Q.According to David Tilman, greater the diversity, greater is the primary productivity. Can you think of a very low diversity man-made ecosystem that has high productivity?

Of course. David Tilman's finding—that higher species diversity leads to higher primary productivity—comes from his long-term grassland experiments. It describes a natural ecosystem pattern. But the question asks you to think of a man-made ecosystem that breaks this rule: low diversity, yet high productivity.

The classic NCERT example is an agricultural field or a monoculture plantation.

Here is why it fits:

• Very low diversity: A wheat field or a sugarcane farm typically contains just one crop species. All other plants are removed as weeds.
• High productivity: These ecosystems are incredibly productive. They are designed to channel all available sunlight, water, and nutrients into a single, fast-growing crop. …

David Tilman’s finding — that greater species diversity often leads to higher primary productivity in natural ecosystems — rests on the idea that diverse communities use resources more efficiently. Different species occupy different niches, complement each other, and buffer against environmental fluctuations. But this principle applies most strongly to natural ecosystems. When we turn to man-made systems, the picture changes dramatically.

Consider a modern agricultural field, say a vast wheat farm in Punjab or a rice paddy in West Bengal. This is an ecosystem deliberately stripped of diversity. It contains essentially one crop species — a monoculture — with perhaps a few hardy weeds that farmers try to eliminate. Biodiversity here is extremely low. Yet the primary productivity (the rate at which plants convert sunlight into biomass) is remarkably high. A wheat field can produce tonnes of grain per hectare each season.

How does this square with Tilman’s observation? The answer lies in human inputs. In a natural grassland, high diversity ensures that if one species suffers from drought or disease, others compensate, maintaining overall productivity. In a monoculture field, we replace that natural resilience with artificial support. We irrigate, fertilise heavily, control pests with pesticides, and remove competing weeds. The crop is bred for maximum yield under these pampered conditions. So the high productivity is not a property of the ecosystem itself — it is a temporary, energy-intensive achievement sustained by constant human effort.

Notice the trap: Tilman's finding describes a natural self-sustaining system, so the answer must be a system that substitutes human inputs ( …