Explain why a food chain can have no more than three or four links?

As more organisms participate in a food chain, less energy is transferred from the prey to the predator. On average about 10% of the energy on one level is transferred to the next level. The other 90% is either lost as heat or cannot be digested.

Example: A plant contains 10,000 kilocalories. The plant is eaten by an insect that will only gain 1,000 kilocalories of energy. Next, the insect is eaten by a fish which gains 100 kilocalories of energy. A bird consumes the fish and gains 10 kilocalories of energy. Finally, a mountain lion catches the bird and gains only 1 kilocalorie of energy.