The Millennium Ecosystem Assessment describes the following major categories of ecosystem services-provisioning, supporting, regulating, preserving and cultural. Which one of the following is supporting service?

The answer is C. Supporting services are the processes that keep the ecosystem working, such as nutrient cycling, soil formation, primary production and pollination. They underpin every other service, but people do not usually consume them directly.

• A (food and water) is a provisioning service, the goods we take from ecosystems.
• B (climate and disease control) is a regulating service, the benefits from regulating natural processes.
• D (maintenance of diversity) is listed in the question's own scheme under the separate 'preserving' category. In the 2005 MEA text, genetic diversity is often grouped under supporting services, which is why this option looks tempting.

For Mains, this framework links to ecosystem-based adaptation, payment for ecosystem services and the economic valuation of biodiversity loss. Cultural services, such as recreation and spiritual value, complete the list.

Trap: Aspirants pick D because 'diversity' sounds basic to the ecosystem. Remember that the question uses its own five-category scheme, and nutrient cycling is the cleanest textbook example of a supporting service.