Q.Primary productivity varies from ecosystem to ecosystem. Explain?

Primary productivity — the rate at which plants capture and store energy — is not uniform across ecosystems. It depends on a combination of factors that control how well plants can photosynthesise.

The most important factors are:

• Sunlight — the energy source for photosynthesis. Ecosystems near the equator receive more intense and consistent sunlight, so they tend to have higher productivity than those at higher latitudes.
• Water availability — a limiting factor in deserts and dry grasslands. Even with abundant sunlight, productivity drops sharply if water is scarce.
• Temperature — affects enzyme activity and metabolic rates. Warm, tropical climates support year-round growth, while cold tundra or alpine regions have very short growing seasons. …

The idea of primary productivity is central to understanding how ecosystems function. It measures how much organic matter (biomass) is produced by plants through photosynthesis in a given time and area. Think of it as the engine that drives the entire food web—herbivores eat plants, carnivores eat herbivores, and so on. But this engine does not run at the same speed everywhere. A lush tropical rainforest and a barren desert are both ecosystems, yet their primary productivities are worlds apart. Why?

The NCERT textbook explains that primary productivity depends on several factors that vary from place to place. The most obvious is sunlight. Plants need light for photosynthesis, so ecosystems that receive abundant sunlight year-round—like tropical forests near the equator—tend to have high productivity. In contrast, high-latitude tundra or deep forests under a dense canopy receive less light, limiting growth.

Water availability is equally critical. Deserts have plenty of sunlight but very little water, so plant growth is stunted. Wetlands, estuaries, and tropical rainforests, on the other hand, have ample water, allowing plants to photosynthesize for longer periods. The NCERT notes that productivity is highest where both sunlight and water are abundant—conditions found in tropical rainforests and coral reefs.

Temperature also plays a decisive role. Photosynthesis is a chemical process that slows down in cold conditions. In polar regions or high mountains, the growing season is short, and low temperatures limit enzyme activity. Warm ecosystems, such as tropical forests and grasslands, support faster growth rates.

Soil nutrients are another key factor. Even with plenty of sun and water, poor soil can choke productivity. Tropical rainforests, ironically, often have thin, nutrient-poor soils because heavy rains leach away minerals. Yet their productivity remains high because nutrients are recycled quickly through the dense litter layer. In contrast, temperate forests and agricultural lands may have richer soils but less sunlight or shorter growing seasons.

Build a factor-by-ecosystem grid — rows sunlight/water/temperature/nutrients, columns desert/tropical-forest/tundra/estuary — and fill in high/low for each cell. Reading down any single column then gives that ecosystem's over …