural ecosystems can no longer absorb all these emissions--especially since they are disappearing at an alarming rate. Deforestation alone releases such tremendous amounts of CO2 and other gases, such as methane, that it has become the second-leading cause of global warming.