I've been reading The Permaculture Way, by Graham Bell, as well as Gaia's Garden: A Home-scale guide to Permaculture, by Toby Hemenway. In both these books, the use and reuse of energy is shown as a cycle rather than a concept of consumption.
We usually think of energy in terms of kilowatts that come from a power plant, gasoline and mpg, and for some of us, propane tanks. The various forms of energy we know of are all derivatives of our benefactor, the sun. It shines and gives plants energy, producing food for herbivores, in turn creating food for carnivores and omnivores. The oil, coal and gasses the earth provides are all forms of the suns energy stored from millions and billions of years.
So in fact, we can look at all life as different manifestations of the suns energy. The plants, the animals that eat the plants, the animals that eat the animals, the decomposers who eat everything, are all created by the energy of the sun.
We can reuse the energy stored in our "waste." Leftover food for example, can be returned to the cycle instead of being sent to the landfill. Yes, it will eventually be consumed by some other life form, but why not let that energy be spent on something nearer to home, and that can actually provide you a benefit. Send it to your compost, mulch it, give it to other animals, give it to your neighbor with a garden who will greatly appreciate it. These are much more efficient ways to recycle the energy. If we can find ways to keep the energy in a system longer, then more benefit can come from it.