Energy Flow

Sunlight is the energy source. Producers (plants) capture roughly 1-2% of incoming sunlight via photosynthesis. From there:

When a primary consumer eats a producer:

• Some plant material is not eaten (roots, woody stems).
• Some is eaten but not digested — leaves the body as faeces.
• Most of the digested energy is used in RESPIRATION → released as HEAT.
• Only ~10% goes into building NEW BODY TISSUE — the part available for the next predator.

The same losses happen at each level. So:

Trophic level	% of original energy
Producer	100%
Primary consumer	~10%
Secondary consumer	~1%
Tertiary consumer	~0.1%

After about 4 levels, there's barely any energy left. THIS is why food chains rarely have more than 4-5 levels.

If you raise a field of wheat, the wheat captures the Sun's energy. You can either:

• Eat the wheat directly → you receive ~10% of the original solar energy. Many people fed.
• Feed the wheat to a cow, then eat the cow → you receive ~1% of the original energy. Far fewer people fed per acre.

In practice, a hectare of land produces:

• About 22,000 kg of potatoes (vegetable diet).
• About 220 kg of beef (meat diet from cattle fed grain).

That's about 100× the human-edible food when growing plants directly.

This is one (powerful) argument for reducing meat in the diet, especially red meat: the same land supports many more people if eaten lower in the chain. It also reduces water use and greenhouse-gas emissions.

This doesn't mean meat is 'wrong' — animals can be grazed on land unsuitable for crops, and provide important nutrients. But the energy math is unambiguous: lower trophic level = more efficient food.

A crucial distinction:

• Energy enters as sunlight, is degraded to heat at each step, and ultimately escapes the ecosystem. It FLOWS one-way.
• Matter (carbon, nitrogen, phosphorus, water) is RECYCLED. Decomposers break down dead organisms and waste → nutrients return to soil → producers re-use them.

That's why the SAME ATOMS of carbon may have been in a dinosaur, a mushroom, the Mediterranean Sea, and now in your bones. But energy doesn't recycle — every ecosystem needs a continuous input of sunlight.

This is a great MYP criterion-D connection: the Sun powers all life on Earth (except a tiny number of deep-sea chemosynthetic communities). If the Sun went out, life would end within a few generations as the energy stored in plants ran down.