Concept of Ecosystem - Productivity

The rate at which biomass is synthesised by a trophic level per unit area per unit time is called its productivity.

Net Primary Productivity (NPP) = Gross Primary Productivity (GPP) - Respiratory loss by plants (R)

Productivity is the rate of biomass production (organic matter) per unit area over a given time period.

It is a rate, not a quantity – it always involves a time component.

Measured in units of:

• Mass per unit area per unit time: g m⁻² yr⁻¹ or kg m⁻² yr⁻¹
• Energy per unit area per unit time: kcal m⁻² yr⁻¹

These units allow comparison of productivity across different ecosystems.

The following factors determine how productive an ecosystem is:

1. Sunlight: Primary driving force; more sunlight = higher rate of photosynthesis = higher GPP
2. Temperature: Warm temperatures (within the optimal range) increase enzyme activity and photosynthetic rate
3. Water availability: Limiting factor in terrestrial ecosystems; scarcity drastically reduces GPP
4. CO₂ concentration: Essential raw material for photosynthesis
5. Nutrients (macronutrients & micronutrients): Nitrogen, phosphorus, and other minerals are required for enzyme synthesis and chlorophyll production
6. Plant species composition: Dense canopy and diverse plant types increase photosynthetic coverage
7. Human activities: Deforestation, pollution, and soil degradation reduce productivity indirectly

• Productivity is the rate of biomass (organic matter) formation at any trophic level per unit area over time, primarily through photosynthesis or chemosynthesis. Unit: g m⁻² yr⁻¹ or g m⁻² day⁻¹.
• Primary productivity is the amount of biomass produced by plants; it includes Gross Primary Productivity (total production) and Net Primary Productivity (usable biomass left after plant respiration).
• Net Primary Productivity (NPP) is important because it represents the energy available to herbivores and other consumers in the ecosystem.
• Secondary productivity is the rate at which organic matter is formed by consumers (heterotrophs) and depends on primary productivity.
• Productivity varies across ecosystems depending on factors like plant species, nutrient availability, climate, and photosynthetic efficiency; globally, oceans contribute a significant share.