2.2.2—First law of thermodynamics

In an ecosystem, energy is transferred and transformed—not created or destroyed—and each transfer leaves less usable energy available to the next trophic level.

The first law accounts for all energy, while the second-law idea explains usefulness: respiration and other processes disperse part of the input as heat. So energy still exists after a transfer, but the ecosystem must keep receiving new energy to maintain biological work.

A plant stores 100 kJ of light energy as biomass. A herbivore eats it; perhaps 20 kJ becomes new herbivore biomass while the rest is used in respiration or leaves as heat. Nothing vanished—the usable store became smaller.

It was transferred to respiration, movement, waste and heat; ‘lost’ means unavailable to the next trophic level, not destroyed.

Do not say energy cycles like nutrients. Matter can be reused in biogeochemical cycles; usable energy flows through and must be continually supplied.