Which of the following has the lowest meant net primary productivity?

The correct answer is C) Temperate grasslands. Net primary productivity (NPP) is gross primary productivity minus the energy producers use in their own respiration, so it measures the organic matter actually available to herbivores and decomposers. Water, warmth and length of growing season are the main drivers.

Among the options, temperate grasslands have the lowest NPP, roughly 600 g/m²/yr in the standard Whittaker and Odum estimates. Tropical rain forests sit at the top, around 2200 g/m²/yr, and temperate forests are at about 1200 to 1250. Tropical savanna lies near 900, higher than temperate grassland because its warm season is longer.

Connect this to Mains GS-III on ecosystems and food security: grassland productivity underpins pastoral and rangeland economies, and these figures explain why such systems degrade quickly under overgrazing.

Trap: Students pick tropical savanna because it looks harsh and dry, but savanna still gets enough warmth and rain to outrank temperate grassland. Compare the options against each other, not by how dry they look.