A typical ecosystem like the one pictured here might have 200,000 caterpillars, 10000 frogs, 500 snakes, and 35 owls. Why do you think there are fewer and fewer organisms at each level of the food chain?
This is due to the inefficiencies of passing energy from one energy level to the next. Studies have shown that only 10% of energy is passed from a lower trophic level to the next higher level as biomass. The rest is lost in inefficiencies such as heat. This is why the graphical model of energy flow in a community is shaped like a pyramid with the lowest energy levels at the bottom and highest energy levels at the top.
