Life is the constant struggle against entropy. As any organism goes through life, it must fight against the universal tendency towards disorder. In order to live, an organism must increase the order within itself, while increasing the disorder of the of the universe.
As energy passes from the sun to the earth, it is absorbed by primary producers, the plants. The primary producers turn this energy (along with water and CO2) into glucose (which
the plant uses) and oxygen (which escapes into the atmosphere and adds to entropy). Suppose a deer (primary consumer) comes along, spies our lovely plant and eats it. Only 10% of the energy absorbed by the plant from the sun goes to the deer. Now Suppose an enormous saber-toothed tiger comes along and devours our poor deer. The saber-toothed tiger is a secondary consumer (Teddy Roosevelt would be a tertiary consumer, and so on). Only 1% of the energy from the plant goes to the saber-toothed tiger. That’s a 90% loss of energy as it flows through a food chain. This means that in order to survive, each organism must constantly replenish the energy which is lost at each tropic level. Inevitably, some animals will lose this struggle, and starve.