๐ฟ Chapter 01 ยท Topic 04 ยท Energy Flow
Energy Flow & Productivity in Ecosystems
Unidirectional energy flow, GPP vs NPP, secondary productivity, ecological efficiency, standing crop, and energy flow models.
Energy Flow
Energy flows through an ecosystem in a unidirectional manner โ from the sun to producers, then to consumers at successive trophic levels. Unlike nutrients, energy cannot be recycled.
- Source: Solar energy (sunlight) is the primary energy source for almost all ecosystems
- Entry point: Producers (plants) capture solar energy via photosynthesis and convert it to chemical energy
- Flow direction: Sun โ Producers โ Primary consumers โ Secondary consumers โ Tertiary consumers
- Loss at each level: ~90% energy lost as heat; only ~10% transferred to next level
- Unidirectional: Energy cannot flow backward from consumers to producers
Key Contrast: Energy flow is unidirectional and non-cyclic. Nutrient/matter flow is cyclic (biogeochemical cycles). This is a very common exam question.
Gross Primary Productivity (GPP) vs Net Primary Productivity (NPP)
Primary Productivity = rate at which producers synthesise organic matter (biomass) through photosynthesis.
| Term | Definition | Formula |
|---|---|---|
| GPP (Gross Primary Productivity) | Total rate of photosynthesis including the organic matter used in respiration | GPP = NPP + R |
| NPP (Net Primary Productivity) | Rate of organic matter accumulation after subtracting plant respiration; available to consumers | NPP = GPP โ R |
| R (Respiration) | Energy used by producers for their own metabolic activities | R = GPP โ NPP |
NPP is what matters for the food chain โ it is the energy actually available to herbivores and higher trophic levels. GPP includes energy the plant uses for itself.
Productivity by Ecosystem (approximate NPP):
| Ecosystem | NPP (g C/mยฒ/yr) | Rank |
|---|---|---|
| Tropical rainforest | ~2,000 | Highest terrestrial |
| Estuaries / Wetlands | ~1,500โ2,000 | Highest overall |
| Temperate forest | ~1,200 | Moderate |
| Grassland | ~600 | Moderate |
| Open ocean | ~125 | Low (but large area) |
| Desert | ~40 | Lowest |
Secondary Productivity
Secondary Productivity = rate of formation of new organic matter by consumers (heterotrophs) through assimilation of food.
- Measured at the level of primary consumers (herbivores), secondary consumers, etc.
- Always less than primary productivity due to energy losses at each trophic level
- Formula: Secondary Productivity = Assimilation โ Respiration
- Includes growth, reproduction, and storage of energy in consumer biomass
Ecological Efficiency
Ecological efficiency measures how effectively energy is transferred between trophic levels.
| Type of Efficiency | Definition | Typical Value |
|---|---|---|
| Lindeman’s Efficiency | Energy at trophic level n+1 / Energy at trophic level n ร 100 | ~10% |
| Assimilation Efficiency | Energy assimilated / Energy ingested ร 100 | 60โ90% (plants); 20โ50% (animals) |
| Production Efficiency | Net production / Assimilation ร 100 | Varies by organism |
| Consumption Efficiency | Ingestion at level n+1 / Production at level n ร 100 | Varies |
Lindeman’s Efficiency = 10% is the most important value for exams. It means: if producers have 1,000 kcal, herbivores get 100 kcal, carnivores get 10 kcal, top carnivores get 1 kcal.
Standing Crop vs Standing State
| Term | Definition |
|---|---|
| Standing Crop | Total amount of living organic matter (biomass) present in an ecosystem at a given time; measured in g/mยฒ or kcal/mยฒ |
| Standing State | Total amount of nutrients (inorganic substances) present in the soil at a given time; measured in g/mยฒ |
Memory Tip: Standing Crop = living biomass (organic). Standing State = nutrients in soil (inorganic). Crop = living; State = stored nutrients.
Energy Flow Models
1. Lindeman’s Model (1942)
- First quantitative model of energy flow in an ecosystem
- Based on study of Cedar Bog Lake, Minnesota, USA
- Proposed the 10% law of energy transfer
- Showed that energy decreases at each successive trophic level
2. Odum’s Model (H.T. Odum)
- More comprehensive model showing energy flow through all components
- Includes inputs (solar energy), outputs (heat), and internal cycling
- Distinguishes between grazing food chain and detritus food chain pathways
- Shows that most energy in terrestrial ecosystems flows through the detritus pathway
Important: In most terrestrial ecosystems, more energy flows through the detritus food chain than the grazing food chain. In aquatic (open ocean) ecosystems, the grazing food chain dominates.
Revision Checklist
โ
Energy flow is unidirectional and non-cyclic
โ Nutrients/matter flow is cyclic (biogeochemical cycles)
โ Primary source of energy = sunlight
โ GPP = NPP + Respiration
โ NPP = GPP โ Respiration (energy available to consumers)
โ Highest NPP: Tropical rainforest (terrestrial); Estuaries (overall)
โ Lowest NPP: Desert
โ Secondary productivity = rate of organic matter formation by consumers
โ Lindeman’s Efficiency = 10% energy transfer per trophic level
โ Standing Crop = living biomass; Standing State = soil nutrients
โ Lindeman’s model (1942) โ Cedar Bog Lake study
โ Most terrestrial energy flows through detritus food chain
โ Most aquatic (open ocean) energy flows through grazing food chain
โ Nutrients/matter flow is cyclic (biogeochemical cycles)
โ Primary source of energy = sunlight
โ GPP = NPP + Respiration
โ NPP = GPP โ Respiration (energy available to consumers)
โ Highest NPP: Tropical rainforest (terrestrial); Estuaries (overall)
โ Lowest NPP: Desert
โ Secondary productivity = rate of organic matter formation by consumers
โ Lindeman’s Efficiency = 10% energy transfer per trophic level
โ Standing Crop = living biomass; Standing State = soil nutrients
โ Lindeman’s model (1942) โ Cedar Bog Lake study
โ Most terrestrial energy flows through detritus food chain
โ Most aquatic (open ocean) energy flows through grazing food chain