Productivity: GPP, NPP and secondary productivity
Primary productivity is the rate at which plants make organic matter. Gross primary productivity minus respiration gives net primary productivity, the food available to consumers.
Primary production and productivity
Every ecosystem needs a constant input of solar energy. Primary production is the amount of biomass (organic matter) made per unit area over a time period by plants during photosynthesis. It is expressed as weight (g m⁻²) or energy (kcal m⁻²). The rate of biomass production is called productivity. It is expressed as g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹, so that different ecosystems can be compared.
GPP vs NPP
Secondary productivity is the rate of formation of new organic matter by consumers.
What decides primary productivity?
- The plant species living in the area.
- Environmental factors such as sunlight, temperature and water.
- The availability of nutrients in the soil or water.
- The photosynthetic capacity of the plants.
- So productivity varies between ecosystems: a tropical rain forest makes far more organic matter per square metre than a desert.
Why are the oceans less productive?
Your textbook gives the annual NPP of the whole biosphere as about 170 billion tons (dry weight) of organic matter. The oceans cover about 70 per cent of the Earth's surface, yet they produce only about 55 billion tons. The rest, about 115 billion tons, comes from land. Your textbook leaves the reason for class discussion, and its exercises name light as the limiting factor for productivity in aquatic ecosystems. The usual answer gives two reasons: sunlight fades quickly with depth, so only the upper layer can photosynthesise, and the open ocean is poor in nutrients such as nitrogen and phosphorus.
Worked example: from GPP to NPP
A grassland has a gross primary productivity of 20,000 kcal m⁻² yr⁻¹. The grasses use 8,000 kcal m⁻² yr⁻¹ in respiration. Find the NPP, and the percentage of GPP that is available to herbivores and decomposers.
- NPP = GPP − R.
- NPP = 20,000 − 8,000 = 12,000 kcal m⁻² yr⁻¹.
- Percentage of GPP = (12,000 ÷ 20,000) × 100 = 60%.
NPP = 12,000 kcal m⁻² yr⁻¹, which is 60% of GPP. This is the food available to herbivores and decomposers.
A well-watered, fertilised paddy field in the Kaveri delta has a high NPP, which is why it can feed so many people. A dry patch of the Thar desert has a very low NPP because water is scarce.
2 marks: “Distinguish between gross primary productivity and net primary productivity.” GPP is the rate of production of organic matter during photosynthesis. NPP is GPP minus respiration losses (NPP = GPP − R) and is the biomass available to consumers.
2 marks: “The oceans cover about 70% of the Earth's surface but have low productivity. Suggest two reasons.” Light, the limiting factor for aquatic productivity, reaches only the upper layer, so photosynthesis is limited with depth; the open ocean is also poor in nutrients such as nitrogen and phosphorus.
3 marks: “Define primary productivity. Name the factors that affect it and explain why it differs between ecosystems.” The rate of biomass production by plants per unit area per unit time (g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹). It depends on the plant species, environmental factors, nutrients and photosynthetic capacity. These vary between ecosystems, so productivity varies too.
Gross is the whole salary; Net is take-home pay after Respiration's tax.