Energy pyramid & the 10% rule

Only about 10% of the energy stored in one trophic level ends up stored in the biomass of the next; the other 90% is used for respiration, lost as heat, or left in uneaten and undigested material. If producers store 10,000 kJ, primary consumers get about 1,000 kJ, secondary consumers 100 kJ, and tertiary consumers 10 kJ, so 9,990 kJ (99.9%) never reaches the top. Real transfer efficiencies range from roughly 5% to 20%.

Because the energy runs out. At 10% efficiency a fifth trophic level receives 0.1⁴ = 0.01% of the producers’ energy: 1,000,000 kJ captured by plants leaves just 100 kJ for a quaternary consumer, too little to support a viable population of large predators. That is also why top predators are rare and why eating lower on the food chain feeds more people from the same land.

Most of it is released as heat when organisms respire to power movement, growth, and body temperature; the rest stays in parts that are never eaten, or leaves as feces and dead tissue that decomposers use. Energy is not destroyed, it is just no longer available to the next consumer, which is why an ecosystem needs a constant input of sunlight while nutrients such as carbon and nitrogen can be recycled.