Energy enters most ecosystems as sunlight. Plants capture it through photosynthesis and convert it into chemical energy stored in sugars. Herbivores eat the plants and use some of that energy to grow and move. Carnivores eat the herbivores. At each step, most of the energy is lost as heat. Only about 10 percent passes from one trophic level to the next. That inefficiency shapes everything about ecosystems.

Because energy dwindles at each transfer, food chains are short. A field might support thousands of grasshoppers but only a few hawks. Top predators need large territories to find enough food. The 10 percent rule is a rough average. It varies by ecosystem and organism. Cold-blooded animals are more efficient than warm-blooded ones because they do not spend energy on body heat. That is why a snake can survive on far less food than a bird of the same size.

Key points about energy flow

• Energy enters as sunlight and leaves as heat.
• Only about 10 percent passes between trophic levels.
• Food chains rarely exceed four or five links.
• Top predators are scarce for energetic reasons.

Matter cycles. Energy does not. The carbon in your body has been part of many organisms before you. The energy you used to read this sentence is gone for good.