Why can’t food chains go on forever?

Because at some point in that food chain, the energy consumption has it's limit, and the predator will not have anything to hunt it. There is a limited amount of energy in a food chain that can go around, and once you reach the predator, that energy doesn't quite run out but it hits that limit where the chain has to repeat itself.

Explanation:

the element that provides strength to the exoskeletons of clams and oysters is phosphorus.

this is so because calcium carbonate is the main component in the shells. calcium carbonate is strengthened by phosphorus.

explanation:

as many times as you want