Flow of Energy

Every ecosystem needs a continuous input of energy to sustain life processes – growth, reproduction, and metabolism. Unlike nutrients (carbon, nitrogen, and water), which cycle repeatedly through the ecosystem, energy passes through only once and is progressively lost as heat. This one-way passage of energy from the Sun through producers, consumers, and decomposers is called the 'flow of energy'.

The portion of sunlight (roughly 400–700 nm wavelength) that plants can actually use for photosynthesis.

1. Solar Input:
   The Sun is the ultimate source of energy for nearly all ecosystems.
2. Radiation Breakdown:
   Total incoming solar radiation is split: ~40% is reflected into space, ~15% is absorbed by the atmosphere, and ~45% reaches the Earth's surface.
3. Energy Capture:
   From the 45% that reaches the surface, green plants trap only about 1% via photosynthesis. The remaining 99% is used up by evaporation, heating the soil and air, or is reflected away.

• Producers (green plants, algae, and cyanobacteria) convert solar energy into chemical energy (food) via photosynthesis. They are correctly called energy converters/transducers, not energy creators, since energy cannot be created – only transformed.
• This chemical energy is passed to primary consumers (herbivores), then to secondary consumers (carnivores), and further to tertiary consumers, before finally reaching decomposers, which break down dead organic matter and release residual energy as heat.

Only about 10% of the energy available at one trophic level is transferred to the next; the remaining 90% is lost as heat during respiration, movement, and other metabolic activities.

• Explains the pyramid shape of energy in ecosystems (energy pyramids are always upright).
• Limits food chains to typically 3–4 trophic levels, since energy becomes insufficient beyond that.

1. Solar energy is the ultimate source of energy for all ecosystems, but only a very small fraction reaches and is used by living organisms.
2. Green plants (producers) trap about 1% of solar energy through photosynthesis and convert light energy into chemical energy of food.
3. Energy flow is unidirectional (non-cyclic): it moves from the sun → producers → consumers → decomposers and is finally lost as heat.
4. According to the laws of thermodynamics, energy cannot be created or destroyed, and with every transfer some energy is lost as heat.
5. The Ten Percent Law states that only about 10% of energy at one trophic level is transferred to the next, limiting food chains to few levels.